Difference between pages "Awk by Example, Part 3" and "LVM Fun"

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{{WikiArticle}}
= Introduction =


== String functions and ... checkbooks? ==
LVM (Logical Volume Management) offers a great flexibility in managing your storage and significantly reduces server downtimes by allowing on-line disk space management: The great idea beneath LVM is to '''make the data and its storage loosely coupled''' through several layers of abstraction. You (the system administrator) have the hand of each of those layers making the entire space management process extremely simple and flexible though  various set of coherent commands.  


=== Formatting output ===
Several other well-known binary Linux distributions makes an aggressive use of LVM and several Unixes including HP-UX, AIX and Solaris offers since a while a similar functionality modulo the commands to be used. LVM is not mandatory but its usage can bring you additional flexibility and make your everyday life much more simpler.
While awk's print statement does do the job most of the time, sometimes more is needed. For those times, awk offers two good old friends called printf() and sprintf(). Yes, these functions, like so many other awk parts, are identical to their C counterparts. printf() will print a formatted string to stdout, while sprintf() returns a formatted string that can be assigned to a variable. If you're not familiar with printf() and sprintf(), an introductory C text will quickly get you up to speed on these two essential printing functions. You can view the printf() man page by typing "man 3 printf" on your Linux system.
 
= Concepts =
 
As usual, having a good idea of the concepts lying beneath is mandatory. LVM is not very complicated, but it is easy to become confused, especially because it is a multi-layered system; however LVM designers had the good idea of keeping the command names consistent between all LVM command sets, making your life easier.
 
LVM consists of, mainly, three things:
 
* '''Physical volumes (or ''PV'')''': nothing more than a physical storage space. A physical volume can by anything like a partition on a local hard disk, a partition located on a remote SAN disk, a USB key or whatever else that could offer a storage space (so yes, technically it could be possible to use an optical storage device accessed in packet writing mode). The storage space on a physical volumes is divided (and managed) in small units called '''Physical Extents''' (or ''PE''). Just to give an analogy if you are a bit familiar with RAID, PE are a bit like RAID stripes.
* '''Volume Groups (or ''VG'')''': a group of at least one PV. VG are '''named''' entities and will appear in the system via the device mapper as '''/dev/''volume-group-name'''''.
* '''Logical Volumes (or ''LV'')''': a '''named''' division of a volume group in which a filesystem is created and that can be mounted in the VFS. Just for the record, just as for the PE in PV, a LV is managed as chucks known as Logical Extents (or ''LE''). Most of the time those LE are hidden to the system administrator due to a 1:1 mapping between them and the PE lying be just beneath but a cool fact to know about LEs is that they can be spread over PV just like RAID stripes in a RAID-0 volume. However, researches done on the Web tends to demonstrate system administrators prefer to build RAID volumes with mdadm than use LVM over them for performance reasons.
 
In short words:  LVM logical volumes (LV) are containers that can hold a single filesystem and which are created inside a volume group (VG) itself composed by an aggregation of at least one physical volumes (PV) themselves stored on various media (usb key, harddisk partition and so on). The data is stored in chunks spread over the various PV.
 
{{fancynote|Retain what PV, VG and LV means as we will use those abbreviations in the rest of this article.}}
 
= Your first tour of LVM =
 
== Physical volumes creation ==
 
{{fancynote|We give the same size to all volumes for the sake of the demonstration. This is not mandatory and be possible to have mixed sizes PV inside a same VG. }}
 
To start with, just create three raw disk images:


Here's some sample awk sprintf() and printf() code. As you can see in the following script, everything looks almost identical to C.
<pre>
<pre>
#!/usr/bin/awk -f
# dd if=/dev/zero of=/tmp/hdd1.img bs=2G count=1
BEGIN {
# dd if=/dev/zero of=/tmp/hdd2.img bs=2G count=1
x=1
# dd if=/dev/zero of=/tmp/hdd3.img bs=2G count=1
b="foo"
printf("%s got a %d on the last test\n","Jim",83)
myout=sprintf("%s-%d",b,x)
print myout
}
</pre>
</pre>
This code will print:
 
and associate them to a loopback device:
 
<pre>
<pre>
Jim got a 83 on the last test
# losetup -f
foo-1
/dev/loop0
# losetup /dev/loop0 /tmp/hdd1.img
# losetup /dev/loop1 /tmp/hdd2.img
# losetup /dev/loop2 /tmp/hdd3.img
 
</pre>
</pre>


=== String functions ===
Okay nothing really exciting there, but wait the fun is coming! First check that '''sys-fs/lvm2''' is present on your system and emerge it if not. At this point, we must tell you a secret: although several articles and authors uses the taxonomy "LVM" it denotes "LVM version 2" or "LVM 2" nowadays. You must know that LVM had, in the old good times (RHEL 3.x and earlier), a previous revision known as "LVM version 1". LVM 1 is now considered as an extincted specie and is not compatible with LVM 2, although LVM 2 tools maintain a backward compatibility. 
Awk has a plethora of string functions, and that's a good thing. In awk, you really need string functions, since you can't treat a string as an array of characters as you can in other languages like C, C++, and Python. For example, if you execute the following code:
 
The very frst step in LVM is to create the physical devices or ''PV''. "Wait create ''what''?! Aren't the loopback devices present on the system?" Yes they are present but they are empty, we must initialize them some metadata to make them usable by LVM. This is simply done by:
 
<pre>
<pre>
mystring="How are you doing today?"
# pvcreate /dev/loop0
print mystring[3]
  Physical volume "/dev/loop0" successfully created
# pvcreate /dev/loop1
  Physical volume "/dev/loop1" successfully created
# pvcreate /dev/loop2
  Physical volume "/dev/loop2" successfully created
</pre>
</pre>
You'll receive an error that looks something like this:
 
It is absolutely normal that nothing in particular is printed at the output of each command but we assure you: you have three LVM PVs. You can check them by issuing:
 
<pre>
<pre>
awk: string.gawk:59: fatal: attempt to use scalar as array
# pvs
  PV        VG  Fmt  Attr PSize PFree
  /dev/loop0      lvm2 a-  2.00g 2.00g
  /dev/loop1      lvm2 a-  2.00g 2.00g
  /dev/loop2      lvm2 a-  2.00g 2.00g
</pre>
</pre>
Oh, well. While not as convenient as Python's sequence types, awk's string functions get the job done. Let's take a look at them.


First, we have the basic length() function, which returns the length of a string. Here's how to use it:
 
Some good information there:
* PV: indicates the physical path the PV lies on
* VG indicates the VG the PV belongs to. At this time, we didn't created any VG yet and the column remains empty.
* Fmt: indicates the format of the PV (here it says we have a LVM version 2 PV)
* Attrs: indicates some status information, the 'a' here just says that the PV is accessible.
* PSize and PFree: indicates the PV size and the amount of remaining space for this PV. Here we have three empty PV so it bascially says "2 gigabytes large, 2 out of gigabytes free"
 
It is now time to introduce you to another command: '''pvdisplay'''. Just run it without any arguments:
 
<pre>
<pre>
print length(mystring)
pvdisplay
  "/dev/loop0" is a new physical volume of "2.00 GiB"
  --- NEW Physical volume ---
  PV Name              /dev/loop0
  VG Name             
  PV Size              2.00 GiB
  Allocatable          NO
  PE Size              0 
  Total PE              0
  Free PE              0
  Allocated PE          0
  PV UUID              b9i1Hi-llka-egCF-2vU2-f7tp-wBqh-qV4qEk
 
  "/dev/loop1" is a new physical volume of "2.00 GiB"
  --- NEW Physical volume ---
  PV Name              /dev/loop1
  VG Name             
  PV Size              2.00 GiB
  Allocatable          NO
  PE Size              0 
  Total PE              0
  Free PE              0
  Allocated PE          0
  PV UUID              i3mdBO-9WIc-EO2y-NqRr-z5Oa-ItLS-jbjq0E
 
  "/dev/loop2" is a new physical volume of "2.00 GiB"
  --- NEW Physical volume ---
  PV Name              /dev/loop2
  VG Name             
  PV Size              2.00 GiB
  Allocatable          NO
  PE Size              0 
  Total PE              0
  Free PE              0
  Allocated PE          0
  PV UUID              dEwVuO-a5vQ-ipcH-Rvlt-5zWt-iAB2-2F0XBf
</pre>
</pre>
This code will print the value:
 
The third three lines of each PV shows:
* what is the storage device beneath a PV
* the VG it is tied to
* the size of this PV.
''Allocatable'' indicates whether the PV is used to store data. As the PV is not a member of a VG, it cannot not be used (yet) hence the "NO" shown. Another set of information is the lines starting with ''PE''. ''PE'' stands for ''' ''Physical Extents'' ''' (data stripe) and is the finest granularity LVM can manipulate. The size of a PE is "0" here because we have a blank PV however it typically holds 32 MB of data. Following ''PE Size'' are ''Total PE'' which show the the total '''number''' of PE available on this PV and ''Free PE'' the number of PE remaining available for use. ''Allocated PE'' just show the difference between ''Total PE'' and ''Free PE''.
 
The latest line (''PV UUID'') is a unique identifier used internally by LVM to name the PV. You have to know that it exists because it is sometimes useful when having to recover from corruption or do weird things with PV however most of the time you don't have to worry about its existence.
{{fancynote|It is possible to force how LVM handles the alignments on the physical storage. This is useful when dealing with 4K sectors drives that lies on their physical sectors size. Refer to the manual page. }}
 
== Volume group creation ==
 
We have the blank PV at this time but to make them a bit more usable for storage we must tell to LVM how they are grouped to form a VG (storage pool) where LV will be created. A nice aspect of VGs resides in the fact that they are not "written in the stone" once created: you can still add, remove or exchange PV (in the case the device the PV is stored on fails for example) inside a VG at a later time. To create our first volume group named ''vgtest'':
 
<pre>
<pre>
24
# vgcreate vgtest /dev/loop0 /dev/loop1 /dev/loop2
  Volume group "vgtest" successfully created
</pre>
</pre>
OK, let's keep going. The next string function is called index, and will return the position of the occurrence of a substring in another string, or it will return 0 if the string isn't found. Using mystring, we can call it this way:
 
Just like we did before with PV, we can get a list of what are the VG known by the system. This is done through the command '''vgs''':
 
<pre>
<pre>
print index(mystring,"you")
# vgs
  VG    #PV #LV #SN Attr  VSize VFree
  vgtest  3  0  0 wz--n- 5.99g 5.99g
</pre>
</pre>
Awk prints:
 
'''vgs''' show you a tabluar view of information:
* '''VG:''' the name of the VG
* '''#PV:''' the number of PV composing the VG
* '''#LV:''' the number of logical volumes (LV) located inside the VG
* '''Attrs:''' a status field. w, z and n here means that VG is:
** '''w:''' '''w'''ritable
** '''z:''' resi'''z'''able
** '''n:''' using the allocation policy '''''n'''ormal'' (tweaking allocation policies is beyond the scope of this article, we will use the default value ''normal'' in the rest of this article)
* VSize and VFree gives statistics on how full a VG is versus its size
 
Note the dashes in ''Attrs'', they mean that the attribute is not active:
* First dash (3rd position) indicates if the VG would have been exported (a 'x' would have been showed at this position in that case).
* Second dash (4th position) indicates if the VG would have been partial (a 'p' would have been showed at this position in that case).
* Third dash (rightmost position) indicates if the VG is a clustered (a 'c' would have been showed at this position in that case). 
 
Exporting a VG and clustered VG are a bit more advanced aspects of LVM and won't be covered here especially the clustered VGs which are used in the case of a shared storage space used in a cluster of machines. Talking about clustered VGs management in particular would require and entire article in itself. '''For now the only detail you have to worry about those dashes in ''Attrs'' is to see a dash at the 4th position of ''Attrs'' instead of a ''p'''''. Seeing ''p'' there would be a bad news: the VG would have missing parts (PV) making it not usable.
 
{{fancynote|In the exact same manner you can see a detailed information about physical volumes with '''pvdisplay''', you can see detailed information of a volume group with '''vgdisplay'''. We will demonstrate that latter command in the paragraphs to follow.}}
 
Before leaving the volume group aspect, do you remember the '''pvs''' command shown in the previous paragraphs? Try it gain:
 
<pre>
<pre>
9
# pvs
  PV        VG    Fmt  Attr PSize PFree
  /dev/loop0 vgtest lvm2 a-  2.00g 2.00g
  /dev/loop1 vgtest lvm2 a-  2.00g 2.00g
  /dev/loop2 vgtest lvm2 a-  2.00g 2.00g
</pre>
</pre>
We move on to two more easy functions, tolower() and toupper(). As you might guess, these functions will return the string with all characters converted to lowercase or uppercase respectively. Notice that tolower() and toupper() return the new string, and don't modify the original. This code:
 
Now it shows the VG our PVs belong to :-)
 
== Logical volumes creation ==
 
Now the final steps: we will create the storage areas (logical volumes or ''LV'') inside the VG where we will then create filesystems on. Just like a VG has a name, a LV has also a name which is unique in the VG.
 
{{fancynote|Two LV can be given the same name as long as they are located on a different VG.}}
 
To divide our VG like below:
 
* lvdata1: 2 GB
* lvdata2: 1 GB
* lvdata3 : 10% of the VG size
* lvdata4 : All of remaining free space in the VG
 
We use the following commands (notice the capital 'L' and the small 'l' to declare absolute or relative sizes):
 
<pre>
<pre>
print tolower(mystring)
# lvcreate -n lvdata1 -L 2GB vgtest
print toupper(mystring)
  Logical volume "lvdata1" created
print mystring
#  lvcreate -n lvdata2 -L 1GB vgtest
  Logical volume "lvdata2" created
# lvcreate -n lvdata3 -l 10%VG vgtest
  Logical volume "lvdata2" created
</pre>
</pre>
....will produce this output:
 
What is going on so far? Let's check with the pvs/vgs counterpart known as '''lvs''':
 
<pre>
<pre>
how are you doing today?
# lvs
HOW ARE YOU DOING TODAY?
  LV      VG    Attr  LSize  Origin Snap%  Move Log Copy%  Convert
How are you doing today?
  lvdata1 vgtest -wi-a-  2.00g                                     
  lvdata2 vgtest -wi-a-  1.00g                                     
  lvdata3 vgtest -wi-a- 612.00m
#
</pre>
</pre>
So far so good, but how exactly do we select a substring or even a single character from a string? That's where substr() comes in. Here's how to call substr():
 
Notice the size of ''lvdata3'', it is roughly 600MB (10% of 6GB). How much free space remains in the VG? Time to see what '''vgs''' and '''vgdisplay''' returns:
 
<pre>
<pre>
mysub=substr(mystring,startpos,maxlen)
# vgs
  VG    #PV #LV #SN Attr  VSize VFree
  vgtest  3  3  0 wz--n- 5.99g 2.39g
# vgdisplay
  --- Volume group ---
  VG Name              vgtest
  System ID           
  Format                lvm2
  Metadata Areas        3
  Metadata Sequence No  4
  VG Access            read/write
  VG Status            resizable
  MAX LV                0
  Cur LV                3
  Open LV              0
  Max PV                0
  Cur PV                3
  Act PV                3
  VG Size              5.99 GiB
  PE Size              4.00 MiB
  Total PE              1533
  Alloc PE / Size      921 / 3.60 GiB
  Free  PE / Size      612 / 2.39 GiB
  VG UUID              baM3vr-G0kh-PXHy-Z6Dj-bMQQ-KK6R-ewMac2
</pre>
</pre>
mystring should be either a string variable or a literal string from which you'd like to extract a substring. startpos should be set to the starting character position, and maxlen should contain the maximum length of the string you'd like to extract. Notice that I said maximum length; if length(mystring) is shorter than startpos+maxlen, your result will be truncated. substr() won't modify the original string, but returns the substring instead. Here's an example:
 
Basically it say we have 1533 PE (chunks) available for a total size of 5.99 GiB. On those 1533, 921 are used (for a size of 3.60 GiB) and 612 remains free (for a size of 2.39 GiB). So we expect to see lvdata4 having an approximative size of 2.4 GiB. Before creating it, have a look at some statistics at the PV level:
 
<pre>
<pre>
print substr(mystring,9,3)
# pvs
  PV        VG    Fmt  Attr PSize PFree 
  /dev/loop0 vgtest lvm2 a-  2.00g      0
  /dev/loop1 vgtest lvm2 a-  2.00g 404.00m
  /dev/loop2 vgtest lvm2 a-  2.00g  2.00g
 
# pvdisplay
  --- Physical volume ---
  PV Name              /dev/loop0
  VG Name              vgtest
  PV Size              2.00 GiB / not usable 4.00 MiB
  Allocatable          yes (but full)
  PE Size              4.00 MiB
  Total PE              511
  Free PE              0
  Allocated PE          511
  PV UUID              b9i1Hi-llka-egCF-2vU2-f7tp-wBqh-qV4qEk
 
  --- Physical volume ---
  PV Name              /dev/loop1
  VG Name              vgtest
  PV Size              2.00 GiB / not usable 4.00 MiB
  Allocatable          yes
  PE Size              4.00 MiB
  Total PE              511
  Free PE              101
  Allocated PE          410
  PV UUID              i3mdBO-9WIc-EO2y-NqRr-z5Oa-ItLS-jbjq0E
 
  --- Physical volume ---
  PV Name              /dev/loop2
  VG Name              vgtest
  PV Size              2.00 GiB / not usable 4.00 MiB
  Allocatable          yes
  PE Size              4.00 MiB
  Total PE              511
  Free PE              511
  Allocated PE          0
  PV UUID              dEwVuO-a5vQ-ipcH-Rvlt-5zWt-iAB2-2F0XBf
</pre>
</pre>
Awk will print:
 
Quite interesting! Did you notice? The first PV is full, the second is more or less full and the third is empty. This is due to the allocation policy used for the VG: it fills its first PV then its second PV and then its third PV (this, by the way, gives you a chance to recover from a dead physical storage if by luck none of your PE was present on it).
 
It is now time to create our last LV, again notice the small 'l' to specify a relative size:
 
<pre>
<pre>
you
# lvcreate -n lvdata4 -l 100%FREE vgtest
  Logical volume "lvdata4" created
# lvs
  LV      VG    Attr  LSize  Origin Snap%  Move Log Copy%  Convert
  lvdata1 vgtest -wi-a-  2.00g                                     
  lvdata2 vgtest -wi-a-  1.00g                                     
  lvdata3 vgtest -wi-a- 612.00m                                     
  lvdata4 vgtest -wi-a-  2.39g
</pre>
</pre>
If you regularly program in a language that uses array indices to access parts of a string (and who doesn't), make a mental note that substr() is your awk substitute. You'll need to use it to extract single characters and substrings; because awk is a string-based language, you'll be using it often.


Now, we move on to some meatier functions, the first of which is called match(). match() is a lot like index(), except instead of searching for a substring like index() does, it searches for a regular expression. The match() function will return the starting position of the match, or zero if no match is found. In addition, match() will set two variables called RSTART and RLENGTH. RSTART contains the return value (the location of the first match), and RLENGTH specifies its span in characters (or -1 if no match was found). Using RSTART, RLENGTH, substr(), and a small loop, you can easily iterate through every match in your string. Here's an example match() call:
Now the $100 question: if '''pvdisplay''' and '''vgdisplay''' commands exist, does command named '''lvdisplay''' exist as well? Yes absolutely! Indeed the command sets are coherent between abstraction levels (PV/VG/LV) and they are named in the exact same manner modulo their first 2 letters:
 
* PV: pvs/pvdisplay/pvchange....
* VG: vgs/vgdisplay/vgchange....
* LG: lvs/lvdisplay/lvchange....
 
Back to our '''lvdisplay''' command, here is how it shows up:
 
<pre>
<pre>
print match(mystring,/you/), RSTART, RLENGTH
# lvdisplay
  --- Logical volume ---
  LV Name                /dev/vgtest/lvdata1
  VG Name                vgtest
  LV UUID                fT22is-cmSL-uhwM-zwCd-jeIe-DWO7-Hkj4k3
  LV Write Access        read/write
  LV Status              available
  # open                0
  LV Size                2.00 GiB
  Current LE            512
  Segments              2
  Allocation            inherit
  Read ahead sectors    auto
  - currently set to    256
  Block device          253:0
 
  --- Logical volume ---
  LV Name                /dev/vgtest/lvdata2
  VG Name                vgtest
  LV UUID                yd07wA-hj77-rOth-vxW8-rwo9-AX7q-lcyb3p
  LV Write Access        read/write
  LV Status              available
  # open                0
  LV Size                1.00 GiB
  Current LE            256
  Segments              1
  Allocation            inherit
  Read ahead sectors    auto
  - currently set to    256
  Block device          253:1
 
  --- Logical volume ---
  LV Name                /dev/vgtest/lvdata3
  VG Name                vgtest
  LV UUID                ocMCL2-nkcQ-Fwdx-pss4-qeSm-NtqU-J7vAXG
  LV Write Access        read/write
  LV Status              available
  # open                0
  LV Size                612.00 MiB
  Current LE            153
  Segments              1
  Allocation            inherit
  Read ahead sectors    auto
  - currently set to    256
  Block device          253:2
 
  --- Logical volume ---
  LV Name                /dev/vgtest/lvdata4
  VG Name                vgtest
  LV UUID                iQ2rV7-8Em8-85ts-anan-PePb-gk18-A31bP6
  LV Write Access        read/write
  LV Status              available
  # open                0
  LV Size                2.39 GiB
  Current LE            612
  Segments              2
  Allocation            inherit
  Read ahead sectors    auto
  - currently set to    256
  Block device          253:3
</pre>
</pre>
Awk will print:
 
Nothing extremely useful to comment for an overview beyond showing at the exception of two things:
# '''LVs are accessed via the device mapper''' (see the lines starting by ''LV Name'' and notice how the name is composed). So '''lvdata1''' will be accessed via ''/dev/vgtest/lvdata1'', ''lvdata2'' will be accessed via ''/dev/vgtest/lvdata2'' and so on.
# just like PV are managed in sets of data chunks (the so famous Physical Extents or PEs), LVs are managed in a set of data chunks known as Logical Extents or LEs. Most of the time you don't have to worry about the existence of LEs because they fits withing a single PE although it is possible to make them smaller hence having several LE within a single PE. Demonstration: if you consider the first LV, '''lvdisplay''' says it has a size of 2 GiB and holds 512 logical extents. Dividing 2GiB by 512 gives 4 MiB as the size of a LE which is the exact same size used for PEs as seen when demonstrating the '''pvdisplay''' command some paragraphs above. So in our case we have a 1:1 match between a LE and the underlying PE.
 
Oh another great point to underline: you can display the PV in relation with a LV :-) Just give a special option to '''lvdisplay''':
 
<pre>
<pre>
9 9 3
# lvdisplay -m
  --- Logical volume ---
  LV Name                /dev/vgtest/lvdata1
  VG Name                vgtest
  (...)
  Current LE            512
  Segments              2
  (...)
  --- Segments ---
  Logical extent 0 to 510:
    Type                linear
    Physical volume    /dev/loop0
    Physical extents    0 to 510
 
  Logical extent 511 to 511:
    Type                linear
    Physical volume    /dev/loop1
    Physical extents    0 to 0
 
 
  --- Logical volume ---
  LV Name                /dev/vgtest/lvdata2
  VG Name                vgtest
  (...)
  Current LE            256
  Segments              1
  (...)
 
  --- Segments ---
  Logical extent 0 to 255:
    Type                linear
    Physical volume    /dev/loop1
    Physical extents    1 to 256
 
 
  --- Logical volume ---
  LV Name                /dev/vgtest/lvdata3
  VG Name                vgtest
  (...)
  Current LE            153
  Segments              1
  (...)
 
  --- Segments ---
  Logical extent 0 to 152:
    Type                linear
    Physical volume    /dev/loop1
    Physical extents    257 to 409
 
 
  --- Logical volume ---
  LV Name                /dev/vgtest/lvdata4
  VG Name                vgtest
  (...)
  Current LE            612
  Segments              2
  (...)
 
  --- Segments ---
  Logical extent 0 to 510:
    Type                linear
    Physical volume    /dev/loop2
    Physical extents    0 to 510
 
  Logical extent 511 to 611:
    Type                linear
    Physical volume    /dev/loop1
    Physical extents    410 to 510
</pre>
</pre>


=== String substitution ===
To go one step further let's analyze a bit how the PE are used: the first LV has 512 LEs (remember: one LE fits within one PE here so 1 LE = 1 PE). Amongst those 512 LEs, 511 of them (0 to 510) are stored on /dev/loop0 and the 512th LE is on /dev/loop1. Huh? Something seems to be wrong here, '''pvdisplay''' said that /dev/loop0 was holding 512 PV so why an extent has been placed on the second storage device? Indeed its not a misbehaviour and absolutely normal: LVM uses some metadata internally with regards the PV, VG and LV thus making some of storage space unavailable for the payload. This explains why 1 PE has been "eaten" to store that metadata. Also notice the linear allocation process: ''/dev/loop0'' has been used, then when being full ''/dev/loop1'' has also been used then the turn of /''dev/loop2'' came.
Now, we're going to look at a couple of string substitution functions, sub() and gsub(). These guys differ slightly from the functions we've looked at so far in that they actually modify the original string. Here's a template that shows how to call sub():
 
Now everything is in place, if you want just check again with '''vgs/pvs/vgdisplay/pvdisplay''' and will notice that the VG is now 100% full and all of the underlying PV are also 100% full.
 
== Filesystems creation  and mounting  ==  
 
Now we have our LVs it could be fun if we could do something useful with them. In the case you missed it, LVs are accessed via the device mapper which uses a combination of the VG and LV names thus:
* lvdata1 is accessible via /dev/vgtest/lvdata1
* lvdata2 is accessible via /dev/vgtest/lvdata2
* and so on!
 
Just like any traditional storage device, the newly created LVs are seen as block devices as well just as if they were a kind of harddisk (don't worry about the "dm-..", it is just an internal block device automatically allocated by the device mapper for you):
<pre>
<pre>
sub(regexp,replstring,mystring)
# ls -l /dev/vgtest
total 0
lrwxrwxrwx 1 root root 7 Dec 27 12:54 lvdata1 -> ../dm-0
lrwxrwxrwx 1 root root 7 Dec 27 12:54 lvdata2 -> ../dm-1
lrwxrwxrwx 1 root root 7 Dec 27 12:54 lvdata3 -> ../dm-2
lrwxrwxrwx 1 root root 7 Dec 27 12:54 lvdata4 -> ../dm-3
 
# ls -l /dev/dm-[0-3]
brw-rw---- 1 root disk 253, 0 Dec 27 12:54 /dev/dm-0
brw-rw---- 1 root disk 253, 1 Dec 27 12:54 /dev/dm-1
brw-rw---- 1 root disk 253, 2 Dec 27 12:54 /dev/dm-2
brw-rw---- 1 root disk 253, 3 Dec 27 12:54 /dev/dm-3
</pre>
</pre>
When you call sub(), it'll find the first sequence of characters in mystring that matches regexp, and it'll replace that sequence with replstring. sub() and gsub() have identical arguments; the only way they differ is that sub() will replace the first regexp match (if any), and gsub() will perform a global replace, swapping out all matches in the string. Here's an example sub() and gsub() call:
 
So if LVs are block device a filesystem can be created on them just like if they were a real harddisk or hardisk partitions? Absolutely! Now let's create ext4 filesystems on our LVs:
 
<pre>
<pre>
sub(/o/,"O",mystring)
# mkfs.ext4 /dev/vgtest/lvdata1
print mystring
 
mystring="How are you doing today?"
mke2fs 1.42 (29-Nov-2011)
gsub(/o/,"O",mystring)
Discarding device blocks: done                           
print mystring
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
131072 inodes, 524288 blocks
26214 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=536870912
16 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
        32768, 98304, 163840, 229376, 294912
 
Allocating group tables: done                           
Writing inode tables: done                           
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done
 
# mkfs.ext4 /dev/vgtest/lvdata1
(...)
# mkfs.ext4 /dev/vgtest/lvdata2
(...)
# mkfs.ext4 /dev/vgtest/lvdata3
(..)
</pre>
</pre>
We had to reset mystring to its original value because the first sub() call modified mystring directly. When executed, this code will cause awk to output:
 
Once the creation ended we must create the mount points and mount the newly created filesystems on them:
 
<pre>
<pre>
HOw are you doing today?
# mkdir /mnt/data-01
HOw are yOu dOing tOday?
# mkdir /mnt/data-02
# mkdir /mnt/data-03
# mkdir /mnt/data-04
# mount /dev/vgtest/lvdata1 /mnt/data01
# mount /dev/vgtest/lvdata2 /mnt/data02
# mount /dev/vgtest/lvdata3 /mnt/data03
# mount /dev/vgtest/lvdata4 /mnt/data04
</pre>
</pre>
Of course, more complex regular expressions are possible. I'll leave it up to you to test out some complicated regexps.


We wrap up our string function coverage by introducing you to a function called split(). split()'s job is to "chop up" a string and place the various parts into an integer-indexed array. Here's an example split() call:
Finally we can check that everything is in order:
 
<pre>
<pre>
numelements=split("Jan,Feb,Mar,Apr,May,Jun,Jul,Aug,Sep,Oct,Nov,Dec",mymonths,",")
# df -h
Filesystem                    Size  Used Avail Use% Mounted on
(...)
/dev/mapper/vgtest-lvdata1    2.0G  96M  1.9G  5% /mnt/data01
/dev/mapper/vgtest-lvdata2  1022M  47M  924M  5% /mnt/data02
/dev/mapper/vgtest-lvdata3    611M  25M  556M  5% /mnt/data03
/dev/mapper/vgtest-lvdata4    2.4G  100M  2.2G  5% /mnt/data04
</pre>
</pre>
When calling split(), the first argument contains the literal string or string variable to be chopped. In the second argument, you should specify the name of the array that split() will stuff the chopped parts into. In the third element, specify the separator that will be used to chop the strings up. When split() returns, it'll return the number of string elements that were split. split() assigns each one to an array index starting with one, so the following code:
 
Did you notice the device has changed? Indeed everything is in order, mount just uses another set of symlinks which point to the exact same block devices:
 
<pre>
<pre>
print mymonths[1],mymonths[numelements]
# ls -l /dev/mapper/vgtest-lvdata[1-4]
lrwxrwxrwx 1 root root 7 Dec 28 20:12 /dev/mapper/vgtest-lvdata1 -> ../dm-0
lrwxrwxrwx 1 root root 7 Dec 28 20:13 /dev/mapper/vgtest-lvdata2 -> ../dm-1
lrwxrwxrwx 1 root root 7 Dec 28 20:13 /dev/mapper/vgtest-lvdata3 -> ../dm-2
lrwxrwxrwx 1 root root 7 Dec 28 20:13 /dev/mapper/vgtest-lvdata4 -> ../dm-3
</pre>
</pre>
....will print:
 
== Renaming a volume group and its logical volumes ==
 
So far we have four LVs named lvdata1 to lvdata4 mounted on /mnt/data01 to /mnt/data04. It would be more adequate to :
# make the number in our LV names being like "01" instead of "1"
# rename our volume groupe to "vgdata" instead of "vgtest"
 
To show how dynamic is the LVM world, we will rename our VG and LV on the fly using two commands: '''vgrename''' for acting at the VG level and its counterpart '''lvrename''' to act at the LV level. Starting by the VG or the LVs makes strictly no difference, you can start either way and get the same result. In our example we have chosen to start with the VG:
 
<pre>
<pre>
Jan Dec
# vgrename vgtest vgdata
  Volume group "vgtest" successfully renamed to "vgdata"
# lvrename vgdata/lvdata1 vgdata/lvdata01
  Renamed "lvdata1" to "lvdata01" in volume group "vgdata"
# lvrename vgdata/lvdata2 vgdata/lvdata02
  Renamed "lvdata2" to "lvdata02" in volume group "vgdata"
# lvrename vgdata/lvdata3 vgdata/lvdata03
  Renamed "lvdata3" to "lvdata03" in volume group "vgdata"
# lvrename vgdata/lvdata4 vgdata/lvdata04
  Renamed "lvdata4" to "lvdata04" in volume group "vgdata"
</pre>
</pre>


=== Special string forms ===
What happened? Simple:
A quick note -- when calling length(), sub(), or gsub(), you can drop the last argument and awk will apply the function call to $0 (the entire current line). To print the length of each line in a file, use this awk script:
 
<pre>
<pre>
{
# vgs
     print length()
  VG     #PV #LV #SN Attr  VSize VFree
}
  vgdata  3  4  0 wz--n- 5.99g    0
# lvs
  LV      VG    Attr  LSize  Origin Snap%  Move Log Copy%  Convert
  lvdata01 vgdata -wi-ao  2.00g                                     
  lvdata02 vgdata -wi-ao  1.00g                                     
  lvdata03 vgdata -wi-ao 612.00m                                     
  lvdata04 vgdata -wi-ao  2.39g
</pre>
</pre>


=== Financial fun ===
Sounds good, our VG and LVs have been renamed! What a command like ''mount'' will say?
A few weeks ago, I decided to write my own checkbook balancing program in awk. I decided that I'd like to have a simple tab-delimited text file into which I can enter my most recent deposits and withdrawals. The idea was to hand this data to an awk script that would automatically add up all the amounts and tell me my balance. Here's how I decided to record all my transactions into my "ASCII checkbook":
 
<pre>
<pre>
23 Aug 2000    food    -   -   Y    Jimmy's Buffet    30.25
# mount
(...)
/dev/mapper/vgtest-lvdata1 on /mnt/data01 type ext4 (rw)
/dev/mapper/vgtest-lvdata2 on /mnt/data02 type ext4 (rw)
/dev/mapper/vgtest-lvdata3 on /mnt/data03 type ext4 (rw)
/dev/mapper/vgtest-lvdata4 on /mnt/data04 type ext4 (rw)
</pre>
</pre>
Every field in this file is separated by one or more tabs. After the date (field 1, $1), there are two fields called "expense category" and "income category". When I'm entering an expense like on the above line, I put a four-letter nickname in the exp field, and a "-" (blank entry) in the inc field. This signifies that this particular item is a "food expense" :) Here's what a deposit looks like:
 
Ooops... It is not exactly a bug, mount still shows the symlinks used at the time the LVs were mounted in the VFS and has not updated its information. However once again everything is correct because the underlying  block devices (/dev/dm-0 to /dev/dm-3) did not changed at all. To see the right information the LVs must be unmounted and mounted again:
 
<pre>
<pre>
23 Aug 2000    -   inco    -   Y    Boss Man        2001.00
# umount /mnt/data01
(...)
# umount /mnt/data04
# mount /dev/vgdata/lvdata01 /mnt/data01
(...)
# mount /dev/vgdata/lvdata04 /mnt/data04
# mount
/dev/mapper/vgdata-lvdata01 on /mnt/data01 type ext4 (rw)
/dev/mapper/vgdata-lvdata02 on /mnt/data02 type ext4 (rw)
/dev/mapper/vgdata-lvdata03 on /mnt/data03 type ext4 (rw)
/dev/mapper/vgdata-lvdata04 on /mnt/data04 type ext4 (rw)
</pre>
</pre>
In this case, I put a "-" (blank) in the exp category, and put "inco" in the inc category. "inco" is my nickname for generic (paycheck-style) income. Using category nicknames allows me to generate a breakdown of my income and expenditures by category. As far as the rest of the records, all the other fields are fairly self-explanatory. The cleared? field ("Y" or "N") records whether the transaction has been posted to my account; beyond that, there's a transaction description, and a positive dollar amount.


The algorithm used to compute the current balance isn't too hard. Awk simply needs to read in each line, one by one. If an expense category is listed but there is no income category (denoted by "-"), then this item is a debit. If an income category is listed, but no expense category (denoted by "-") is present, then the dollar amount is a credit. And, if there is both an expense and income category listed, then this amount is a "category transfer"; that is, the dollar amount will be subtracted from the expense category and added to the income category. Again, all these categories are virtual, but are very useful for tracking income and expenditures, as well as for budgeting.
{{fancynote|Using /dev/''volumegroup''/''logicalvolume'' or /dev/''volumegroup''-''logicalvolume'' makes no difference at all, those are two sets of symlinks pointing on the '''exact''' same block device. }}
 
= Expanding and shrinking the storage space  =
 
Did you notice in the previous section we have never talked on topic like "create this partition at the beginning" or "allocate 10 sectors more". In LVM you do not have to worry about that kind of problematics: your only concern is more "Do I have the space to allocate a new LV or how can I extend an existing LV?". '''LVM takes cares of the low levels aspects for you, just focus on what you want to do with your storage space.'''
 
The most common problem with computers is the shortage of space on a volume, most of the time production servers can run months or years without requiring a reboot for various reasons (kernel upgrade, hardware failure...) however they regularly requires to extend their storage space because we do generate more and more data as the time goes. With "traditional" approach like fiddling directly with hard drives partitions, storage space manipulation can easily become a headache mainly because it requires coherent copy to be made and thus application downtimes. Don't expect the situation to be more enjoyable with a SAN storage rather a directly attached storage device... Basically the problems remains the same.
 
== Expanding a storage space ==
 
The most common task for a system administrator is to expand the available storage space. In the LVM world this implies:
* Creating a new PV
* Adding the PV to the VG (thus extending the VG capacity)
* Extending the existing LVs or create new ones
* Extending the structures of the filesystems located on a LV in the case a LV is extended (Not all of the filesystems around support that capability).
 
=== Bringing a new PV in the VG ===
 
In the exact same manner we have created our first PV let's create our additional storage device, associate it to a loopback device and then create a PV on it:


=== The code ===
Time to look at the code. We'll start off with the first line, the BEGIN block and a function definition:
<pre>
<pre>
#!/usr/bin/awk -f
# dd if=/dev/zero of=/tmp/hdd4.img bs=2G count=1
BEGIN {
# losetup /dev/loop3 /tmp/hdd4.img
    FS="\t+"
# pvcreate /dev/loop3
    months="Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec"
</pre>
}


function monthdigit(mymonth) {
A '''pvs''' should report the new PV with 2 GB of free space:
     return (index(months,mymonth)+3)/4
 
}
<pre>
# pvs
  PV        VG     Fmt  Attr PSize PFree
  /dev/loop0 vgdata lvm2 a-  2.00g    0
  /dev/loop1 vgdata lvm2 a-  2.00g    0
  /dev/loop2 vgdata lvm2 a-  2.00g    0
  /dev/loop3        lvm2 a-  2.00g 2.00g
</pre>
</pre>
Adding the first "#!..." line to any awk script will allow it to be directly executed from the shell, provided that you "chmod +x myscript" first. The remaining lines define our BEGIN block, which gets executed before awk starts processing our checkbook file. We set FS (the field separator) to "\t+", which tells awk that the fields will be separated by one or more tabs. In addition, we define a string called months that's used by our monthdigit() function, which appears next.


The last three lines show you how to define your own awk function. The format is simple -- type "function", then the function name, and then the parameters separated by commas, inside parentheses. After this, a "{ }" code block contains the code that you'd like this function to execute. All functions can access global variables (like our months variable). In addition, awk provides a "return" statement that allows the function to return a value, and operates similarly to the "return" found in C, Python, and other languages. This particular function converts a month name in a 3-letter string format into its numeric equivalent. For example, this:
Excellent! The next step consist of adding this newly created PV inside our VG ''vgdata'', this is where the '''vgextend''' command comes at our rescue:
 
<pre>
<pre>
print monthdigit("Mar")
# vgextend vgdata /dev/loop3
  Volume group "vgdata" successfully extended
# vgs
  VG    #PV #LV #SN Attr  VSize VFree
  vgdata  4  4  0 wz--n- 7.98g 2.00g
</pre>
</pre>
....will print this:
 
Great, ''vgdata'' is now 8 GB large instead of 6 GB and have 2 GB of free space to allocate to either new LVs either existing LVs.
 
=== Extending the LV and its filesystem ===
 
Bringing new LV would demonstrate nothing more nevertheless extending our existing LVs is much more interesting. How can we use our 2GB extra free space? We can, for example, split it in two allocating a 50% to our first (''lvdata01'') and third (''lvdata03'') LV adding 1GB of space to both. The best of the story is that operation is very simple and is realized with a command named '''lvextend''':
 
<pre>
<pre>
3
# lvextend vgdata/lvdata01 -l +50%FREE
  Extending logical volume lvdata01 to 3.00 GiB
  Logical volume lvdata01 successfully resized
# lvextend vgdata/lvdata03 -l +50%FREE
  Extending logical volume lvdata03 to 1.10 GiB
  Logical volume lvdata03 successfully resized
</pre>
</pre>
Now, let's move on to some more functions.


=== Financial functions ===
Ouaps!! We did a mistake there: lvdata01 has the expected size (2GB + 1GB for a grand total of 3 GB) but lvdata03 only grown of 512 MB (for a grand total size of 1.1 GB). Our mistake was obvious: once the first gigabyte (50% of 2GB) of extra space has been given to lvdata01, only one gigabyte remained free on the VG thus when we said "allocate 50% of the remaining gigabyte to ''lvdata03''" LVM added only 512 MB leaving the other half of this gigabyte unused. The '''vgs''' command can confirm this:
Here are three more functions that perform the bookkeeping for us. Our main code block, which we'll see soon, will process each line of the checkbook file sequentially, calling one of these functions so that the appropriate transactions are recorded in an awk array. There are three basic kinds of transactions, credit (doincome), debit (doexpense) and transfer (dotransfer). You'll notice that all three functions accept one argument, called mybalance. mybalance is a placeholder for a two-dimensional array, which we'll pass in as an argument. Up until now, we haven't dealt with two-dimensional arrays; however, as you can see below, the syntax is quite simple. Just separate each dimension with a comma, and you're in business.


We'll record information into "mybalance" as follows. The first dimension of the array ranges from 0 to 12, and specifies the month, or zero for the entire year. Our second dimension is a four-letter category, like "food" or "inco"; this is the actual category we're dealing with. So, to find the entire year's balance for the food category, you'd look in mybalance[0,"food"]. To find June's income, you'd look in mybalance[6,"inco"].
<pre>
<pre>      
# vgs
function doincome(mybalance) {
  VG     #PV #LV #SN Attr  VSize VFree 
     mybalance[curmonth,$3] += amount
  vgdata  4  4  0 wz--n- 7.98g 512.00m
    mybalance[0,$3] += amount       
</pre>
}


function doexpense(mybalance) {
Nevermind about that voluntary mistake we will keep that extra space for a later paragraph :-) What happened to the storage space visible from the operating system?
    mybalance[curmonth,$2] -= amount
    mybalance[0,$2] -= amount       
}


function dotransfer(mybalance) {
<pre>
    mybalance[0,$2] -= amount
# df -h | grep lvdata01
    mybalance[curmonth,$2] -= amount
/dev/mapper/vgdata-lvdata01  2.0G  96M  1.9G  5% /mnt/data01
    mybalance[0,$3] += amount
    mybalance[curmonth,$3] += amount
}
</pre>
</pre>
When doincome() or any of the other functions are called, we record the transaction in two places -- mybalance[0,category] and mybalance[curmonth, category], the entire year's category balance and the current month's category balance, respectively. This allows us to easily generate either an annual or monthly breakdown of income/expenditures later on.


If you look at these functions, you'll notice that the array referenced by mybalance is passed in by reference. In addition, we also refer to several global variables: curmonth, which holds the numeric value of the month of the current record, $2 (the expense category), $3 (the income category), and amount ($7, the dollar amount). When doincome() and friends are called, all these variables have already been set correctly for the current record (line) being processed.
Obviously resizing a LV does not "automagically" resize the filesystem structures to take into account the new LV size making that step part of our duty. Happily for us, ext3 can be resized and better it can be grown when mounted in the VFS. This is known as ''online resizing'' and a few others filesystems supports that capability, among them we can quote ext2 (ext3 without a journal), ext4 (patches integrated very recently as of Nov/Dec 2011), XFS, ResiserFS and BTRFS. To our knowledge, only BTRFS support both online resizing '''and''' online shrinking as of Decembrer 2011, all of the others require a filesystem to be unmounted first before being shrunk.  
 
{{fancynote|Consider using the option -r when invoking lvextend, it asks the command to perform a filesystem resize.}}
 
Now let's extend (grow) the ext3 filesystem located on lvdata01. As said above, ext3 support online resizing hence we do not need to kick it out of the VFS first:


=== The main block ===
Here's the main code block that contains the code that parses each line of input data. Remember, because we have set FS correctly, we can refer to the first field as $1, the second field as $2, etc. When doincome() and friends are called, the functions can access the current values of curmonth, $2, $3 and amount from inside the function. Take a look at the code and meet me on the other side for an explanation.
<pre>
<pre>
{
# resize2fs /dev/vgdata/lvdata01
    curmonth=monthdigit(substr($1,4,3))
resize2fs 1.42 (29-Nov-2011)
    amount=$7
Filesystem at /dev/vgdata/lvdata01 is mounted on /mnt/data01; on-line resizing required
     
old_desc_blocks = 1, new_desc_blocks = 1
    #record all the categories encountered
Performing an on-line resize of /dev/vgdata/lvdata01 to 785408 (4k) blocks.
    if ( $2 != "-" )
The filesystem on /dev/vgdata/lvdata01 is now 785408 blocks long.
        globcat[$2]="yes"
    if ( $3 != "-" )
        globcat[$3]="yes"


    #tally up the transaction properly
# df -h | grep lvdata01
    if ( $2 == "-" ) {
/dev/mapper/vgdata-lvdata01  3.0G  96M  2.8G  4% /mnt/data01
        if ( $3 == "-" ) {
            print "Error: inc and exp fields are both blank!"
            exit 1
        } else {
            #this is income
            doincome(balance)
            if ( $5 == "Y" )
                doincome(balance2)
        }
    } else if ( $3 == "-" ) {
        #this is an expense
        doexpense(balance)
        if ( $5 == "Y" )
            doexpense(balance2)
    } else {
        #this is a transfer
        dotransfer(balance)
        if ( $5 == "Y" )
            dotransfer(balance2)
    }                       
}
</pre>
</pre>
In the main block, the first two lines set curmonth to an integer between 1 and 12, and set amount to field 7 (to make the code easier to understand). Then, we have four interesting lines, where we write values into an array called globcat. globcat, or the global categories array, is used to record all those categories encountered in the file -- "inco", "misc", "food", "util", etc. For example, if $2 == "inco", we set globcat["inco"] to "yes". Later on, we can iterate through our list of categories with a simple "for (x in globcat)" loop.


On the next twenty or so lines, we analyze fields $2 and $3, and record the transaction appropriately. If $2=="-" and $3!="-", we have some income, so we call doincome(). If the situation is reversed, we call doexpense(); and if both $2 and $3 contain categories, we call dotransfer(). Each time, we pass the "balance" array to these functions so that the appropriate data is recorded there.
''Et voila!'' Our  LV has now plenty of new space usable :-) '''We do not bother about ''how'' the storage is organized by LVM amongst the underlying storage devices and it is not our problem after all. We only worry about having our storage requirements being satisfied without any further details. From our point of view everything is seen just as if we were manipulating a single storage device subdivided in several partitions of a dynamic size and always organized in a set of contiguous blocks.'''


You'll also notice several lines that say "if ( $5 == "Y" ), record that same transaction in balance2". What exactly are we doing here? You'll recall that $5 contains either a "Y" or a "N", and records whether the transaction has been posted to the account. Because we record the transaction to balance2 only if the transaction has been posted, balance2 will contain the actual account balance, while "balance" will contain all transactions, whether they have been posted or not. You can use balance2 to verify your data entry (since it should match with your current account balance according to your bank), and use "balance" to make sure that you don't overdraw your account (since it will take into account any checks you have written that have not yet been cashed).
Now let's shuffle the cards a bit more: when we examined how the LEs of our LVs were allocated, we saw that ''lvdata01'' (named lvdata1 at this time) consisted of 512 LEs or 512 PEs (because of the 1:1 mapping between those)  spread over two PVs. As we have extended it to use an additional PV, we should see it using 3 segments:
 
* Segment 1: located on the PV stored on /dev/loop0 (LE/PE #0 to #510)
* Segment 2: located on the PV stored on /dev/loop1 (LE/PE #511)
* Segment 3: located on the PV stored on /dev/loop1 (LE/PE #512 and followers)
 
Is it the case? Let's check:


=== Generating the report ===
After the main block repeatedly processes each input record, we now have a fairly comprehensive record of debits and credits broken down by category and by month. Now, all we need to do is define an END block that will generate a report, in this case a modest one:
<pre>
<pre>
END {
# lvdisplay -m  vgdata/lvdata01
     bal=0
  --- Logical volume ---
     bal2=0       
  LV Name                /dev/vgdata/lvdata01
     for (x in globcat) {
  VG Name                vgdata
        bal=bal+balance[0,x]
  LV UUID                fT22is-cmSL-uhwM-zwCd-jeIe-DWO7-Hkj4k3
        bal2=bal2+balance2[0,x]    
  LV Write Access        read/write
     }
  LV Status              available
     printf("Your available funds: %10.2f\n", bal)
  # open                1
     printf("Your account balance: %10.2f\n", bal2)       
  LV Size                3.00 GiB
}
  Current LE            767
  Segments              3
  Allocation            inherit
  Read ahead sectors    auto
  - currently set to     256
  Block device          253:0
 
  --- Segments ---
  Logical extent 0 to 510:
     Type                linear
     Physical volume    /dev/loop0
    Physical extents    0 to 510
    
  Logical extent 511 to 511:
     Type                linear
     Physical volume    /dev/loop1
    Physical extents    0 to 0
 
  Logical extent 512 to 766:
     Type                linear
    Physical volume    /dev/loop3
    Physical extents    0 to 254
</pre>
</pre>
This report prints out a summary that looks something like this:
 
Bingo! Note that if it is true here (LVM uses linear allocation) would not be true in the general case.
 
{{fancywarning|'''Never mix a local storage device with a SAN disk within the same volume group''' and especially if that later is your system volume. It will bring you a lot of troubles if the SAN disk goes offline or bring weird performance fluctuations as PEs allocated on the SAN will get faster response times than those located on  a local disk. }}
 
== Shrinking a storage space ==
 
On some occasions it can be useful to reduce the size of a LV or the size of the VG itself. The principle is similar to what has been demonstrated in the previous section:
 
# umount the filesystem belong to the LV to be processed (if your filesystem does not support online shrinking)
# reduce the filesystem size (if the LV is not to be flushed)
# reduce the LV size - OR - remove the LV
# remove a PV from the volume group if no longer used to store extents
 
The simplest case to start with is how a LV can be removed: a good candidate for removal is ''lvdata03'', we failed to resize it and the better would be to scrap it. First unmount it:
 
<pre>
# lvs
  LV      VG    Attr  LSize Origin Snap%  Move Log Copy%  Convert
  lvdata01 vgdata -wi-ao 3.00g                                     
  lvdata02 vgdata -wi-ao 1.00g                                     
  lvdata03 vgdata -wi-ao 1.10g                                     
  lvdata04 vgdata -wi-ao 2.39g                                     
# umount /dev/vgdata/lvdata03
# lvs
  LV      VG    Attr  LSize Origin Snap%  Move Log Copy%  Convert
  lvdata01 vgdata -wi-ao 3.00g                                     
  lvdata02 vgdata -wi-ao 1.00g                                     
  lvdata03 vgdata -wi-a- 1.10g                                     
  lvdata04 vgdata -wi-ao 2.39g
</pre>
 
Noticed the little change with '''lvs'''? It lies in the ''Attr'' field: once the ''lvdata03'' has been unmounted, '''lvs''' tells us the LV is not '''o'''pened anymore (the little o at the rightmost position has been replaced by a dash). The LV still exists but nothing is using it.
 
To remove ''lvdata03'' use the command '''lvremove''' and confirm the removal by entering 'y' when asked:
 
<pre>
<pre>
Your available funds:   1174.22
# lvremove vgdata/lvdata03
Your account balance:    2399.33
Do you really want to remove active logical volume lvdata03? [y/n]: y
  Logical volume "lvdata03" successfully removed
# lvs
  LV      VG    Attr  LSize Origin Snap%  Move Log Copy%  Convert
  lvdata01 vgdata -wi-ao 3.00g                                     
  lvdata02 vgdata -wi-ao 1.00g                                     
  lvdata04 vgdata -wi-ao 2.39g
# vgs
  VG    #PV #LV #SN Attr  VSize VFree
  vgdata  4  3  0 wz--n- 7.98g 1.60g
</pre>
</pre>
In our END block, we used the "for (x in globcat)" construct to iterate through every category, tallying up a master balance based on all the transactions recorded. We actually tally up two balances, one for available funds, and another for the account balance. To execute the program and process your own financial goodies that you've entered into a file called '''mycheckbook.txt''', put all the above code into a text file called '''balance''' and do <span style="color:green;">"chmod +x balance"</span>, and then type <span style="color:green;">"./balance mycheckbook.txt"</span>. The balance script will then add up all your transactions and print out a two-line balance summary for you.


=== Upgrades ===
Notice the 1.60 of space has been freed in the VG. What can we do next? Shrinking ''lvdata04'' by 50% giving roughly 1.2GB or 1228MB (1.2*1024) of its size could be a good idea so here we go. First we need to umount the filesystem from the VFS because ext3 '''does not support''' online shrinking.
I use a more advanced version of this program to manage my personal and business finances. My version (which I couldn't include here due to space limitations) prints out a monthly breakdown of income and expenses, including annual totals, net income and a bunch of other stuff. Even better, it outputs the data in HTML format, so that I can view it in a Web browser :) If you find this program useful, I encourage you to add these features to this script. You won't need to configure it to record any additional information; all the information you need is already in balance and balance2. Just upgrade the END block, and you're in business!
 
<pre>
# umount /dev/vgdata/lvdata04
# e2fsck -f /dev/vgdata/lvdata04
e2fsck 1.42 (29-Nov-2011)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/vgdata/lvdata04: 11/156800 files (0.0% non-contiguous), 27154/626688 blocks
# resize2fs -p /dev/vgdata/lvdata04 -L 1228M
# lvreduce /dev/vgdata/lvdata04 -L 1228
  WARNING: Reducing active logical volume to 1.20 GiB
  THIS MAY DESTROY YOUR DATA (filesystem etc.)
Do you really want to reduce lvdata04? [y/n]: y
  Reducing logical volume lvdata04 to 1.20 GiB
  Logical volume lvdata04 successfully resized
oxygen ~ # e2fsck -f /dev/vgdata/lvdata04
e2fsck 1.42 (29-Nov-2011)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/vgdata/lvdata04: 11/78400 files (0.0% non-contiguous), 22234/314368 blocks
</pre>
 
Not very practical indeed, we can tell '''lvreduce''' to handle the underlying filesystem shrinkage for us. Let's shrink again this time giving a 1 GB volume (1024 MB) in absolute size:
 
<pre>
# lvreduce /dev/vgdata/lvdata04 -r -L 1024
fsck from util-linux 2.20.1
/dev/mapper/vgdata-lvdata04: clean, 11/78400 files, 22234/314368 blocks
resize2fs 1.42 (29-Nov-2011)
Resizing the filesystem on /dev/mapper/vgdata-lvdata04 to 262144 (4k) blocks.
The filesystem on /dev/mapper/vgdata-lvdata04 is now 262144 blocks long.
 
  Reducing logical volume lvdata04 to 1.00 GiB
  Logical volume lvdata04 successfully resized
# lvs
  LV      VG    Attr  LSize Origin Snap%  Move Log Copy%  Convert
  lvdata01 vgdata -wi-ao 3.00g                                     
  lvdata02 vgdata -wi-ao 1.00g                                     
  lvdata04 vgdata -wi-a- 1.00g
</pre>
 
{{fancynote|Notice the number of 4k blocks shown: 4096*262144/1024^2 gives 1,073,741,824 bytes either 1 GB.}}
 
Time to mount the volume again:
 
<pre>
# mount /dev/vgdata/lvdata04 /mnt/data04
# df -h | grep lvdata04
/dev/mapper/vgdata-lvdata04  1021M  79M  891M  9% /mnt/data04
</pre>
 
And what is going on at the VG level?
 
<pre>
# vgs
  VG    #PV #LV #SN Attr  VSize VFree
  vgdata  4  3  0 wz--n- 7.98g 2.99g
</pre>
 
Wow, we have near 3 GB of free space inside, a bit more than one of our PV. It could be great if we can free one of the those and of course LVM gives you the possibility to do that. Before going further, let's check what happened at the PVs level:
 
<pre>
# pvs   
  PV        VG    Fmt  Attr PSize PFree 
  /dev/loop0 vgdata lvm2 a-  2.00g      0
  /dev/loop1 vgdata lvm2 a-  2.00g 1016.00m
  /dev/loop2 vgdata lvm2 a-  2.00g 1020.00m
  /dev/loop3 vgdata lvm2 a-  2.00g    1.00g
</pre>
 
Did you noticed? 1 GB of space has been freed on the last PV (/dev/loop3) since ''lvdata04'' has been shrunk not counting the space freed on ''/dev/loop1'' and ''/dev/loop2'' after the removal of lvdata02.
 
 
Next steo: can we remove a PV directly (the command to remove a PV from a VG is '''vgreduce''')?
 
<pre>
# vgreduce vgdata /dev/loop0
  Physical volume "/dev/loop0" still in use
</pre>
 
Of course not, all of our PVs supports the content of our LVs and we must find a manner to move all of the PE (physical extents) actually hold by the PV /dev/loop0 elsewhere withing the VG. But wait a minute, the victory is there yet: we do have some free space in the  /dev/loop0 and we will get more and more free space in it as the displacement process will progress. What is going to happen if, from a concurrent session, we create others LV in ''vgdata'' at the same time the content of  /dev/loop0 is moved? Simple: it can be filled again with the PEs newly allocated.
 
So before proceeding to the displacement of what ''/dev/loop0'' contents, we must say to LVM: "please don't allocate anymore PEs on ''/dev/loop0''". This is achieved via the parameter ''-x'' of the command '''pvchange''':
<pre>
# pvchange -x n /dev/loop0
  Physical volume "/dev/loop0" changed
  1 physical volume changed / 0 physical volumes not changed
</pre>
 
The value ''n'' given to ''-x'' marks the PV as ''unallocable'' (i.e. not usable for future PE allocations). Let's check again the PVs with '''pvs''' and '''pvdisplay''':
 
<pre>
# pvs
  PV        VG    Fmt  Attr PSize PFree 
  /dev/loop0 vgdata lvm2 --  2.00g      0
  /dev/loop1 vgdata lvm2 a-  2.00g 1016.00m
  /dev/loop2 vgdata lvm2 a-  2.00g 1020.00m
  /dev/loop3 vgdata lvm2 a-  2.00g    1.00g
 
# pvdisplay /dev/loop0
  --- Physical volume ---
  PV Name              /dev/loop0
  VG Name              vgdata
  PV Size              2.00 GiB / not usable 4.00 MiB
  Allocatable          NO
  PE Size              4.00 MiB
  Total PE              511
  Free PE              0
  Allocated PE          511
  PV UUID              b9i1Hi-llka-egCF-2vU2-f7tp-wBqh-qV4qEk
</pre>
 
Great news here, the ''Attrs'' field shows a dash instead of 'a' at the leftmost position meaning the PV is effectively ''not allocatable''. However '''marking a PV not allocatable does not wipe the existing PEs stored on it'''. In other words, it means that data present on the PV remains '''absolutely intact'''. Another positive point lies the remaining capacities of the PVs composing ''vgdata'': the sum of free space available on ''/dev/loop1'', ''/dev/loop2'' and ''/dev/loop3'' is 3060MB (1016MB + 1020MB + 1024MB) so largely sufficient to hold the 2048 MB (2 GB) actually stored on the PV ''/dev/loop0''.
 
Now we have frozen the allocation of PEs on /dev/loop0 we can make LVM move all of PEs located in this PV on the others PVs composing the VG ''vgdata''. Again, we don't have to worry about the gory details like where LVM will precisely relocate the PEs actually hold by ''/dev/loop0'', our '''only''' concerns is to get all of them moved out of ''/dev/loop0''. That job gets done by:
 
<pre>
# pvmove /dev/loop0
  /dev/loop0: Moved: 5.9%
  /dev/loop0: Moved: 41.3%
  /dev/loop0: Moved: 50.1%
  /dev/loop0: Moved: 100.0%
</pre>
 
We don't have to tell LVM the VG name because it already knows that ''/dev/loop0'' belongs to ''vgdata'' and what are the others PVs belonging to that VG usable to host the PEs coming from ''/dev/loop0''. It is absolutely normal for the process to takes some minutes (real life cases can go up to several hours even with SAN disks located on high-end storage hardware which is much more faster than local SATA or even SAS drive).
 
At the end of the moving process, we can see that the PV ''/dev/loop0'' is totally free:
 
<pre>
# pvs
  PV        VG    Fmt  Attr PSize PFree 
  /dev/loop0 vgdata lvm2 a-  2.00g    2.00g
  /dev/loop1 vgdata lvm2 a-  2.00g 1016.00m
  /dev/loop2 vgdata lvm2 a-  2.00g      0
  /dev/loop3 vgdata lvm2 a-  2.00g      0
 
# pvdisplay /dev/loop0
  --- Physical volume ---
  PV Name              /dev/loop0
  VG Name              vgdata
  PV Size              2.00 GiB / not usable 4.00 MiB
  Allocatable          yes
  PE Size              4.00 MiB
  Total PE              511
  Free PE              511
  Allocated PE          0
  PV UUID              b9i1Hi-llka-egCF-2vU2-f7tp-wBqh-qV4qEk
</pre>
 
511 PEs free out of a maximum 511 PEs so all of its containt has been successfully spread on the others PVs (the volume is also still marked as "unallocatable", this is normal). Now it is ready to be detached from the VG ''vgdata'' with the help of '''vgreduce''' :
 
<pre>
# vgreduce vgdata /dev/loop0
  Removed "/dev/loop0" from volume group "vgdata"
</pre>
 
What happened to ''vgdata''?
<pre>
# vgs
  VG    #PV #LV #SN Attr  VSize VFree 
  vgdata  3  3  0 wz--n- 5.99g 1016.00m
</pre>
 
Its storage space falls to ~6GB! What would tell '''pvs'''?
 
<pre>
# pvs
  PV        VG    Fmt  Attr PSize PFree 
  /dev/loop0        lvm2 a-  2.00g    2.00g
  /dev/loop1 vgdata lvm2 a-  2.00g 1016.00m
  /dev/loop2 vgdata lvm2 a-  2.00g      0
  /dev/loop3 vgdata lvm2 a-  2.00g      0
</pre>
 
''/dev/loop0'' is now a standalone device detached from any VG. However it still contains some LVM metadata that remains to be wiped with the help of the '''pvremove''' command:
 
{{fancywarning|pvremove/pvmove '''do not destroy the disk content'''. Please *do* a secure erase of the storage device with ''shred'' or any similar tool before disposing of it. }}
 
<pre>
# pvdisplay /dev/loop0
  "/dev/loop0" is a new physical volume of "2.00 GiB"
  --- NEW Physical volume ---
  PV Name              /dev/loop0
  VG Name             
  PV Size              2.00 GiB
  Allocatable          NO
  PE Size              0 
  Total PE              0
  Free PE              0
  Allocated PE          0
  PV UUID              b9i1Hi-llka-egCF-2vU2-f7tp-wBqh-qV4qEk
 
# pvremove /dev/loop0
  Labels on physical volume "/dev/loop0" successfully wiped
# pvdisplay /dev/loop0
  No physical volume label read from /dev/loop0
  Failed to read physical volume "/dev/loop0"
</pre>
 
Great! Things are just simple than that. In their day to day reality, system administrators drive their show in a extremely close similar manner: they do additional tasks like taking backups of data located on the LVs before doing any risky operation or plan applications shutdown periods prior starting a manipulation with a LVM volume to take extra precautions.
 
== Replacing a PV (storage device) by another ==
 
The principle a mix of what has been said in the above sections. The principle is basically:
# Create a new PV
# Associate it to the VG
# Move the contents of the PV to be removed on the remaining PVs composing the VG
# Remove the PV from the VG and wipe it
 
The strategy in this paragraph is to reuse ''/dev/loop0'' and make it replace ''/dev/loop2'' (both devices are of the same size, however we also could have used a bigger ''/dev/loop0'' as well).
 
Here we go! First we need to (re-)create the LVM metadata to make ''/dev/loop0'' usable by LVM:
 
<pre>
# pvcreate /dev/loop0
  Physical volume "/dev/loop0" successfully created
</pre>
 
Then this brand new PV is added to the VG ''vgdata'' thus increasing its size of 2 GB:
 
<pre>
# vgextend vgdata  /dev/loop0
  Volume group "vgdata" successfully extended
# vgs
  VG    #PV #LV #SN Attr  VSize VFree
  vgdata  4  3  0 wz--n- 7.98g 2.99g
# pvs
  PV        VG    Fmt  Attr PSize PFree 
  /dev/loop0 vgdata lvm2 a-  2.00g    2.00g
  /dev/loop1 vgdata lvm2 a-  2.00g 1016.00m
  /dev/loop2 vgdata lvm2 a-  2.00g      0
  /dev/loop3 vgdata lvm2 a-  2.00g      0
</pre>
 
Now we have to suspend the allocation of PEs on ''/dev/loop2'' prior to moving its PEs (and freeing some space on it):
 
<pre>
# pvchange -x n /dev/loop2
  Physical volume "/dev/loop2" changed
  1 physical volume changed / 0 physical volumes not changed
# pvs
  PV        VG    Fmt  Attr PSize PFree 
  /dev/loop0 vgdata lvm2 a-  2.00g    2.00g
  /dev/loop1 vgdata lvm2 a-  2.00g 1016.00m
  /dev/loop2 vgdata lvm2 --  2.00g      0
  /dev/loop3 vgdata lvm2 a-  2.00g      0
</pre>
 
Then we move all of the the PEs on ''/dev/loop2'' to the rest of the VG:
 
<pre>
# pvmove /dev/loop2
  /dev/loop2: Moved: 49.9%
  /dev/loop2: Moved: 100.0%
# pvs
  PV        VG    Fmt  Attr PSize PFree 
  /dev/loop0 vgdata lvm2 a-  2.00g      0
  /dev/loop1 vgdata lvm2 a-  2.00g 1016.00m
  /dev/loop2 vgdata lvm2 --  2.00g    2.00g
  /dev/loop3 vgdata lvm2 a-  2.00g      0
</pre>
 
Then we remove ''/dev/loop2'' from the VG and we wipe its LVM metadata:
 
<pre>
# vgreduce vgdata /dev/loop2
  Removed "/dev/loop2" from volume group "vgdata"
# pvremove /dev/loop2
  Labels on physical volume "/dev/loop2" successfully wiped
</pre>
 
Final state of the PVs composing ''vgdata'':
<pre>
# pvs
  PV        VG    Fmt  Attr PSize PFree 
  /dev/loop0 vgdata lvm2 a-  2.00g      0
  /dev/loop1 vgdata lvm2 a-  2.00g 1016.00m
  /dev/loop3 vgdata lvm2 a-  2.00g      0
</pre>
 
''/dev/loop0'' took the place of ''/dev/loop2'' :-)
 
= More advanced topics =
 
== Backing up the layout ==
 
== Freezing a VG ==
 
== LVM snapshots ==
 
== Linear/Stripped/Mirrored Logical volumes ==


I hope you've enjoyed this series. For more information on awk, check out the resources listed below.
= LVM and Funtoo =


== Resources ==
* Read Daniel's other awk articles on Funtoo: Awk By Example, [[Awk by example, Part1|Part 1]] and [[Awk by example, Part2|Part 2]].
* If you'd like a good old-fashioned book, [http://www.oreilly.com/catalog/sed2/ O'Reilly's sed & awk, 2nd Edition] is a wonderful choice.
* Be sure to check out the [http://www.faqs.org/faqs/computer-lang/awk/faq/ comp.lang.awk FAQ]. It also contains lots of additional awk links.
* Patrick Hartigan's [http://sparky.rice.edu/~hartigan/awk.html awk tutorial] is packed with handy awk scripts.
* [http://www.tasoft.com/tawk.html Thompson's TAWK Compiler] compiles awk scripts into fast binary executables. Versions are available for Windows, OS/2, DOS, and UNIX.
* [http://www.gnu.org/software/gawk/manual/gawk.html The GNU Awk User's Guide] is available for online reference.


[[ Category:Linux Core Concepts ]]
[[Category:Labs]]
[[Category:Filesystems]]
[[Category:Articles]]
[[Category:Articles]]

Revision as of 05:10, May 14, 2013

Introduction

LVM (Logical Volume Management) offers a great flexibility in managing your storage and significantly reduces server downtimes by allowing on-line disk space management: The great idea beneath LVM is to make the data and its storage loosely coupled through several layers of abstraction. You (the system administrator) have the hand of each of those layers making the entire space management process extremely simple and flexible though various set of coherent commands.

Several other well-known binary Linux distributions makes an aggressive use of LVM and several Unixes including HP-UX, AIX and Solaris offers since a while a similar functionality modulo the commands to be used. LVM is not mandatory but its usage can bring you additional flexibility and make your everyday life much more simpler.

Concepts

As usual, having a good idea of the concepts lying beneath is mandatory. LVM is not very complicated, but it is easy to become confused, especially because it is a multi-layered system; however LVM designers had the good idea of keeping the command names consistent between all LVM command sets, making your life easier.

LVM consists of, mainly, three things:

  • Physical volumes (or PV): nothing more than a physical storage space. A physical volume can by anything like a partition on a local hard disk, a partition located on a remote SAN disk, a USB key or whatever else that could offer a storage space (so yes, technically it could be possible to use an optical storage device accessed in packet writing mode). The storage space on a physical volumes is divided (and managed) in small units called Physical Extents (or PE). Just to give an analogy if you are a bit familiar with RAID, PE are a bit like RAID stripes.
  • Volume Groups (or VG): a group of at least one PV. VG are named entities and will appear in the system via the device mapper as /dev/volume-group-name.
  • Logical Volumes (or LV): a named division of a volume group in which a filesystem is created and that can be mounted in the VFS. Just for the record, just as for the PE in PV, a LV is managed as chucks known as Logical Extents (or LE). Most of the time those LE are hidden to the system administrator due to a 1:1 mapping between them and the PE lying be just beneath but a cool fact to know about LEs is that they can be spread over PV just like RAID stripes in a RAID-0 volume. However, researches done on the Web tends to demonstrate system administrators prefer to build RAID volumes with mdadm than use LVM over them for performance reasons.

In short words: LVM logical volumes (LV) are containers that can hold a single filesystem and which are created inside a volume group (VG) itself composed by an aggregation of at least one physical volumes (PV) themselves stored on various media (usb key, harddisk partition and so on). The data is stored in chunks spread over the various PV.

   Note

Retain what PV, VG and LV means as we will use those abbreviations in the rest of this article.

Your first tour of LVM

Physical volumes creation

   Note

We give the same size to all volumes for the sake of the demonstration. This is not mandatory and be possible to have mixed sizes PV inside a same VG.

To start with, just create three raw disk images: 

# dd if=/dev/zero of=/tmp/hdd1.img bs=2G count=1
# dd if=/dev/zero of=/tmp/hdd2.img bs=2G count=1
# dd if=/dev/zero of=/tmp/hdd3.img bs=2G count=1

and associate them to a loopback device: 

# losetup -f
/dev/loop0 
# losetup /dev/loop0 /tmp/hdd1.img
# losetup /dev/loop1 /tmp/hdd2.img
# losetup /dev/loop2 /tmp/hdd3.img

Okay nothing really exciting there, but wait the fun is coming! First check that sys-fs/lvm2 is present on your system and emerge it if not. At this point, we must tell you a secret: although several articles and authors uses the taxonomy "LVM" it denotes "LVM version 2" or "LVM 2" nowadays. You must know that LVM had, in the old good times (RHEL 3.x and earlier), a previous revision known as "LVM version 1". LVM 1 is now considered as an extincted specie and is not compatible with LVM 2, although LVM 2 tools maintain a backward compatibility.

The very frst step in LVM is to create the physical devices or PV. "Wait create what?! Aren't the loopback devices present on the system?" Yes they are present but they are empty, we must initialize them some metadata to make them usable by LVM. This is simply done by: 

# pvcreate /dev/loop0
  Physical volume "/dev/loop0" successfully created
# pvcreate /dev/loop1
  Physical volume "/dev/loop1" successfully created
# pvcreate /dev/loop2
  Physical volume "/dev/loop2" successfully created

It is absolutely normal that nothing in particular is printed at the output of each command but we assure you: you have three LVM PVs. You can check them by issuing: 

# pvs
  PV         VG   Fmt  Attr PSize PFree
  /dev/loop0      lvm2 a-   2.00g 2.00g
  /dev/loop1      lvm2 a-   2.00g 2.00g
  /dev/loop2      lvm2 a-   2.00g 2.00g


Some good information there:

  • PV: indicates the physical path the PV lies on
  • VG indicates the VG the PV belongs to. At this time, we didn't created any VG yet and the column remains empty.
  • Fmt: indicates the format of the PV (here it says we have a LVM version 2 PV)
  • Attrs: indicates some status information, the 'a' here just says that the PV is accessible.
  • PSize and PFree: indicates the PV size and the amount of remaining space for this PV. Here we have three empty PV so it bascially says "2 gigabytes large, 2 out of gigabytes free"

It is now time to introduce you to another command: pvdisplay. Just run it without any arguments: 

 pvdisplay
  "/dev/loop0" is a new physical volume of "2.00 GiB"
  --- NEW Physical volume ---
  PV Name               /dev/loop0
  VG Name               
  PV Size               2.00 GiB
  Allocatable           NO
  PE Size               0   
  Total PE              0
  Free PE               0
  Allocated PE          0
  PV UUID               b9i1Hi-llka-egCF-2vU2-f7tp-wBqh-qV4qEk
   
  "/dev/loop1" is a new physical volume of "2.00 GiB"
  --- NEW Physical volume ---
  PV Name               /dev/loop1
  VG Name               
  PV Size               2.00 GiB
  Allocatable           NO
  PE Size               0   
  Total PE              0
  Free PE               0
  Allocated PE          0
  PV UUID               i3mdBO-9WIc-EO2y-NqRr-z5Oa-ItLS-jbjq0E
   
  "/dev/loop2" is a new physical volume of "2.00 GiB"
  --- NEW Physical volume ---
  PV Name               /dev/loop2
  VG Name               
  PV Size               2.00 GiB
  Allocatable           NO
  PE Size               0   
  Total PE              0
  Free PE               0
  Allocated PE          0
  PV UUID               dEwVuO-a5vQ-ipcH-Rvlt-5zWt-iAB2-2F0XBf

The third three lines of each PV shows:

  • what is the storage device beneath a PV
  • the VG it is tied to
  • the size of this PV.

Allocatable indicates whether the PV is used to store data. As the PV is not a member of a VG, it cannot not be used (yet) hence the "NO" shown. Another set of information is the lines starting with PE. PE stands for Physical Extents (data stripe) and is the finest granularity LVM can manipulate. The size of a PE is "0" here because we have a blank PV however it typically holds 32 MB of data. Following PE Size are Total PE which show the the total number of PE available on this PV and Free PE the number of PE remaining available for use. Allocated PE just show the difference between Total PE and Free PE.

The latest line (PV UUID) is a unique identifier used internally by LVM to name the PV. You have to know that it exists because it is sometimes useful when having to recover from corruption or do weird things with PV however most of the time you don't have to worry about its existence.

   Note

It is possible to force how LVM handles the alignments on the physical storage. This is useful when dealing with 4K sectors drives that lies on their physical sectors size. Refer to the manual page.

Volume group creation

We have the blank PV at this time but to make them a bit more usable for storage we must tell to LVM how they are grouped to form a VG (storage pool) where LV will be created. A nice aspect of VGs resides in the fact that they are not "written in the stone" once created: you can still add, remove or exchange PV (in the case the device the PV is stored on fails for example) inside a VG at a later time. To create our first volume group named vgtest: 

# vgcreate vgtest /dev/loop0 /dev/loop1 /dev/loop2
  Volume group "vgtest" successfully created

Just like we did before with PV, we can get a list of what are the VG known by the system. This is done through the command vgs: 

# vgs
  VG     #PV #LV #SN Attr   VSize VFree
  vgtest   3   0   0 wz--n- 5.99g 5.99g

vgs show you a tabluar view of information:

  • VG: the name of the VG
  • #PV: the number of PV composing the VG
  • #LV: the number of logical volumes (LV) located inside the VG
  • Attrs: a status field. w, z and n here means that VG is:
    • w: writable
    • z: resizable
    • n: using the allocation policy normal (tweaking allocation policies is beyond the scope of this article, we will use the default value normal in the rest of this article)
  • VSize and VFree gives statistics on how full a VG is versus its size

Note the dashes in Attrs, they mean that the attribute is not active:

  • First dash (3rd position) indicates if the VG would have been exported (a 'x' would have been showed at this position in that case).
  • Second dash (4th position) indicates if the VG would have been partial (a 'p' would have been showed at this position in that case).
  • Third dash (rightmost position) indicates if the VG is a clustered (a 'c' would have been showed at this position in that case).

Exporting a VG and clustered VG are a bit more advanced aspects of LVM and won't be covered here especially the clustered VGs which are used in the case of a shared storage space used in a cluster of machines. Talking about clustered VGs management in particular would require and entire article in itself. For now the only detail you have to worry about those dashes in Attrs is to see a dash at the 4th position of Attrs instead of a p. Seeing p there would be a bad news: the VG would have missing parts (PV) making it not usable.

   Note

In the exact same manner you can see a detailed information about physical volumes with pvdisplay, you can see detailed information of a volume group with vgdisplay. We will demonstrate that latter command in the paragraphs to follow.

Before leaving the volume group aspect, do you remember the pvs command shown in the previous paragraphs? Try it gain: 

# pvs
  PV         VG     Fmt  Attr PSize PFree
  /dev/loop0 vgtest lvm2 a-   2.00g 2.00g
  /dev/loop1 vgtest lvm2 a-   2.00g 2.00g
  /dev/loop2 vgtest lvm2 a-   2.00g 2.00g

Now it shows the VG our PVs belong to :-)

Logical volumes creation

Now the final steps: we will create the storage areas (logical volumes or LV) inside the VG where we will then create filesystems on. Just like a VG has a name, a LV has also a name which is unique in the VG.

   Note

Two LV can be given the same name as long as they are located on a different VG.

To divide our VG like below:

  • lvdata1: 2 GB
  • lvdata2: 1 GB
  • lvdata3 : 10% of the VG size
  • lvdata4 : All of remaining free space in the VG

We use the following commands (notice the capital 'L' and the small 'l' to declare absolute or relative sizes): 

# lvcreate -n lvdata1 -L 2GB vgtest
  Logical volume "lvdata1" created
#  lvcreate -n lvdata2 -L 1GB vgtest
  Logical volume "lvdata2" created
# lvcreate -n lvdata3 -l 10%VG vgtest
  Logical volume "lvdata2" created

What is going on so far? Let's check with the pvs/vgs counterpart known as lvs: 

# lvs
  LV      VG     Attr   LSize   Origin Snap%  Move Log Copy%  Convert
  lvdata1 vgtest -wi-a-   2.00g                                      
  lvdata2 vgtest -wi-a-   1.00g                                      
  lvdata3 vgtest -wi-a- 612.00m
#

Notice the size of lvdata3, it is roughly 600MB (10% of 6GB). How much free space remains in the VG? Time to see what vgs and vgdisplay returns: 

# vgs
  VG     #PV #LV #SN Attr   VSize VFree
  vgtest   3   3   0 wz--n- 5.99g 2.39g
# vgdisplay 
  --- Volume group ---
  VG Name               vgtest
  System ID             
  Format                lvm2
  Metadata Areas        3
  Metadata Sequence No  4
  VG Access             read/write
  VG Status             resizable
  MAX LV                0
  Cur LV                3
  Open LV               0
  Max PV                0
  Cur PV                3
  Act PV                3
  VG Size               5.99 GiB
  PE Size               4.00 MiB
  Total PE              1533
  Alloc PE / Size       921 / 3.60 GiB
  Free  PE / Size       612 / 2.39 GiB
  VG UUID               baM3vr-G0kh-PXHy-Z6Dj-bMQQ-KK6R-ewMac2

Basically it say we have 1533 PE (chunks) available for a total size of 5.99 GiB. On those 1533, 921 are used (for a size of 3.60 GiB) and 612 remains free (for a size of 2.39 GiB). So we expect to see lvdata4 having an approximative size of 2.4 GiB. Before creating it, have a look at some statistics at the PV level: 

# pvs
  PV         VG     Fmt  Attr PSize PFree  
  /dev/loop0 vgtest lvm2 a-   2.00g      0 
  /dev/loop1 vgtest lvm2 a-   2.00g 404.00m
  /dev/loop2 vgtest lvm2 a-   2.00g   2.00g

# pvdisplay
  --- Physical volume ---
  PV Name               /dev/loop0
  VG Name               vgtest
  PV Size               2.00 GiB / not usable 4.00 MiB
  Allocatable           yes (but full)
  PE Size               4.00 MiB
  Total PE              511
  Free PE               0
  Allocated PE          511
  PV UUID               b9i1Hi-llka-egCF-2vU2-f7tp-wBqh-qV4qEk
   
  --- Physical volume ---
  PV Name               /dev/loop1
  VG Name               vgtest
  PV Size               2.00 GiB / not usable 4.00 MiB
  Allocatable           yes 
  PE Size               4.00 MiB
  Total PE              511
  Free PE               101
  Allocated PE          410
  PV UUID               i3mdBO-9WIc-EO2y-NqRr-z5Oa-ItLS-jbjq0E
   
  --- Physical volume ---
  PV Name               /dev/loop2
  VG Name               vgtest
  PV Size               2.00 GiB / not usable 4.00 MiB
  Allocatable           yes 
  PE Size               4.00 MiB
  Total PE              511
  Free PE               511
  Allocated PE          0
  PV UUID               dEwVuO-a5vQ-ipcH-Rvlt-5zWt-iAB2-2F0XBf

Quite interesting! Did you notice? The first PV is full, the second is more or less full and the third is empty. This is due to the allocation policy used for the VG: it fills its first PV then its second PV and then its third PV (this, by the way, gives you a chance to recover from a dead physical storage if by luck none of your PE was present on it).

It is now time to create our last LV, again notice the small 'l' to specify a relative size: 

# lvcreate -n lvdata4 -l 100%FREE vgtest
  Logical volume "lvdata4" created
# lvs
  LV      VG     Attr   LSize   Origin Snap%  Move Log Copy%  Convert
  lvdata1 vgtest -wi-a-   2.00g                                      
  lvdata2 vgtest -wi-a-   1.00g                                      
  lvdata3 vgtest -wi-a- 612.00m                                      
  lvdata4 vgtest -wi-a-   2.39g

Now the $100 question: if pvdisplay and vgdisplay commands exist, does command named lvdisplay exist as well? Yes absolutely! Indeed the command sets are coherent between abstraction levels (PV/VG/LV) and they are named in the exact same manner modulo their first 2 letters:

  • PV: pvs/pvdisplay/pvchange....
  • VG: vgs/vgdisplay/vgchange....
  • LG: lvs/lvdisplay/lvchange....

Back to our lvdisplay command, here is how it shows up: 

# lvdisplay 
  --- Logical volume ---
  LV Name                /dev/vgtest/lvdata1
  VG Name                vgtest
  LV UUID                fT22is-cmSL-uhwM-zwCd-jeIe-DWO7-Hkj4k3
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                2.00 GiB
  Current LE             512
  Segments               2
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:0
   
  --- Logical volume ---
  LV Name                /dev/vgtest/lvdata2
  VG Name                vgtest
  LV UUID                yd07wA-hj77-rOth-vxW8-rwo9-AX7q-lcyb3p
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                1.00 GiB
  Current LE             256
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:1
   
  --- Logical volume ---
  LV Name                /dev/vgtest/lvdata3
  VG Name                vgtest
  LV UUID                ocMCL2-nkcQ-Fwdx-pss4-qeSm-NtqU-J7vAXG
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                612.00 MiB
  Current LE             153
  Segments               1
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:2
   
  --- Logical volume ---
  LV Name                /dev/vgtest/lvdata4
  VG Name                vgtest
  LV UUID                iQ2rV7-8Em8-85ts-anan-PePb-gk18-A31bP6
  LV Write Access        read/write
  LV Status              available
  # open                 0
  LV Size                2.39 GiB
  Current LE             612
  Segments               2
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:3

Nothing extremely useful to comment for an overview beyond showing at the exception of two things:

  1. LVs are accessed via the device mapper (see the lines starting by LV Name and notice how the name is composed). So lvdata1 will be accessed via /dev/vgtest/lvdata1, lvdata2 will be accessed via /dev/vgtest/lvdata2 and so on.
  2. just like PV are managed in sets of data chunks (the so famous Physical Extents or PEs), LVs are managed in a set of data chunks known as Logical Extents or LEs. Most of the time you don't have to worry about the existence of LEs because they fits withing a single PE although it is possible to make them smaller hence having several LE within a single PE. Demonstration: if you consider the first LV, lvdisplay says it has a size of 2 GiB and holds 512 logical extents. Dividing 2GiB by 512 gives 4 MiB as the size of a LE which is the exact same size used for PEs as seen when demonstrating the pvdisplay command some paragraphs above. So in our case we have a 1:1 match between a LE and the underlying PE.

Oh another great point to underline: you can display the PV in relation with a LV :-) Just give a special option to lvdisplay: 

# lvdisplay -m
  --- Logical volume ---
  LV Name                /dev/vgtest/lvdata1
  VG Name                vgtest
  (...)
  Current LE             512
  Segments               2
  (...)
  --- Segments ---
  Logical extent 0 to 510:
    Type                linear
    Physical volume     /dev/loop0
    Physical extents    0 to 510
   
  Logical extent 511 to 511:
    Type                linear
    Physical volume     /dev/loop1
    Physical extents    0 to 0
   
   
  --- Logical volume ---
  LV Name                /dev/vgtest/lvdata2
  VG Name                vgtest
  (...)
  Current LE             256
  Segments               1
  (...)
   
  --- Segments ---
  Logical extent 0 to 255:
    Type                linear
    Physical volume     /dev/loop1
    Physical extents    1 to 256
   
   
  --- Logical volume ---
  LV Name                /dev/vgtest/lvdata3
  VG Name                vgtest
  (...)
  Current LE             153
  Segments               1
  (...)
   
  --- Segments ---
  Logical extent 0 to 152:
    Type                linear
    Physical volume     /dev/loop1
    Physical extents    257 to 409
   
   
  --- Logical volume ---
  LV Name                /dev/vgtest/lvdata4
  VG Name                vgtest
  (...)
  Current LE             612
  Segments               2
  (...)
   
  --- Segments ---
  Logical extent 0 to 510:
    Type                linear
    Physical volume     /dev/loop2
    Physical extents    0 to 510
   
  Logical extent 511 to 611:
    Type                linear
    Physical volume     /dev/loop1
    Physical extents    410 to 510

To go one step further let's analyze a bit how the PE are used: the first LV has 512 LEs (remember: one LE fits within one PE here so 1 LE = 1 PE). Amongst those 512 LEs, 511 of them (0 to 510) are stored on /dev/loop0 and the 512th LE is on /dev/loop1. Huh? Something seems to be wrong here, pvdisplay said that /dev/loop0 was holding 512 PV so why an extent has been placed on the second storage device? Indeed its not a misbehaviour and absolutely normal: LVM uses some metadata internally with regards the PV, VG and LV thus making some of storage space unavailable for the payload. This explains why 1 PE has been "eaten" to store that metadata. Also notice the linear allocation process: /dev/loop0 has been used, then when being full /dev/loop1 has also been used then the turn of /dev/loop2 came.

Now everything is in place, if you want just check again with vgs/pvs/vgdisplay/pvdisplay and will notice that the VG is now 100% full and all of the underlying PV are also 100% full.

Filesystems creation and mounting

Now we have our LVs it could be fun if we could do something useful with them. In the case you missed it, LVs are accessed via the device mapper which uses a combination of the VG and LV names thus:

  • lvdata1 is accessible via /dev/vgtest/lvdata1
  • lvdata2 is accessible via /dev/vgtest/lvdata2
  • and so on!

Just like any traditional storage device, the newly created LVs are seen as block devices as well just as if they were a kind of harddisk (don't worry about the "dm-..", it is just an internal block device automatically allocated by the device mapper for you): 

# ls -l /dev/vgtest
total 0
lrwxrwxrwx 1 root root 7 Dec 27 12:54 lvdata1 -> ../dm-0
lrwxrwxrwx 1 root root 7 Dec 27 12:54 lvdata2 -> ../dm-1
lrwxrwxrwx 1 root root 7 Dec 27 12:54 lvdata3 -> ../dm-2
lrwxrwxrwx 1 root root 7 Dec 27 12:54 lvdata4 -> ../dm-3

# ls -l /dev/dm-[0-3]
brw-rw---- 1 root disk 253, 0 Dec 27 12:54 /dev/dm-0
brw-rw---- 1 root disk 253, 1 Dec 27 12:54 /dev/dm-1
brw-rw---- 1 root disk 253, 2 Dec 27 12:54 /dev/dm-2
brw-rw---- 1 root disk 253, 3 Dec 27 12:54 /dev/dm-3

So if LVs are block device a filesystem can be created on them just like if they were a real harddisk or hardisk partitions? Absolutely! Now let's create ext4 filesystems on our LVs: 

# mkfs.ext4 /dev/vgtest/lvdata1

mke2fs 1.42 (29-Nov-2011)
Discarding device blocks: done                            
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
131072 inodes, 524288 blocks
26214 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=536870912
16 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks: 
        32768, 98304, 163840, 229376, 294912

Allocating group tables: done                            
Writing inode tables: done                            
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done

# mkfs.ext4 /dev/vgtest/lvdata1
(...)
# mkfs.ext4 /dev/vgtest/lvdata2
(...)
# mkfs.ext4 /dev/vgtest/lvdata3
(..)

Once the creation ended we must create the mount points and mount the newly created filesystems on them: 

# mkdir /mnt/data-01
# mkdir /mnt/data-02
# mkdir /mnt/data-03
# mkdir /mnt/data-04
# mount /dev/vgtest/lvdata1 /mnt/data01
# mount /dev/vgtest/lvdata2 /mnt/data02
# mount /dev/vgtest/lvdata3 /mnt/data03
# mount /dev/vgtest/lvdata4 /mnt/data04

Finally we can check that everything is in order: 

# df -h
Filesystem                    Size  Used Avail Use% Mounted on
(...)
/dev/mapper/vgtest-lvdata1    2.0G   96M  1.9G   5% /mnt/data01
/dev/mapper/vgtest-lvdata2   1022M   47M  924M   5% /mnt/data02
/dev/mapper/vgtest-lvdata3    611M   25M  556M   5% /mnt/data03
/dev/mapper/vgtest-lvdata4    2.4G  100M  2.2G   5% /mnt/data04

Did you notice the device has changed? Indeed everything is in order, mount just uses another set of symlinks which point to the exact same block devices: 

# ls -l /dev/mapper/vgtest-lvdata[1-4]
lrwxrwxrwx 1 root root 7 Dec 28 20:12 /dev/mapper/vgtest-lvdata1 -> ../dm-0
lrwxrwxrwx 1 root root 7 Dec 28 20:13 /dev/mapper/vgtest-lvdata2 -> ../dm-1
lrwxrwxrwx 1 root root 7 Dec 28 20:13 /dev/mapper/vgtest-lvdata3 -> ../dm-2
lrwxrwxrwx 1 root root 7 Dec 28 20:13 /dev/mapper/vgtest-lvdata4 -> ../dm-3

Renaming a volume group and its logical volumes

So far we have four LVs named lvdata1 to lvdata4 mounted on /mnt/data01 to /mnt/data04. It would be more adequate to :

  1. make the number in our LV names being like "01" instead of "1"
  2. rename our volume groupe to "vgdata" instead of "vgtest"

To show how dynamic is the LVM world, we will rename our VG and LV on the fly using two commands: vgrename for acting at the VG level and its counterpart lvrename to act at the LV level. Starting by the VG or the LVs makes strictly no difference, you can start either way and get the same result. In our example we have chosen to start with the VG: 

# vgrename vgtest vgdata
  Volume group "vgtest" successfully renamed to "vgdata"
# lvrename vgdata/lvdata1 vgdata/lvdata01
  Renamed "lvdata1" to "lvdata01" in volume group "vgdata"
# lvrename vgdata/lvdata2 vgdata/lvdata02
  Renamed "lvdata2" to "lvdata02" in volume group "vgdata"
# lvrename vgdata/lvdata3 vgdata/lvdata03
  Renamed "lvdata3" to "lvdata03" in volume group "vgdata"
# lvrename vgdata/lvdata4 vgdata/lvdata04
  Renamed "lvdata4" to "lvdata04" in volume group "vgdata"

What happened? Simple: 

# vgs
  VG     #PV #LV #SN Attr   VSize VFree
  vgdata   3   4   0 wz--n- 5.99g    0 
# lvs
  LV       VG     Attr   LSize   Origin Snap%  Move Log Copy%  Convert
  lvdata01 vgdata -wi-ao   2.00g                                      
  lvdata02 vgdata -wi-ao   1.00g                                      
  lvdata03 vgdata -wi-ao 612.00m                                      
  lvdata04 vgdata -wi-ao   2.39g

Sounds good, our VG and LVs have been renamed! What a command like mount will say? 

# mount
(...)
/dev/mapper/vgtest-lvdata1 on /mnt/data01 type ext4 (rw)
/dev/mapper/vgtest-lvdata2 on /mnt/data02 type ext4 (rw)
/dev/mapper/vgtest-lvdata3 on /mnt/data03 type ext4 (rw)
/dev/mapper/vgtest-lvdata4 on /mnt/data04 type ext4 (rw)

Ooops... It is not exactly a bug, mount still shows the symlinks used at the time the LVs were mounted in the VFS and has not updated its information. However once again everything is correct because the underlying block devices (/dev/dm-0 to /dev/dm-3) did not changed at all. To see the right information the LVs must be unmounted and mounted again: 

# umount /mnt/data01
(...)
# umount /mnt/data04
# mount /dev/vgdata/lvdata01 /mnt/data01 
(...)
# mount /dev/vgdata/lvdata04 /mnt/data04
# mount
/dev/mapper/vgdata-lvdata01 on /mnt/data01 type ext4 (rw)
/dev/mapper/vgdata-lvdata02 on /mnt/data02 type ext4 (rw)
/dev/mapper/vgdata-lvdata03 on /mnt/data03 type ext4 (rw)
/dev/mapper/vgdata-lvdata04 on /mnt/data04 type ext4 (rw)
   Note

Using /dev/volumegroup/logicalvolume or /dev/volumegroup-logicalvolume makes no difference at all, those are two sets of symlinks pointing on the exact same block device.

Expanding and shrinking the storage space

Did you notice in the previous section we have never talked on topic like "create this partition at the beginning" or "allocate 10 sectors more". In LVM you do not have to worry about that kind of problematics: your only concern is more "Do I have the space to allocate a new LV or how can I extend an existing LV?". LVM takes cares of the low levels aspects for you, just focus on what you want to do with your storage space.

The most common problem with computers is the shortage of space on a volume, most of the time production servers can run months or years without requiring a reboot for various reasons (kernel upgrade, hardware failure...) however they regularly requires to extend their storage space because we do generate more and more data as the time goes. With "traditional" approach like fiddling directly with hard drives partitions, storage space manipulation can easily become a headache mainly because it requires coherent copy to be made and thus application downtimes. Don't expect the situation to be more enjoyable with a SAN storage rather a directly attached storage device... Basically the problems remains the same.

Expanding a storage space

The most common task for a system administrator is to expand the available storage space. In the LVM world this implies:

  • Creating a new PV
  • Adding the PV to the VG (thus extending the VG capacity)
  • Extending the existing LVs or create new ones
  • Extending the structures of the filesystems located on a LV in the case a LV is extended (Not all of the filesystems around support that capability).

Bringing a new PV in the VG

In the exact same manner we have created our first PV let's create our additional storage device, associate it to a loopback device and then create a PV on it: 

# dd if=/dev/zero of=/tmp/hdd4.img bs=2G count=1
# losetup /dev/loop3 /tmp/hdd4.img
# pvcreate /dev/loop3

A pvs should report the new PV with 2 GB of free space: 

# pvs
  PV         VG     Fmt  Attr PSize PFree
  /dev/loop0 vgdata lvm2 a-   2.00g    0 
  /dev/loop1 vgdata lvm2 a-   2.00g    0 
  /dev/loop2 vgdata lvm2 a-   2.00g    0 
  /dev/loop3        lvm2 a-   2.00g 2.00g

Excellent! The next step consist of adding this newly created PV inside our VG vgdata, this is where the vgextend command comes at our rescue: 

# vgextend vgdata /dev/loop3
  Volume group "vgdata" successfully extended
# vgs
  VG     #PV #LV #SN Attr   VSize VFree
  vgdata   4   4   0 wz--n- 7.98g 2.00g

Great, vgdata is now 8 GB large instead of 6 GB and have 2 GB of free space to allocate to either new LVs either existing LVs.

Extending the LV and its filesystem

Bringing new LV would demonstrate nothing more nevertheless extending our existing LVs is much more interesting. How can we use our 2GB extra free space? We can, for example, split it in two allocating a 50% to our first (lvdata01) and third (lvdata03) LV adding 1GB of space to both. The best of the story is that operation is very simple and is realized with a command named lvextend: 

# lvextend vgdata/lvdata01 -l +50%FREE 
  Extending logical volume lvdata01 to 3.00 GiB
  Logical volume lvdata01 successfully resized
# lvextend vgdata/lvdata03 -l +50%FREE
  Extending logical volume lvdata03 to 1.10 GiB
  Logical volume lvdata03 successfully resized

Ouaps!! We did a mistake there: lvdata01 has the expected size (2GB + 1GB for a grand total of 3 GB) but lvdata03 only grown of 512 MB (for a grand total size of 1.1 GB). Our mistake was obvious: once the first gigabyte (50% of 2GB) of extra space has been given to lvdata01, only one gigabyte remained free on the VG thus when we said "allocate 50% of the remaining gigabyte to lvdata03" LVM added only 512 MB leaving the other half of this gigabyte unused. The vgs command can confirm this: 

# vgs
  VG     #PV #LV #SN Attr   VSize VFree  
  vgdata   4   4   0 wz--n- 7.98g 512.00m

Nevermind about that voluntary mistake we will keep that extra space for a later paragraph :-) What happened to the storage space visible from the operating system? 

# df -h | grep lvdata01
/dev/mapper/vgdata-lvdata01   2.0G   96M  1.9G   5% /mnt/data01

Obviously resizing a LV does not "automagically" resize the filesystem structures to take into account the new LV size making that step part of our duty. Happily for us, ext3 can be resized and better it can be grown when mounted in the VFS. This is known as online resizing and a few others filesystems supports that capability, among them we can quote ext2 (ext3 without a journal), ext4 (patches integrated very recently as of Nov/Dec 2011), XFS, ResiserFS and BTRFS. To our knowledge, only BTRFS support both online resizing and online shrinking as of Decembrer 2011, all of the others require a filesystem to be unmounted first before being shrunk.

   Note

Consider using the option -r when invoking lvextend, it asks the command to perform a filesystem resize.

Now let's extend (grow) the ext3 filesystem located on lvdata01. As said above, ext3 support online resizing hence we do not need to kick it out of the VFS first: 

# resize2fs /dev/vgdata/lvdata01
resize2fs 1.42 (29-Nov-2011)
Filesystem at /dev/vgdata/lvdata01 is mounted on /mnt/data01; on-line resizing required
old_desc_blocks = 1, new_desc_blocks = 1
Performing an on-line resize of /dev/vgdata/lvdata01 to 785408 (4k) blocks.
The filesystem on /dev/vgdata/lvdata01 is now 785408 blocks long.

# df -h | grep lvdata01
/dev/mapper/vgdata-lvdata01   3.0G   96M  2.8G   4% /mnt/data01

Et voila! Our LV has now plenty of new space usable :-) We do not bother about how the storage is organized by LVM amongst the underlying storage devices and it is not our problem after all. We only worry about having our storage requirements being satisfied without any further details. From our point of view everything is seen just as if we were manipulating a single storage device subdivided in several partitions of a dynamic size and always organized in a set of contiguous blocks.

Now let's shuffle the cards a bit more: when we examined how the LEs of our LVs were allocated, we saw that lvdata01 (named lvdata1 at this time) consisted of 512 LEs or 512 PEs (because of the 1:1 mapping between those) spread over two PVs. As we have extended it to use an additional PV, we should see it using 3 segments:

  • Segment 1: located on the PV stored on /dev/loop0 (LE/PE #0 to #510)
  • Segment 2: located on the PV stored on /dev/loop1 (LE/PE #511)
  • Segment 3: located on the PV stored on /dev/loop1 (LE/PE #512 and followers)

Is it the case? Let's check: 

# lvdisplay -m  vgdata/lvdata01
  --- Logical volume ---
  LV Name                /dev/vgdata/lvdata01
  VG Name                vgdata
  LV UUID                fT22is-cmSL-uhwM-zwCd-jeIe-DWO7-Hkj4k3
  LV Write Access        read/write
  LV Status              available
  # open                 1
  LV Size                3.00 GiB
  Current LE             767
  Segments               3
  Allocation             inherit
  Read ahead sectors     auto
  - currently set to     256
  Block device           253:0
   
  --- Segments ---
  Logical extent 0 to 510:
    Type                linear
    Physical volume     /dev/loop0
    Physical extents    0 to 510
   
  Logical extent 511 to 511:
    Type                linear
    Physical volume     /dev/loop1
    Physical extents    0 to 0
   
  Logical extent 512 to 766:
    Type                linear
    Physical volume     /dev/loop3
    Physical extents    0 to 254

Bingo! Note that if it is true here (LVM uses linear allocation) would not be true in the general case.

   Warning

Never mix a local storage device with a SAN disk within the same volume group and especially if that later is your system volume. It will bring you a lot of troubles if the SAN disk goes offline or bring weird performance fluctuations as PEs allocated on the SAN will get faster response times than those located on a local disk.

Shrinking a storage space

On some occasions it can be useful to reduce the size of a LV or the size of the VG itself. The principle is similar to what has been demonstrated in the previous section:

  1. umount the filesystem belong to the LV to be processed (if your filesystem does not support online shrinking)
  2. reduce the filesystem size (if the LV is not to be flushed)
  3. reduce the LV size - OR - remove the LV
  4. remove a PV from the volume group if no longer used to store extents

The simplest case to start with is how a LV can be removed: a good candidate for removal is lvdata03, we failed to resize it and the better would be to scrap it. First unmount it: 

# lvs
  LV       VG     Attr   LSize Origin Snap%  Move Log Copy%  Convert
  lvdata01 vgdata -wi-ao 3.00g                                      
  lvdata02 vgdata -wi-ao 1.00g                                      
  lvdata03 vgdata -wi-ao 1.10g                                      
  lvdata04 vgdata -wi-ao 2.39g                                      
# umount /dev/vgdata/lvdata03
# lvs
  LV       VG     Attr   LSize Origin Snap%  Move Log Copy%  Convert
  lvdata01 vgdata -wi-ao 3.00g                                      
  lvdata02 vgdata -wi-ao 1.00g                                      
  lvdata03 vgdata -wi-a- 1.10g                                      
  lvdata04 vgdata -wi-ao 2.39g

Noticed the little change with lvs? It lies in the Attr field: once the lvdata03 has been unmounted, lvs tells us the LV is not opened anymore (the little o at the rightmost position has been replaced by a dash). The LV still exists but nothing is using it.

To remove lvdata03 use the command lvremove and confirm the removal by entering 'y' when asked: 

# lvremove vgdata/lvdata03
Do you really want to remove active logical volume lvdata03? [y/n]: y
  Logical volume "lvdata03" successfully removed
# lvs
  LV       VG     Attr   LSize Origin Snap%  Move Log Copy%  Convert
  lvdata01 vgdata -wi-ao 3.00g                                      
  lvdata02 vgdata -wi-ao 1.00g                                      
  lvdata04 vgdata -wi-ao 2.39g
# vgs
  VG     #PV #LV #SN Attr   VSize VFree
  vgdata   4   3   0 wz--n- 7.98g 1.60g

Notice the 1.60 of space has been freed in the VG. What can we do next? Shrinking lvdata04 by 50% giving roughly 1.2GB or 1228MB (1.2*1024) of its size could be a good idea so here we go. First we need to umount the filesystem from the VFS because ext3 does not support online shrinking. 

# umount /dev/vgdata/lvdata04
# e2fsck -f /dev/vgdata/lvdata04
e2fsck 1.42 (29-Nov-2011)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/vgdata/lvdata04: 11/156800 files (0.0% non-contiguous), 27154/626688 blocks
# resize2fs -p /dev/vgdata/lvdata04 -L 1228M
# lvreduce /dev/vgdata/lvdata04 -L 1228
  WARNING: Reducing active logical volume to 1.20 GiB
  THIS MAY DESTROY YOUR DATA (filesystem etc.)
Do you really want to reduce lvdata04? [y/n]: y
  Reducing logical volume lvdata04 to 1.20 GiB
  Logical volume lvdata04 successfully resized
oxygen ~ # e2fsck -f /dev/vgdata/lvdata04 
e2fsck 1.42 (29-Nov-2011)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
/dev/vgdata/lvdata04: 11/78400 files (0.0% non-contiguous), 22234/314368 blocks

Not very practical indeed, we can tell lvreduce to handle the underlying filesystem shrinkage for us. Let's shrink again this time giving a 1 GB volume (1024 MB) in absolute size: 

# lvreduce /dev/vgdata/lvdata04 -r -L 1024
fsck from util-linux 2.20.1
/dev/mapper/vgdata-lvdata04: clean, 11/78400 files, 22234/314368 blocks
resize2fs 1.42 (29-Nov-2011)
Resizing the filesystem on /dev/mapper/vgdata-lvdata04 to 262144 (4k) blocks.
The filesystem on /dev/mapper/vgdata-lvdata04 is now 262144 blocks long.

  Reducing logical volume lvdata04 to 1.00 GiB
  Logical volume lvdata04 successfully resized
# lvs
  LV       VG     Attr   LSize Origin Snap%  Move Log Copy%  Convert
  lvdata01 vgdata -wi-ao 3.00g                                      
  lvdata02 vgdata -wi-ao 1.00g                                      
  lvdata04 vgdata -wi-a- 1.00g
   Note

Notice the number of 4k blocks shown: 4096*262144/1024^2 gives 1,073,741,824 bytes either 1 GB.

Time to mount the volume again: 

# mount /dev/vgdata/lvdata04 /mnt/data04 
# df -h | grep lvdata04
/dev/mapper/vgdata-lvdata04  1021M   79M  891M   9% /mnt/data04

And what is going on at the VG level? 

# vgs
  VG     #PV #LV #SN Attr   VSize VFree
  vgdata   4   3   0 wz--n- 7.98g 2.99g

Wow, we have near 3 GB of free space inside, a bit more than one of our PV. It could be great if we can free one of the those and of course LVM gives you the possibility to do that. Before going further, let's check what happened at the PVs level: 

# pvs     
  PV         VG     Fmt  Attr PSize PFree   
  /dev/loop0 vgdata lvm2 a-   2.00g       0 
  /dev/loop1 vgdata lvm2 a-   2.00g 1016.00m
  /dev/loop2 vgdata lvm2 a-   2.00g 1020.00m
  /dev/loop3 vgdata lvm2 a-   2.00g    1.00g

Did you noticed? 1 GB of space has been freed on the last PV (/dev/loop3) since lvdata04 has been shrunk not counting the space freed on /dev/loop1 and /dev/loop2 after the removal of lvdata02.


Next steo: can we remove a PV directly (the command to remove a PV from a VG is vgreduce)? 

# vgreduce vgdata /dev/loop0 
  Physical volume "/dev/loop0" still in use

Of course not, all of our PVs supports the content of our LVs and we must find a manner to move all of the PE (physical extents) actually hold by the PV /dev/loop0 elsewhere withing the VG. But wait a minute, the victory is there yet: we do have some free space in the /dev/loop0 and we will get more and more free space in it as the displacement process will progress. What is going to happen if, from a concurrent session, we create others LV in vgdata at the same time the content of /dev/loop0 is moved? Simple: it can be filled again with the PEs newly allocated.

So before proceeding to the displacement of what /dev/loop0 contents, we must say to LVM: "please don't allocate anymore PEs on /dev/loop0". This is achieved via the parameter -x of the command pvchange: 

# pvchange -x n /dev/loop0 
  Physical volume "/dev/loop0" changed
  1 physical volume changed / 0 physical volumes not changed

The value n given to -x marks the PV as unallocable (i.e. not usable for future PE allocations). Let's check again the PVs with pvs and pvdisplay: 

# pvs
  PV         VG     Fmt  Attr PSize PFree   
  /dev/loop0 vgdata lvm2 --   2.00g       0 
  /dev/loop1 vgdata lvm2 a-   2.00g 1016.00m
  /dev/loop2 vgdata lvm2 a-   2.00g 1020.00m
  /dev/loop3 vgdata lvm2 a-   2.00g    1.00g

# pvdisplay /dev/loop0
  --- Physical volume ---
  PV Name               /dev/loop0
  VG Name               vgdata
  PV Size               2.00 GiB / not usable 4.00 MiB
  Allocatable           NO
  PE Size               4.00 MiB
  Total PE              511
  Free PE               0
  Allocated PE          511
  PV UUID               b9i1Hi-llka-egCF-2vU2-f7tp-wBqh-qV4qEk

Great news here, the Attrs field shows a dash instead of 'a' at the leftmost position meaning the PV is effectively not allocatable. However marking a PV not allocatable does not wipe the existing PEs stored on it. In other words, it means that data present on the PV remains absolutely intact. Another positive point lies the remaining capacities of the PVs composing vgdata: the sum of free space available on /dev/loop1, /dev/loop2 and /dev/loop3 is 3060MB (1016MB + 1020MB + 1024MB) so largely sufficient to hold the 2048 MB (2 GB) actually stored on the PV /dev/loop0.

Now we have frozen the allocation of PEs on /dev/loop0 we can make LVM move all of PEs located in this PV on the others PVs composing the VG vgdata. Again, we don't have to worry about the gory details like where LVM will precisely relocate the PEs actually hold by /dev/loop0, our only concerns is to get all of them moved out of /dev/loop0. That job gets done by: 

# pvmove /dev/loop0
  /dev/loop0: Moved: 5.9%
  /dev/loop0: Moved: 41.3%
  /dev/loop0: Moved: 50.1%
  /dev/loop0: Moved: 100.0%

We don't have to tell LVM the VG name because it already knows that /dev/loop0 belongs to vgdata and what are the others PVs belonging to that VG usable to host the PEs coming from /dev/loop0. It is absolutely normal for the process to takes some minutes (real life cases can go up to several hours even with SAN disks located on high-end storage hardware which is much more faster than local SATA or even SAS drive).

At the end of the moving process, we can see that the PV /dev/loop0 is totally free: 

# pvs
  PV         VG     Fmt  Attr PSize PFree   
  /dev/loop0 vgdata lvm2 a-   2.00g    2.00g
  /dev/loop1 vgdata lvm2 a-   2.00g 1016.00m
  /dev/loop2 vgdata lvm2 a-   2.00g       0 
  /dev/loop3 vgdata lvm2 a-   2.00g       0 

# pvdisplay /dev/loop0
  --- Physical volume ---
  PV Name               /dev/loop0
  VG Name               vgdata
  PV Size               2.00 GiB / not usable 4.00 MiB
  Allocatable           yes 
  PE Size               4.00 MiB
  Total PE              511
  Free PE               511
  Allocated PE          0
  PV UUID               b9i1Hi-llka-egCF-2vU2-f7tp-wBqh-qV4qEk

511 PEs free out of a maximum 511 PEs so all of its containt has been successfully spread on the others PVs (the volume is also still marked as "unallocatable", this is normal). Now it is ready to be detached from the VG vgdata with the help of vgreduce : 

# vgreduce vgdata /dev/loop0
  Removed "/dev/loop0" from volume group "vgdata"

What happened to vgdata? 

# vgs
  VG     #PV #LV #SN Attr   VSize VFree   
  vgdata   3   3   0 wz--n- 5.99g 1016.00m

Its storage space falls to ~6GB! What would tell pvs? 

# pvs
  PV         VG     Fmt  Attr PSize PFree   
  /dev/loop0        lvm2 a-   2.00g    2.00g
  /dev/loop1 vgdata lvm2 a-   2.00g 1016.00m
  /dev/loop2 vgdata lvm2 a-   2.00g       0 
  /dev/loop3 vgdata lvm2 a-   2.00g       0

/dev/loop0 is now a standalone device detached from any VG. However it still contains some LVM metadata that remains to be wiped with the help of the pvremove command:

   Warning

pvremove/pvmove do not destroy the disk content. Please *do* a secure erase of the storage device with shred or any similar tool before disposing of it. 

# pvdisplay /dev/loop0
  "/dev/loop0" is a new physical volume of "2.00 GiB"
  --- NEW Physical volume ---
  PV Name               /dev/loop0
  VG Name               
  PV Size               2.00 GiB
  Allocatable           NO
  PE Size               0   
  Total PE              0
  Free PE               0
  Allocated PE          0
  PV UUID               b9i1Hi-llka-egCF-2vU2-f7tp-wBqh-qV4qEk

# pvremove /dev/loop0
  Labels on physical volume "/dev/loop0" successfully wiped
# pvdisplay /dev/loop0
  No physical volume label read from /dev/loop0
  Failed to read physical volume "/dev/loop0"

Great! Things are just simple than that. In their day to day reality, system administrators drive their show in a extremely close similar manner: they do additional tasks like taking backups of data located on the LVs before doing any risky operation or plan applications shutdown periods prior starting a manipulation with a LVM volume to take extra precautions.

Replacing a PV (storage device) by another

The principle a mix of what has been said in the above sections. The principle is basically:

  1. Create a new PV
  2. Associate it to the VG
  3. Move the contents of the PV to be removed on the remaining PVs composing the VG
  4. Remove the PV from the VG and wipe it

The strategy in this paragraph is to reuse /dev/loop0 and make it replace /dev/loop2 (both devices are of the same size, however we also could have used a bigger /dev/loop0 as well).

Here we go! First we need to (re-)create the LVM metadata to make /dev/loop0 usable by LVM: 

# pvcreate /dev/loop0
  Physical volume "/dev/loop0" successfully created

Then this brand new PV is added to the VG vgdata thus increasing its size of 2 GB: 

# vgextend vgdata  /dev/loop0
  Volume group "vgdata" successfully extended
# vgs
  VG     #PV #LV #SN Attr   VSize VFree
  vgdata   4   3   0 wz--n- 7.98g 2.99g
# pvs
  PV         VG     Fmt  Attr PSize PFree   
  /dev/loop0 vgdata lvm2 a-   2.00g    2.00g
  /dev/loop1 vgdata lvm2 a-   2.00g 1016.00m
  /dev/loop2 vgdata lvm2 a-   2.00g       0 
  /dev/loop3 vgdata lvm2 a-   2.00g       0

Now we have to suspend the allocation of PEs on /dev/loop2 prior to moving its PEs (and freeing some space on it): 

# pvchange -x n /dev/loop2
  Physical volume "/dev/loop2" changed
  1 physical volume changed / 0 physical volumes not changed
# pvs
  PV         VG     Fmt  Attr PSize PFree   
  /dev/loop0 vgdata lvm2 a-   2.00g    2.00g
  /dev/loop1 vgdata lvm2 a-   2.00g 1016.00m
  /dev/loop2 vgdata lvm2 --   2.00g       0 
  /dev/loop3 vgdata lvm2 a-   2.00g       0

Then we move all of the the PEs on /dev/loop2 to the rest of the VG: 

# pvmove /dev/loop2 
  /dev/loop2: Moved: 49.9%
  /dev/loop2: Moved: 100.0%
# pvs
  PV         VG     Fmt  Attr PSize PFree   
  /dev/loop0 vgdata lvm2 a-   2.00g       0 
  /dev/loop1 vgdata lvm2 a-   2.00g 1016.00m
  /dev/loop2 vgdata lvm2 --   2.00g    2.00g
  /dev/loop3 vgdata lvm2 a-   2.00g       0

Then we remove /dev/loop2 from the VG and we wipe its LVM metadata: 

# vgreduce vgdata /dev/loop2
  Removed "/dev/loop2" from volume group "vgdata"
# pvremove /dev/loop2
  Labels on physical volume "/dev/loop2" successfully wiped

Final state of the PVs composing vgdata: 

# pvs
  PV         VG     Fmt  Attr PSize PFree   
  /dev/loop0 vgdata lvm2 a-   2.00g       0 
  /dev/loop1 vgdata lvm2 a-   2.00g 1016.00m
  /dev/loop3 vgdata lvm2 a-   2.00g       0

/dev/loop0 took the place of /dev/loop2 :-)

More advanced topics

Backing up the layout

Freezing a VG

LVM snapshots

Linear/Stripped/Mirrored Logical volumes

LVM and Funtoo

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