Learning Linux LVM, Part 2
The cvs.gentoo.org upgrade
In my first LVM article, I explained the concepts behind LVM. Now it's time to put LVM into action. In this article, I'm going to set up LVM on the official Gentoo Linux cvs server, cvs.gentoo.org. Although cvs.gentoo.org has only one hard drive, LVM's flexibility still provides an incredible improvement over the standard static partitioning approach. I'll show you all the steps of the LVM conversion process, so that if you're interested you can perform a similar conversion on one of your machines.
Because implementing LVM is a major change to the system (involving the creation of new partitions and other potentially hazardous actions) it's a really good idea to perform a full system backup before beginning this process. If you're not going to perform a backup, I hope you're using a test box with no important data on it. I should mention that I didn't experience any problems while converting to LVM, but it's best to be prepared in case something goes wrong.
That said, let's continue. Cvs.gentoo.org had a nice new IBM 45 GB hard drive sitting in it; however, when I installed Gentoo Linux on cvs, I only partitioned about 10 gigabytes of the drive, keeping the remaining 35 GB for future filesystems. Such are the little tricks you need to employ when not using LVM -- leaving part of the drive unpartitioned is a primitive but effective way to allow for future expansion. However, with LVM there is a better approach.
The space problem
In the past few weeks, I had been noticing that my root ReiserFS partition had been slowly filling up, as you can see from this df output:
# df Filesystem 1k-blocks Used Available Use% Mounted on /dev/hda3 9765200 6989312 2775888 72% / tmpfs 269052 0 269052 0% /dev/shm
Now, a 72% full root partition isn't exactly a crisis, but it isn't a wonderful situation either. ReiserFS, like many other filesystems, starts slowing down as it gets more and more full, and it was just a matter of time before my root filesystem would fill up completely and filesystem performance would take a hit.
I decided to fix this problem by using LVM to create a new logical volume out of the 35 GB of currently unpartitioned space at the end of my hard drive. Then, I'd create a filesystem on this volume and move a good chunk of the contents of /dev/hda3 to it.
If you're thinking of making a similar transition on one of your machines, the first thing you need to do is find a suitable piece of your root filesystem to move to a logical volume. For me, the choice was easy -- my /home tree was taking up around 5.7 GB. By moving /home to its own LVM logical volume, my root filesystem would then be at about 20% capacity. Since most new data is being added to /home, my root filesystem would likely stay at around 20% capacity as well -- a very healthy situation.
The beginnings of a solution
To begin the conversion, I first had to partition the unused space at the end of my hard drive. Using cfdisk, I created a 35 GB partition (/dev/hda5) and set the partition type of the partition to 8E (the official LVM partition type). After this change, I rebooted to force a reread of my partition table. After the reboot, my partition table looked like this:
# sfdisk -l Disk /dev/hda: 89355 cylinders, 16 heads, 63 sectors/track Units = cylinders of 516096 bytes, blocks of 1024 bytes, counting from 0 Device Boot Start End #cyls #blocks Id System /dev/hda1 * 0+ 247 248- 124960+ 83 Linux /dev/hda2 248 743 496 249984 82 Linux swap /dev/hda3 744 20119 19376 9765504 83 Linux /dev/hda4 20120 89354 69235 34894440 5 Extended /dev/hda5 20120+ 89354 69235- 34894408+ 8e Linux LVM
Now that I had an empty 35 GB partition, I was ready to initialize it for LVM. Here's the procedure -- first, I would initialize the 35 gigabytes as a physical volume; then, I would create a volume group using this physical volume, and finally, I would allocate some of the extents on the volume group, creating a logical volume that would contain my new filesystem and house all the files currently in /home.
To begin the process, I used the pvcreate command to initialize /dev/hda5 as a physical volume:
# pvcreate /dev/hda5 pvcreate -- physical volume "/dev/hda5" successfully created
pvcreate set up a special "accounting" area on /dev/hda5, called the VGDA (volume group descriptor area). LVM uses this area to keep track of how the physical extents are allocated, among other things.
My next step was to create a volume group and add /dev/hda5 to this group. The volume group would act as a pool of extents (chunks of storage blocks). Once the volume group was created, I could create as many logical volumes as I wanted. I decided that my volume group would be called "main":
# vgcreate main /dev/hda5 vgcreate -- INFO: using default physical extent size 4 MB vgcreate -- doing automatic backup of volume group "main" vgcreate -- volume group "main" successfully created and activated
The vgcreate command did a couple of things. In addition to creating the "main" volume group, it also set up /dev/hda5 to use 4 MB extents, the default extent size. This means that any logical volumes I create from this volume group can be expanded and shrunk in 4 MB increments.
It is possible to specify a larger extent size at vgcreate time. Extents can range anywhere from 8 KB up to terabytes in size, and must always be a multiple of two. For example, if I wanted to create a volume group with 32 megabyte extents, I'd type:
# vgcreate -s 32M main /dev/hda5
Use an extent size that makes sense for the amount of storage you have available. If you have petabytes of storage, it would make sense to have extents in the gigabyte range. The size you choose for extents in your volume group does not affect filesystem performance, but it does have an impact on the performance of the LVM tools themselves. The fewer extents they have to manage behind the scenes, the faster the tools operate.
Once your volume group is created, you can view its information by typing vgdisplay:
# vgdisplay --- Volume group --- VG Name main VG Access read/write VG Status available/resizable VG # 0 MAX LV 256 Cur LV 0 Open LV 0 MAX LV Size 255.99 GB Max PV 256 Cur PV 1 Act PV 1 VG Size 33.28 GB PE Size 4 MB Total PE 8519 Alloc PE / Size 0 / 0 Free PE / Size 8519 / 33.28 GB VG UUID 2qC2H2-iA8s-qW6F-cwXx-JVIh-I6VC-VVCGmn
Now that I had my volume group, I was ready to create a logical volume. I decided to initially make it 8 gigabytes in size and call it "lv_home":
# lvcreate -L8G -nlv_home main lvcreate -- doing automatic backup of "main" lvcreate -- logical volume "/dev/main/lv_home" successfully created
Then, I created a filesystem on the volume:
# mkreiserfs /dev/main/lv_home <----------- MKREISERFSv2 -----------> Block size 4096 bytes Block count 2097152 Used blocks 8275 Journal - 8192 blocks (18-8209), journal header is in block 8210 Bitmaps: 17, 32768, 65536, 98304, 131072, 163840, 196608, 229376, 262144, 294912, 327680, 360448, 393216, 425984, 458752, 491520, 524288, 557056, 589824, 622592, 655360, 688128, 720896, 753664, 786432, 819200, 851968, 884736, 917504, 950272, 983040, 1015808, 1048576, 1081344, 1114112, 1146880, 1179648, 1212416, 1245184, 1277952, 1310720, 1343488, 1376256, 1409024, 1441792, 1474560, 1507328, 1540096, 1572864, 1605632, 1638400, 1671168, 1703936, 1736704, 1769472, 1802240, 1835008, 1867776, 1900544, 1933312, 1966080, 1998848, 2031616, 2064384 Root block 8211 Hash function "r5" ATTENTION: ALL DATA WILL BE LOST ON '/dev/main/lv_home'! (y/n)y journal size 8192 (from 18) Initializing journal - 0%....20%....40%....60%....80%....100% Syncing..done.
Now that the filesystem was created, I could mount it at /mnt/newhome:
# mkdir /mnt/newhome # mount /dev/main/lv_home /mnt/newhome # df Filesystem 1k-blocks Used Available Use% Mounted on /dev/hda3 9765200 6989840 2775360 72% / tmpfs 291388 0 291388 0% /dev/shm /dev/main/lv_home 8388348 32840 8355508 1% /mnt/newhome
As you can see above, I was almost ready to copy over all my data in /home. Before I began, I dropped to runlevel 1 to ensure that no users or processes would be accessing or modifying files in /home as they were being copied over:
# init 1
Then, I began copying the files:
# cp -avx /home/* /mnt/newhome
The copy completed in about ten minutes. Then, I backed up my original /home to /home.old, just in case something was wrong with my copy. I created a new mount point, and remounted the new home at /home:
# cd / # mv home home.old # mkdir home # umount /mnt/newhome # mount /dev/main/lv_home /home
Then, it was time to set up the server so that my new /home partition would be available every time the machine started up. First, I modified my /etc/fstab so that it included a new /home entry:
#fs mountpoint type opts dump/pass /dev/hda3 / reiserfs defaults 1 1 /dev/main/lv_home /home reiserfs defaults 2 2 /dev/hda2 none swap sw 0 0 /dev/hda1 /boot reiserfs noauto 0 0 /dev/cdrom /mnt/cdrom iso9660 noauto,ro 0 0
Once I had completed these steps, I rebooted the machine, and to my delight everything worked perfectly. After a day or so of absolutely no problems, I deleted /home.old to free up some space on my root filesystem. Yay! The transition to LVM was a success.
The beauty of LVM
While the transition to LVM is a bit of an ordeal, once the transition is complete, managing filesystems becomes tremendously easier. As an example, I decided to resize my new /home logical volume, adding about 2 gigabytes worth of space to the end of the filesystem. First, I added additional capacity to my "lv_home" logical volume, and then I used the resize_reiserfs utility to expand the filesystem so that it would use this additional capacity. Here are the two commands that did all this:
# lvextend -L+2G /dev/main/lv_home # resize_reiserfs -f /dev/main/lv_home
In about a second, I had enlarged my /home filesystem by 2 GB; amazingly, I didn't need to reboot, drop to runlevel 1, or even unmount /home to perform the resize. Everything continued to work as it had before. Isn't that great? Here's the current state of my filesystems:
# df Filesystem 1k-blocks Used Available Use% Mounted on /dev/hda3 9765200 1413340 8351860 15% / /dev/main/lv_home 10485436 5609836 4875600 54% /home
You can see how LVM really can make an administrator's work a whole lot easier. In the future, I hope to move additional parts of my root filesystem over to LVM, and eventually even convert my root filesystem over to an LVM logical volume. The resources below will help you learn even more about LVM.
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