Difference between pages "ZFS Fun" and "Category:App-crypt"

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{{Fancyimportant|This tutorial is under a heavy revision to be switched from ZFS Fuse to ZFS on Linux.}}
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{{PortageCategory|description=<nowiki>
 
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The app-crypt category contains cryptographic (encryption, decryption,
= Introduction =
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steganography and signing) software.
 
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</nowiki>}}
== ZFS features and limitations ==
+
 
+
ZFS offers an impressive amount of features even putting aside its hybrid nature (both a filesystem and a volume manager -- zvol) covered in detail on [http://en.wikipedia.org/wiki/ZFS Wikipedia]. One of the most fundamental points to keep in mind about ZFS is it '''targets a legendary reliability in terms of preserving data integrity'''. ZFS uses several techniques to detect and repair (self-healing) corrupted data. Simply speaking it makes an aggressive use of checksums and relies on data redundancy, the price to pay is a bit more CPU processing power. However, the [http://en.wikipedia.org/wiki/ZFS Wikipedia article about ZFS] also mention it is strongly discouraged to use ZFS over classic RAID arrays as it can not control the data redundancy, thus ruining most of its benefits.
+
 
+
In short, ZFS has the following features (not exhaustive):
+
 
+
* Storage pool dividable in one or more logical storage entities.
+
* Plenty of space:
+
** 256 zettabytes per storage pool (2^64 storages pools max in a system).
+
** 16 exabytes max for a single file
+
** 2^48 entries max per directory
+
* Virtual block-devices support support over a ZFS pool (zvol) - (extremely cool when jointly used  over a RAID-Z volume)
+
* Read-only Snapshot support (it is possible to get a read-write copy of them, those are named clones)
+
* Encryption support (supported only at ZFS version 30 and upper, ZFS version 31 is shipped with Oracle Solaris 11 so that version is mandatory if you plan to encrypt your ZFS datasets/pools)
+
* Built-in''' RAID-5-like-over-steroid capabilities known as [http://en.wikipedia.org/wiki/Non-standard_RAID_levels#RAID-Z RAID-Z] and RAID-6-like-over-steroid capabilities known as RAID-Z2'''. RAID-Z3 (triple parity) also exists.
+
* Copy-on-Write transactional filesystem
+
* Meta-attributes support (properties) allowing you to you easily drive the show like "That directory is encrypted", "that directory is limited to 5GiB", "That directory is exported via NFS" and so on. Depending on what you define, ZFS do the job for you!
+
* Dynamic striping to optimize data throughput
+
* Variable block length 
+
* Data deduplication
+
* Automatic pool re-silvering
+
* Transparent data compression
+
* Transparent encryption (Solaris 11 and later only)
+
 
+
Most notable limitations are:
+
 
+
* Lack a features ZFS developers knows as "Block Pointer rewrite functionality" (planned to be developed), without it ZFS suffers of currently not being able to:
+
** Pool defragmentation (COW techniques used in ZFS mitigates the problem)
+
** Pool resizing
+
** Data compression (re-applying)
+
** Adding an additional device in a RAID-Z/Z2/Z3 pool to increase it size (however, it is possible to replace in sequence each one of the disks composing a RAID-Z/Z2/Z3)
+
* '''NOT A CLUSTERED FILESYSTEM''' like Lustre, GFS or OCFS2
+
* No data healing if used on a single device (corruption can still be detected), workaround if to force a data duplication on the drive
+
* No support of TRIMming (SSD devices)
+
 
+
== ZFS on well known operating systems ==
+
 
+
=== Linux ===
+
 
+
Despite the source code of ZFS is open, its license (Sun CDDL) is incompatible with the license governing the Linux kernel (GNU GPL v2) thus preventing its direct integration. However a couple of ports exists, but suffers of maturity and lack of features. As of writing (February 2014) two known implementations exists:
+
 
+
* [http://zfs-fuse.net ZFS-fuse]: a totally userland implementation relying on FUSE. This implementation can now be considered as defunct as of February  2014). The original site of ZFS FUSE seems to have disappeared nevertheless the source code is still available on [http://freecode.com/projects/zfs-fuse http://freecode.com/projects/zfs-fuse]. ZFS FUSE stalled at version 0.7.0 in 2011 and never really evolved since then.
+
* [http://zfsonlinux.org ZFS on Linux]: a kernel mode implementation of ZFS in kernel mode which supports a lot of NFS features. The implementation is not as complete as it is under Solaris and its siblings like OpenIndiana (e.g. SMB integration is still missing, no encryption support...) but a lot of functionality is there. This is the implementation used for this article. As ZFS on Linux is an out-of-tree Linux kernel implementation, patches must be waited after each Linux kernel release. ZfsOnLinux currently supports zpools version 28 and '''since its version 0.6.2 is considered as ready for production.'''
+
 
+
=== Solaris/OpenIndiana ===
+
 
+
* '''Oracle Solaris:''' remains the de facto reference platform for ZFS implementation: ZFS on this platform is now considered as mature and usable on production systems. Solaris 11 uses ZFS even for its "system" pool (aka ''rpool''). A great advantage of this: it is now quite easy to revert the effect of a patch at the condition a snapshot has been taken just before applying it. In the "old good" times of Solaris 10 and before, reverting a patch was possible but could be tricky and complex when possible. ZFS is far from being new in Solaris as it takes its roots in 2005 to be, then, integrated in Solaris 10 6/06 introduced in June 2006.
+
 
+
* '''[http://openindiana.org OpenIndiana]:''' is based on the Illuminos kernel (a derivative of the now defunct OpenSolaris) which aims to provide absolute binary compatibility with Sun/Oracle Solaris. Worth mentioning that Solaris kernel and the [https://www.illumos.org Illumos kernel] were both sharing the same code base, however, they now follows a different path since Oracle announced the discontinuation of OpenSolaris (August 13th 2010). Like Oracle Solaris, OpenIndiana uses ZFS for its system pool. The illumos kernel ZFS support lags a bit behind Oracle: it  supports zpool version 28 where as Oracle Solaris 11 has zpool version 31 support, data encryption being supported at zpool version 30.
+
 
+
=== *BSD ===
+
 
+
* '''FreeBSD''': ZFS is present in FreeBSD since FreeBSD 7 (zpool version 6) and FreeBSD can boot on a ZFS volume (zfsboot). ZFS support has been vastly enhanced in FreeBSD 8.x (8.2 supports zpool version 15, version 8.3 supports version 28), FreeBSD 9 and FreeBSD 10 (both supports zpool version 28). ZFS in FreeBSD is now considered as fully functional and mature. FreeBSD derivatives such as the popular [http://www.freenas.org FreeNAS] takes befenits of ZFS and integrated it in their tools. In the case of that latter,  it have, for example, supports for zvol though its Web management interface (FreeNAS >= 8.0.1).
+
 
+
* '''NetBSD''': ZFS has been started to be ported as a GSoC project in 2007 and is present in the NetBSD mainstream since 2009 (zpool version 13).
+
 
+
* '''OpenBSD''': No ZFS support yet and not planned until Oracle changes some policies according to the project FAQ.
+
 
+
== ZFS alternatives ==
+
 
+
* WAFL seems to have severe limitation [http://unixconsult.org/wafl/ZFS%20vs%20WAFL.html] (document is not dated), also an interesting article lies [http://blogs.netapp.com/dave/2008/12/is-wafl-a-files.html here]
+
* BTRFS is advancing every week but it still lacks such features like the capability of emulating a virtual block device over a storage pool (zvol) and built-in support for RAID-5/6 is not complete yet (cf. [https://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg29169.html Btrfs mailing list]). At date of writing, it is still experimental where as ZFS is used on big production servers. 
+
* VxFS has also been targeted by comparisons like [http://blogs.oracle.com/dom/entry/zfs_v_vxfs_iozone this one] (a bit [http://www.symantec.com/connect/blogs/suns-comparision-vxfs-and-zfs-scalability-flawed controversial]). VxFS has been known in the industry since 1993 and is known for its legendary flexibility. Symantec acquired VxFS and proposed a basic version (no clustering for example) of it under the same [http://www.symantec.com/enterprise/sfbasic/index.jsp Veritas Storage Foundation Basic]
+
* An interesting discussion about modern filesystems can be found on [http://www.osnews.com/story/19665/Solaris_Filesystem_Choices OSNews.com]
+
 
+
== ZFS vs BTRFS at a glance ==
+
Some key features in no particular order of importance between ZFS and BTRFS:
+
 
+
{| class="wikitable"
+
!Feature||ZFS!!BTRFS!!Remarks
+
|-
+
|Transactional filesystem||YES||YES
+
|-
+
|Journaling||NO||YES||Not a design flaw, but ZFS is robust ''by design''...  See page 7 of [http://hub.opensolaris.org/bin/download/Community+Group+zfs/docs/zfslast.pdf ''"ZFS The last word on filesystems"''].
+
|-
+
|Dividable pool of data storage||YES||YES
+
|-
+
|Read-only snapshot support||YES||YES
+
|-
+
|Writable snapshot support||YES||YES
+
|-
+
|Sending/Receiving a snapshot over the network||YES||YES
+
|-
+
|Rollback capabilities||YES||YES||While ZFS knows where and how to rollback the data (on-line), BTRFS requires a bit more work from the system administrator (off-line).
+
|-
+
|Virtual block-device emulation||YES||NO
+
|-
+
|Data deduplication||YES||YES||Built-in in ZFS, third party tool ([https://github.com/g2p/bedup bedup]) in BTRFS
+
|-
+
|Data blocks reoptimization||NO||YES||ZFS is missing a "block pointer rewrite functionality", true on all known implementations so far. Not a major performance crippling however. BTRFS can do on-line data defragmentation.
+
|-
+
|Built-in data redundancy support||YES||YES||ZFS has a sort of RAID-5/6 (but better! RAID-Z{1,2,3}) capability, BTRFS only fully supports data mirroring at this point, however some works remains to be done on parity bits handling by BTRFS.
+
|-
+
|Management by attributes||YES||NO||Nearly everything touching ZFS management is related to attributes manipulation (quotas, sharing over NFS, encryption, compression...), BTRFS also retain the concept but it les less aggressively used.
+
|-
+
|Production quality code||NO||NO||ZFS support in Linux is not considered as production quality (yet) although it is very robust. Several operating systems like Solaris/OpenIndiana have a production quality implementation, Solaris/OpenIndiana is now installed in ZFS datasets by defaults.
+
|-
+
|Integrated within the Linux kernel tree||NO||YES||ZFS is released under the CDDL license...
+
|}
+
 
+
= ZFS resource naming restrictions =
+
 
+
Before going further, you must be aware of restrictions concerning the names you can use on a ZFS filesystem. The general rule is: you can can use all of the alphanumeric characters plus the following specials are allowed:
+
* Underscore (_)
+
* Hyphen (-)
+
* Colon (:)
+
* Period (.)
+
 
+
The name used to designate a ZFS pool has no particular restriction except:
+
* it can't use one the reserved words in particular:
+
** ''mirror''
+
** ''raidz'' (''raidz2'', ''raidz3'' and so on)
+
** ''spare''
+
** ''cache''
+
** ''log''
+
* names must begin with an alphanumeric character (same for ZFS datasets).
+
 
+
= Some ZFS concepts =
+
Once again with no particular order of importance:
+
{|class="wikitable"
+
|-
+
!ZFS!!What it is!!Counterparts examples
+
|-
+
|zpool||A  group of one or many physical storage media (hard drive partition, file...). A zpool has to be divided in at least one '''ZFS dataset''' or at least one '''zvol''' to hold any data. Several zpools can coexists in a system at the condition they each hold a unique name. Also note that '''zpools can never be mounted, the only things that can are the ZFS datasets they hold.'''||
+
* Volume group (VG) in LVM
+
* BTRFS volumes
+
|-
+
|dataset||A logical subdivision of a zpool mounted in your host's VFS where your files and directories resides. Several uniquely named ZFS datasets can coexist in a single system at the conditions they each own a unique name within their zpool.||
+
* Logical subvolumes (LV) in LVM formatted with a filesystem like ext3.
+
* BTRFS subvolumes
+
|-
+
|snapshot||A read-only photo of a ZFS dataset state as is taken at a precise moment of time. ZFS has no way to cooperate on its own with applications that read and write data on ZFS datasets, if those latter still hold data at the moment the snapshot is taken, only what has been flushed will be included in the snapshot. Worth mentioning that snapshot do not take diskspace aside of sone metadata at the exact time they are created, they size will grow as more and data blocks (i.e. files) are deleted or changed on their corresponding live ZFS dataset.||
+
* No direct equivalent in LVM.
+
* BTRFS read-only snapshots
+
|-
+
|clone||What is is... A writable physical clone of snapshot||
+
* LVM snapshots
+
* BTRFS snapshots
+
|-
+
|zvol||An emulated block device whose data is hold behind the scene in the zpool the zvol has been created in.||No known equivalent even in BTRFS
+
|-
+
|}
+
 
+
= Your first contact with ZFS  =
+
== Requirements ==
+
* ZFS userland tools installed (package ''sys-fs/zfs'')
+
* ZFS kernel modules built and installed (package ''sys-fs/zfs-kmod''), there is a known issue with kernel 3.13 series see [http://forums.funtoo.org/viewtopic.php?id=2442 this thread on Funtoo's forum]
+
* Disk size of 64 Mbytes as a bare minimum (128 Mbytes is the minimum size of a pool). Multiple disk will be simulated through the use of several raw images accessed via the Linux loopback devices.
+
* At least 512 MB of RAM
+
 
+
== Preparing ==
+
Once your have emerged ''sys-fs/zfs'' and ''sys-fs/zfs-kmod'' you have two options to start using ZFS at this point :
+
* Either you start ''/etc/init.d/zfs'' (will load all of the zfs kernel modules for you plus a couple of other things)
+
* Either you load the zfs kernel modules by hand (will load all of the zfs kernel modules for you)
+
 
+
So :
+
<console>###i## rc-service zfs start</console>
+
 
+
Or:
+
<console>
+
###i## modprobe zfs
+
###i## lsmod | grep zfs
+
zfs                  874072  0
+
zunicode              328120  1 zfs
+
zavl                  12997  1 zfs
+
zcommon                35739  1 zfs
+
znvpair                48570  2 zfs,zcommon
+
spl                    58011  5 zfs,zavl,zunicode,zcommon,znvpair
+
</console>
+
 
+
== Your first ZFS pool ==
+
To start with, four raw disks (2 GB each) are created:
+
 
+
<console>
+
###i## for i in 0 1 2 3; do dd if=/dev/zero of=/tmp/zfs-test-disk0${i}.img bs=2G count=1; done
+
0+1 records in
+
0+1 records out
+
2147479552 bytes (2.1 GB) copied, 40.3722 s, 53.2 MB/s
+
...
+
</console>
+
 
+
Then let's see what loopback devices are in use and which is the first free:
+
 
+
<console>
+
###i## losetup -a
+
###i## losetup -f
+
/dev/loop0
+
</console>
+
 
+
In the above example nothing is used and the first available loopback device is /dev/loop0. Now associate all of the disks with a loopback device (/tmp/zfs-test-disk00.img -> /dev/loop/0, /tmp/zfs-test-disk01.img -> /dev/loop/1 and so on):
+
 
+
<console>
+
###i## for i in 0 1 2 3; do losetup /dev/loop${i} /tmp/zfs-test-disk0${i}.img; done
+
###i## losetup -a
+
/dev/loop0: [000c]:781455 (/tmp/zfs-test-disk00.img)
+
/dev/loop1: [000c]:806903 (/tmp/zfs-test-disk01.img)
+
/dev/loop2: [000c]:807274 (/tmp/zfs-test-disk02.img)
+
/dev/loop3: [000c]:781298 (/tmp/zfs-test-disk03.img)
+
</console>
+
 
+
{{Fancynote|ZFS literature often names zpools "tank", this is not a requirement you can use whatever name of you choice (as we did here...) }}
+
 
+
Every story in ZFS takes its root with a the very first ZFS related command you will be in touch with: '''zpool'''. '''zpool''' as you might guessed manages all ZFS aspects in connection with the physical devices underlying your ZFS storage spaces and the very first task is to use this command to make what is called a ''pool'' (if you have used LVM before, volume groups can be seen as a counter part). Basically what you will do here is to tell ZFS to take a collection of physical storage stuff which can take several forms like a hard drive partition, a USB key partition or even a file and consider all of them as a single pool of storage (we will subdivide it in following paragraphs). No black magic here, ZFS will write some metadata on them behind the scene to be able to track which physical device belongs to what pool of storage.
+
 
+
<console>
+
###i## zpool create myfirstpool /dev/loop0 /dev/loop1 /dev/loop2 /dev/loop3
+
</console>
+
 
+
And.. nothing! Nada! The command silently returned but it ''did'' something, the next section will explain what.
+
 
+
== Your first ZFS dataset ==
+
<console>
+
###i## zpool list
+
NAME          SIZE  ALLOC  FREE    CAP  DEDUP  HEALTH  ALTROOT
+
myfirstpool  7.94G  130K  7.94G    0%  1.00x  ONLINE  -
+
</console>
+
 
+
What does this mean? Several things: First, your zpool is here and has a size of, roughly, 8 Go minus some space eaten by some metadata. Second is is actually usable because the column ''HEALTH'' says ''ONLINE''. Other columns are not meaningful for us for the moment just ignore them. If want more crusty details you can use the zpool command like this:
+
 
+
<console>
+
###i## zpool status
+
  pool: myfirstpool
+
state: ONLINE
+
  scan: none requested
+
config:
+
 
+
        NAME        STATE    READ WRITE CKSUM
+
        myfirstpool  ONLINE      0    0    0
+
          loop0    ONLINE      0    0    0
+
          loop1    ONLINE      0    0    0
+
          loop2    ONLINE      0    0    0
+
          loop3    ONLINE      0    0    0
+
</console>
+
Information is quite intuitive: your pool is seen as being usable (''state'' is similar to ''HEALTH'') and is composed of several devices each one listed as being in a ''healthy'' state ... at least for now because they will be salvaged for demonstration purpose in a later section. For your information the columns ''READ'',''WRITE'' and ''CKSUM'' list the number of operation failures on each of the devices respectfully:
+
* ''READ'' for reading failures. Having a non-zero value is not a good sign... the device is clunky and will soon fail.
+
* ''WRITE'' for writing failures. Having a non-zero value is not a good sign... the device is clunky and will soon fail.
+
* ''CKSUM'' for mismatch between the checksum of the data at the time is had been written and how it has been recomputed when read again (yes, ZFS uses checksums in a agressive manner). Having a non-zero value is not a good sign... corruption happened, ZFS will do its best to recover data by its own but this is definitely not a good sign of a healthy system.
+
 
+
Cool! So far so good you have a new 8 Gb usable brand new storage space on you system. Has been mounted somewhere?
+
 
+
<console>
+
###i## mount | grep myfirstpool
+
/myfirstpool on /myfirstpool type zfs (rw,xattr)
+
</console>
+
 
+
Remember the tables in the section above? A zpool in itself can '''never be mounted''', never ''ever''. It is just a container where ZFS datasets are created then mounted. So what happened here? Obscure black magic? No, of course not! Indeed a ZFS dataset named after the zpool's name should have been created automatically for us then mounted. Is is true? We will check this shortly. For the moment you will be introduced with the second command you will deal with when using ZFS : '''zfs'''. While the '''zpool''' command is used with anything related to zpools, the '''zfs''' is used to anything related to ZFS datasets '''(a ZFS dataset ''always'' resides in a zpool, ''always'' no exception on that).'''
+
 
+
{{Fancynote|'''zfs''' and '''zpool''' commands are the two only ones you will need to remember when dealing with ZFS.}}
+
 
+
So how can we check what ZFS datasets are currently known by the system? As you might already guessed like this:
+
 
+
<console>
+
###i## zfs list
+
NAME          USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool  114K  7.81G    30K  /myfirstpool
+
</console>
+
 
+
Tala! The mystery is busted! the ''zfs'' command tells us that not only a ZFS dataset named ''myfirstpool'' has been created but also it has been mounted in the system's VFS for us. If you check with the ''df'' command, you should also see something like this:
+
 
+
<console>
+
###i## df -h
+
Filesystem      Size  Used Avail Use% Mounted on
+
(...)
+
myfirstpool    7.9G    0  7.9G  0% /myfirstpool
+
</console>
+
 
+
The $100 question:''"what to do with this band new ZFS /myfirstpool dataset ?"''. Copy some files on it of course! We used a Linux kernel source but you can of course use whatever you want:
+
<console>
+
###i## cp -a /usr/src/linux-3.13.5-gentoo /myfirstpool
+
###i## ln -s /myfirstpool/linux-3.13.5-gentoo /myfirstpool/linux
+
###i## ls -lR /myfirstpool
+
/myfirstpool:
+
total 3
+
lrwxrwxrwx  1 root root 32 Mar  2 14:02 linux -> /myfirstpool/linux-3.13.5-gentoo
+
drwxr-xr-x 25 root root 50 Feb 27 20:35 linux-3.13.5-gentoo
+
 
+
/myfirstpool/linux-3.13.5-gentoo:
+
total 31689
+
-rw-r--r--  1 root root    18693 Jan 19 21:40 COPYING
+
-rw-r--r--  1 root root    95579 Jan 19 21:40 CREDITS
+
drwxr-xr-x 104 root root      250 Feb 26 07:39 Documentation
+
-rw-r--r--  1 root root    2536 Jan 19 21:40 Kbuild
+
-rw-r--r--  1 root root      277 Feb 26 07:39 Kconfig
+
-rw-r--r--  1 root root  268770 Jan 19 21:40 MAINTAINERS
+
(...)
+
</console>
+
 
+
A ZFS dataset  behaves like any other filesystem: you can create regular files, symbolic links, pipes, special devices nodes, etc. Nothing mystic here.
+
 
+
Now we have some data in the ZFS dataset let's see what various commands report:
+
<console>
+
###i## df -h
+
Filesystem      Size  Used Avail Use% Mounted on
+
(...)
+
myfirstpool    7.9G  850M  7.0G  11% /myfirstpool
+
</console>
+
<console>
+
###i## zfs list
+
NAME          USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool  850M  6.98G  850M  /myfirstpool
+
</console>
+
<console>
+
###i## zpool list
+
NAME          SIZE  ALLOC  FREE    CAP  DEDUP  HEALTH  ALTROOT
+
myfirstpool  7.94G  850M  7.11G    10%  1.00x  ONLINE  -
+
</console>
+
{{Fancynote|Notice the various sizes reported by '''zpool''' and '''zfs''' commands. In this case it is  the same however it can differ, this is true especially with zpools mounted in RAID-Z.}}
+
 
+
== Unmounting/remounting a ZFS dataset ==
+
 
+
 
+
{{Fancyimportant|'''Only ZFS datasets can be mounted''' inside your host's VFS, no exception on that! Zpools cannot be mounted, never, never, never... please pay attention to the  terminology and keep things clear by not messing up with terms. We will introduce ZFS snapshots and ZFS clones but those are ZFS datasets at the basis so they can also be mounted and unmounted.}}
+
 
+
 
+
If a ZFS dataset behaves just like any other filesystem, can we unmount it?
+
<console>
+
###i## umount /myfirstpool
+
###i## mount | grep myfirstpool
+
</console>
+
 
+
No more ''/myfirstpool'' the line of sight! So yes, it is possible to unmount a ZFS dataset just like you would do with any other filesystem. Is the ZFS dataset still present on the system even it is unmounted? Let's check:
+
 
+
<console>
+
###i## zfs list
+
NAME          USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool  850M  6.98G  850M  /myfirstpool
+
</console>
+
 
+
Hopefully and obviously it is else ZFS would not be very useful. Your next concern would certainly be: "How can we remount it then?" Simple! Like this:
+
<console>
+
###i## zfs mount myfirstpool
+
###i## mount | grep myfirstpool
+
myfirstpool on /myfirstpool type zfs (rw,xattr)
+
</console>
+
 
+
The ZFS dataset is back! :-)
+
 
+
== Your first contact with ZFS management by attributes or the end of /etc/fstab ==
+
At this point you might be curious about how the '''zfs''' command know what it has to mount and ''where'' is has to mount it. You might be familiar with the following syntax of the '''mount''' command that, behind the scenes, scans the file ''/etc/fstab'' and mount the specified entry:
+
<console>
+
###i## mount /boot
+
</console>
+
 
+
Does ''/etc/fstab'' contain something related to our ZFS dataset?
+
 
+
<console>
+
###i## cat /etc/fstab | grep myfirstpool
+
#
+
</console>
+
 
+
Doh!!!... Obvisouly nothing there. Another mystery? Sure not! The answer lies in a extremely powerful feature of ZFS: the attributes. Simply speaking: an attribute is named property of a ZFS dataset that holds a value. Attributes govern various aspects of how the datasets are managed like: ''"Is the data has to be compressed?"'', ''"Is the data has to be encrypted?"'', ''"Is the data has to be exposed to the rest of the world by NFS or SMB/Samba?"'' and of course... '''"Where the dataset has to be mounted?"''. The answer to that latter question can be tell by the following command:
+
 
+
<console>
+
###i## zfs get mountpoint myfirstpool
+
NAME        PROPERTY    VALUE        SOURCE
+
myfirstpool  mountpoint  /myfirstpool  default
+
</console>
+
 
+
Bingo! When you remounted the dataset just some paragraphs ago, ZFS automatically inspected the ''mountpoint'' attribute and saw this dataset has to be mounted in the directory ''/myfirstpool''.
+
 
+
= A step forward with ZFS datasets =
+
 
+
So far you were given a quick tour of what ZFS can do for you and  it is very important at this point to distinguish a ''zpool'' from a ''ZFS dataset'' and to call a dataset for what it is (a dataset) and not for what is is not (a zpool). It is a bit confusing and an editorial choice to have choosen a confusing name just to make you familiar with the one and the other.
+
 
+
== Creating datasets ==
+
 
+
Obviously it is possible to have more than one ZFS dataset within a single zpool. Quizz: what command would you use to subdivide a zpool in datasets? '''zfs''' or '''zpool'''? Stops reading for two seconds and try to figure out this little question. Frankly.
+
 
+
Answer is... '''zfs'''! Although you want to operate on the zpool to logically subdivide it in several datasets, you manage datasets at the end thus you will use the '''zfs''' command. It is not always easy at the beginning, do not be too worry you will soon get the habit when to use one or the other. Creating a dataset in a zpool is easy: just give to the '''zfs''' command the name of the pool you want to divide and the name of the dataset you want to create in it. So let's create three datasets named ''myfirstDS'', ''mysecondDS'' and ''mythirdDS'' in ''myfirstpool''(observe how we use the zpool and datasets' names) :
+
 
+
<console>
+
###i## zfs create myfirstpool/myfirstDS
+
###i## zfs create myfirstpool/mysecondDS
+
###i## zfs create myfirstpool/mythirdDS
+
</console>
+
 
+
What happened? Let's check :
+
 
+
<console>
+
###i## zfs list
+
NAME                    USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool              850M  6.98G  850M  /myfirstpool
+
myfirstpool/myfirstDS    30K  6.98G    30K  /myfirstpool/myfirstDS
+
myfirstpool/mysecondDS    30K  6.98G    30K  /myfirstpool/mysecondDS
+
myfirstpool/mythirdDS    30K  6.98G    30K  /myfirstpool/mythirdDS
+
</console>
+
 
+
Obviously we have there what we asked. Moreover if we inspect the contents of ''/myfirstpool'' we can notice three new directories having the same than just created:
+
 
+
<console>
+
###i## ls -l /myfirstpool
+
total 8
+
lrwxrwxrwx  1 root root 32 Mar  2 14:02 linux -> /myfirstpool/linux-3.13.5-gentoo
+
drwxr-xr-x 25 root root 50 Feb 27 20:35 linux-3.13.5-gentoo
+
drwxr-xr-x  2 root root  2 Mar  2 15:26 myfirstDS
+
drwxr-xr-x  2 root root  2 Mar  2 15:26 mysecondDS
+
drwxr-xr-x  2 root root  2 Mar  2 15:26 mythirdDS
+
</console>
+
No surprise here! As you might have guessed, those three new directories serves as mountpoints:
+
 
+
<console>
+
###i## mount | grep myfirstpool
+
myfirstpool on /myfirstpool type zfs (rw,xattr)
+
myfirstpool/myfirstDS on /myfirstpool/myfirstDS type zfs (rw,xattr)
+
myfirstpool/mysecondDS on /myfirstpool/mysecondDS type zfs (rw,xattr)
+
myfirstpool/mythirdDS on /myfirstpool/mythirdDS type zfs (rw,xattr)
+
</console>
+
 
+
As we did before, we can copy some files in the newly created datasets just like they were regular directories:
+
 
+
<console>
+
###i## cp -a /usr/portage /myfirstpool/mythirdDS
+
###i## ls -l /myfirstpool/mythirdDS/*
+
total 697
+
drwxr-xr-x  48 root root  49 Aug 18  2013 app-accessibility
+
drwxr-xr-x  238 root root  239 Jan 10 06:22 app-admin
+
drwxr-xr-x    4 root root    5 Dec 28 08:54 app-antivirus
+
drwxr-xr-x  100 root root  101 Feb 26 07:19 app-arch
+
drwxr-xr-x  42 root root  43 Nov 26 21:24 app-backup
+
drwxr-xr-x  34 root root  35 Aug 18  2013 app-benchmarks
+
drwxr-xr-x  66 root root  67 Oct 16 06:39 app-cdr(...)
+
</console>
+
 
+
Nothing really too exciting here, we have file in ''mythirdDS''. A bit more interesting output:
+
 
+
<console>
+
###i## zfs list
+
NAME                    USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool            1.81G  6.00G  850M  /myfirstpool
+
myfirstpool/myfirstDS    30K  6.00G    30K  /myfirstpool/myfirstDS
+
myfirstpool/mysecondDS    30K  6.00G    30K  /myfirstpool/mysecondDS
+
myfirstpool/mythirdDS  1002M  6.00G  1002M  /myfirstpool/mythirdDS
+
</console>
+
<console>
+
###i## df -h
+
Filesystem              Size  Used Avail Use% Mounted on
+
(...)
+
myfirstpool            6.9G  850M  6.1G  13% /myfirstpool
+
myfirstpool/myfirstDS  6.1G    0  6.1G  0% /myfirstpool/myfirstDS
+
myfirstpool/mysecondDS  6.1G    0  6.1G  0% /myfirstpool/mysecondDS
+
myfirstpool/mythirdDS  7.0G 1002M  6.1G  15% /myfirstpool/mythirdDS
+
</console>
+
 
+
Noticed the size given for the 'AVAIL' column? At the very beginning of this tutorial we had slightly less than  8 Gb of available space, it now has a value of roughly 6 Gb. The datasets are just a subdivision of the zpool, they '''compete with each others''' for using the available storage within the zpool, no miracle here. To what limit? The pool itself as we never imposed a ''quota'' on datasets. Hopefully '''df''' and '''zfs list''' gives a coherent result.
+
 
+
== Second contact with attributes: quota management ==
+
 
+
Remember how painful is the quota management under Linux? Now you can say goodbye to '''setquota''', '''edquota''' and other '''quotacheck''' commands, ZFS handle this in the snap of fingers! Guess with what? An ZFS dataset attribute of course! ;-) Just to make you drool here is how a 2Gb limit can be set on ''myfirstpool/mythirdDS'' :
+
 
+
<console>
+
###i## zfs set quota=2G myfirstpool/mythirdDS
+
</console>
+
 
+
''Et voila!'' The '''zfs''' command is bit silent however if we check we can see that ''myfirstpool/mythirdDS'' is now capped to 2 Gb (forget about 'REFER' for the moment): around 1 Gb of data has been copied in this dataset thus leaving a big 1 Gb of available space.
+
 
+
<console>
+
###i## zfs list
+
NAME                    USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool            1.81G  6.00G  850M  /myfirstpool
+
myfirstpool/myfirstDS    30K  6.00G    30K  /myfirstpool/myfirstDS
+
myfirstpool/mysecondDS    30K  6.00G    30K  /myfirstpool/mysecondDS
+
myfirstpool/mythirdDS  1002M  1.02G  1002M  /myfirstpool/mythirdDS
+
</console>
+
 
+
Using the '''df''' command:
+
 
+
<console>
+
###i## df -h                               
+
Filesystem              Size  Used Avail Use% Mounted on
+
(...)
+
myfirstpool            6.9G  850M  6.1G  13% /myfirstpool
+
myfirstpool/myfirstDS  6.1G    0  6.1G  0% /myfirstpool/myfirstDS
+
myfirstpool/mysecondDS  6.1G    0  6.1G  0% /myfirstpool/mysecondDS
+
myfirstpool/mythirdDS  2.0G 1002M  1.1G  49% /myfirstpool/mythirdDS
+
</console>
+
 
+
Of course you can use this technique for the home directories of your users /home this also having the a advantage of being much less forgiving than a soft/hard user quota: when the limit is reached, it is reached period and no more data can be written in the dataset. The user must do some cleanup and cannot procastinate anymore :-)
+
 
+
To remove the quota:
+
 
+
<console>
+
###i## zfs set quota=none myfirstpool/mythirdDS
+
</console>
+
 
+
''none'' is simply the original value for the ''quota'' attribute (we did not demonstrate it, you can check by doing a '''zfs get quota  myfirstpool/mysecondDS''' for example).
+
 
+
== Destroying datasets ==
+
{{Fancyimportant|There is no way to resurrect a destroyed ZFS dataset and the data it contained! Once you destroy a dataset the corresponding metadata is cleared and gone forever so be careful when using ''zfs destroy'' notably with the ''-r'' option ... }}
+
 
+
 
+
We have three datasets, but the third is pretty useless and contains a lot of garbage. Is it possible to remove it with a simple '''rm -rf'''? Let's try:
+
 
+
<console>
+
###i## rm -rf /myfirstpool/mythirdDS
+
rm: cannot remove `/myfirstpool/mythirdDS': Device or resource busy
+
</console>
+
 
+
This is perfectly normal, remember that datasets are indeed something '''mounted''' in your VFS. ZFS might be ZFS and do alot for you, it cannot enforce the nature of a mounted filesystem under Linux/Unix. The "ZFS way" to remove a dataset is to use the ''zfs'' command like this at the reserve no process owns open files on it (once again, ZFS can do miracles for you but not that kind of miracles as it has to unmount the dataset before deleting it):
+
 
+
<console>
+
###i## zfs destroy myfirstpool/mythirdDS
+
###i## zfs list
+
NAME                    USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool              444M  7.38G  444M  /myfirstpool
+
myfirstpool/myfirstDS    21K  7.38G    21K  /myfirstpool/myfirstDS
+
myfirstpool/mysecondDS    21K  7.38G    21K  /myfirstpool/mysecondDS
+
</console>
+
 
+
''Et voila''! No more ''myfirstpool/mythirdDS'' dataset. :-)
+
 
+
A bit more subtle case would be to try to destroy a ZFS dataset whenever another ZFS dataset is nested in it. Before doing that nasty experiment  ''myfirstpool/mythirdDS'' must be created again this time with another nested dataset (''myfirstpool/mythirdDS/nestedSD1''):
+
 
+
<console>
+
###i## zfs create myfirstpool/mythirdDS
+
###i## zfs create myfirstpool/mythirdDS/nestedSD1
+
###i## zfs list
+
NAME                              USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool                      851M  6.98G  850M  /myfirstpool
+
myfirstpool/myfirstDS              30K  6.98G    30K  /myfirstpool/myfirstDS
+
myfirstpool/mysecondDS            30K  6.98G    30K  /myfirstpool/mysecondDS
+
myfirstpool/mythirdDS            124K  6.98G    34K  /myfirstpool/mythirdDS
+
myfirstpool/mythirdDS/nestedDS1    30K  6.98G    30K  /myfirstpool/mythirdDS/nestedDS1
+
</console>
+
 
+
Now let's try to destroy ''myfirstpool/mythirdDS'' again:
+
 
+
<console>
+
###i## zfs destroy myfirstpool/mythirdDS
+
cannot destroy 'myfirstpool/mythirdDS': filesystem has children
+
use '-r' to destroy the following datasets:
+
myfirstpool/mythirdDS/nestedDS1
+
</console>
+
 
+
The zfs command detected the situation  and refused to proceed on the deletion without your consent to make a recursive destruction (-r parameter). Before going any step further let's create some more nested datasets plus a couple of directories inside ''myfirstpool/mythirdDS'':
+
 
+
<console>
+
###i## zfs create myfirstpool/mythirdDS/nestedDS1
+
###i## zfs create myfirstpool/mythirdDS/nestedDS2
+
###i## zfs create myfirstpool/mythirdDS/nestedDS3
+
###i## zfs create myfirstpool/mythirdDS/nestedDS3/nestednestedDS
+
###i## mkdir /myfirstpool/mythirdDS/dir1
+
###i## mkdir /myfirstpool/mythirdDS/dir2
+
###i## mkdir /myfirstpool/mythirdDS/dir3
+
</console>
+
<console>
+
###i## zfs list
+
NAME                                            USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool                                      851M  6.98G  850M  /myfirstpool
+
myfirstpool/myfirstDS                            30K  6.98G    30K  /myfirstpool/myfirstDS
+
myfirstpool/mysecondDS                            30K  6.98G    30K  /myfirstpool/mysecondDS
+
myfirstpool/mythirdDS                            157K  6.98G    37K  /myfirstpool/mythirdDS
+
myfirstpool/mythirdDS/nestedDS1                  30K  6.98G    30K  /myfirstpool/mythirdDS/nestedDS1
+
myfirstpool/mythirdDS/nestedDS2                  30K  6.98G    30K  /myfirstpool/mythirdDS/nestedDS2
+
myfirstpool/mythirdDS/nestedDS3                  60K  6.98G    30K  /myfirstpool/mythirdDS/nestedDS3
+
myfirstpool/mythirdDS/nestedDS3/nestednestedDS    30K  6.98G    30K  /myfirstpool/mythirdDS/nestedDS3/nestednestedDS
+
</console>
+
 
+
Now what happens if ''myfirstpool/mythirdDS'' is destroyed again with '-r'?
+
 
+
<console>
+
###i## zfs destroy -r myfirstpool/mythirdDS
+
###i## zfs list                           
+
NAME                    USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool              851M  6.98G  850M  /myfirstpool
+
myfirstpool/myfirstDS    30K  6.98G    30K  /myfirstpool/myfirstDS
+
myfirstpool/mysecondDS    30K  6.98G    30K  /myfirstpool/mysecondDS
+
</console>
+
 
+
''myfirstpool/mythirdDS'' and everything it contained is now gone!
+
 
+
== Snapshotting and rolling back datasets ==
+
 
+
This is, by far, one of the coolest features of ZFS. You can:
+
# take a photo of a dataset (this photo is called a ''snapshot'')
+
# do ''whatever'' you want with the data contained in the dataset
+
# restore (roll back) the dataset in  in the '''exact''' same state it was before you did your changes just as if nothing had ever happened in the middle.
+
 
+
=== Single snapshot ===
+
 
+
{{Fancyimportant|'''Only ZFS datasets''' can be snapshotted and rolled back, not the zpool.}}
+
 
+
 
+
To start with, let's copy some files in ''mysecondDS'':
+
 
+
<console>
+
###i## cp -a /usr/portage /myfirstpool/mysecondDS
+
###i## ls /myfirstpool/mysecondDS/portage
+
total 672
+
drwxr-xr-x  48 root root  49 Aug 18  2013 app-accessibility
+
drwxr-xr-x  238 root root  239 Jan 10 06:22 app-admin
+
drwxr-xr-x    4 root root    5 Dec 28 08:54 app-antivirus
+
drwxr-xr-x  100 root root  101 Feb 26 07:19 app-arch
+
drwxr-xr-x  42 root root  43 Nov 26 21:24 app-backup
+
drwxr-xr-x  34 root root  35 Aug 18  2013 app-benchmarks
+
(...)
+
drwxr-xr-x  62 root root  63 Feb 20 06:47 x11-wm
+
drwxr-xr-x  16 root root  17 Aug 18  2013 xfce-base
+
drwxr-xr-x  64 root root  65 Dec 14 19:09 xfce-extra
+
</console>
+
 
+
Now, let's take a snapshot of ''mysecondDS''. What command would be used? '''zpool''' or '''zfs'''? In that case it is '''zfs''' because we manipulate a ZFS dataset (this time you problably got it right!):
+
 
+
<console>
+
###i## zfs snapshot myfirstpool/mysecondDS@Charlie
+
</console>
+
 
+
{{fancynote|The syntax is always ''pool/dataset@snapshot'', the snapshot's name is left at your discretion however '''you must use an arobase  sign (@)''' to separate the snapshot's name from the rest of the path.}}
+
 
+
Let's check what ''/myfirstpool/mysecondDS'' contains after taking the snapshot:
+
<console>
+
###i## ls -la /myfirstpool/mysecondDS   
+
total 9
+
drwxr-xr-x  3 root root  3 Mar  2 18:22 .
+
drwxr-xr-x  5 root root  6 Mar  2 17:58 ..
+
drwx------ 170 root root 171 Mar  2 18:36 portage
+
</console>
+
 
+
Nothing really new the ''portage'' directory is here nothing more ''a priori''. If you have used BTRFS before reading this tutorial you probably expected to see a ''@Charlie'' lying in ''/myfirstpool/mysecondDS''? So where the check is ''Charlie''? In ZFS a dataset snapshot is not visible from within the VFS tree (if you are not convinced you can search for it with the '''find''' command but it will never find it). Let's check with the '''zfs''' command:
+
 
+
<console>
+
###i## zfs list
+
###i## zfs list -t all   
+
NAME                            USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool                    1.81G  6.00G  850M  /myfirstpool
+
myfirstpool/myfirstDS            30K  6.00G    30K  /myfirstpool/myfirstDS
+
myfirstpool/mysecondDS          1001M  6.00G  1001M  /myfirstpool/mysecondDS
+
</console>
+
 
+
Wow... No sign of the snapshot. What you mus know is that indeed '''zfs list''' shows only datasets by default and omits snapshots. If the command is invoked with the parameter ''-t'' set to ''all'' it will list everything:
+
 
+
<console>
+
###i## zfs list
+
###i## zfs list -t all   
+
NAME                            USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool                    1.81G  6.00G  850M  /myfirstpool
+
myfirstpool/myfirstDS            30K  6.00G    30K  /myfirstpool/myfirstDS
+
myfirstpool/mysecondDS          1001M  6.00G  1001M  /myfirstpool/mysecondDS
+
myfirstpool/mysecondDS@Charlie      0      -  1001M  -
+
</console>
+
 
+
So yes, ''@Charlie'' is here! Also notice here the power of copy-on-write filesystems: ''@Charlie'' takes only a couple of kilobytes (some ZFS metadata) just like any ZFS snapshot at the time they are taken. The reason snapshots occupy very little space in the datasets is because data and metadata blocks are the same and no physical copy of them are made. At the time goes on and more and more changes happens in the original dataset (''myfirstpool/mysecondDS'' here), ZFS will allocate new data and metadata blocks to accommodate the changes but will leave the blocks used by the snapshot untouched and the snapshot will tend to eat more and more pool space. It seems odd at first glance because a snapshot is a frozen in time copy of a ZFS dataset but this the way ZFS manage them. So caveat emptor: remove any unused snapshot to not full your zpool...
+
 
+
Now we have found Charlie, let's do some changes in the ''mysecondDS'':
+
 
+
<console>
+
###i## rm -rf /myfirstpool/mysecondDS/portage/[a-h]*
+
###i## echo "Hello, world" >  /myfirstpool/mysecondDS/hello.txt
+
###i## cp /lib/firmware/radeon/* /myfirstpool/mysecondDS
+
###i## ls -l  /myfirstpool/mysecondDS
+
/myfirstpool/mysecondDS:
+
total 3043
+
-rw-r--r--  1 root root  8704 Mar  2 19:29 ARUBA_me.bin
+
-rw-r--r--  1 root root  8704 Mar  2 19:29 ARUBA_pfp.bin
+
-rw-r--r--  1 root root  6144 Mar  2 19:29 ARUBA_rlc.bin
+
-rw-r--r--  1 root root  24096 Mar  2 19:29 BARTS_mc.bin
+
-rw-r--r--  1 root root  5504 Mar  2 19:29 BARTS_me.bin
+
(...)
+
-rw-r--r--  1 root root  60388 Mar  2 19:29 VERDE_smc.bin
+
-rw-r--r--  1 root root    13 Mar  2 19:28 hello.txt
+
drwx------ 94 root root    95 Mar  2 19:28 portage
+
 
+
/myfirstpool/mysecondDS/portage:
+
total 324
+
drwxr-xr-x  16 root root  17 Oct 26 07:30 java-virtuals
+
drwxr-xr-x 303 root root  304 Jan 21 06:53 kde-base
+
drwxr-xr-x 117 root root  118 Feb 21 06:24 kde-misc
+
drwxr-xr-x  2 root root  756 Feb 23 08:44 licenses
+
drwxr-xr-x  20 root root  21 Jan  7 06:56 lxde-base
+
(...)
+
</console>
+
 
+
Now let's check again what the '''zpool''' command gives:
+
 
+
<console>
+
###i## zfs list -t all                     
+
NAME                            USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool                    1.82G  6.00G  850M  /myfirstpool
+
myfirstpool/myfirstDS            30K  6.00G    30K  /myfirstpool/myfirstDS
+
myfirstpool/mysecondDS          1005M  6.00G  903M  /myfirstpool/mysecondDS
+
myfirstpool/mysecondDS@Charlie  102M      -  1001M  -
+
</console>
+
 
+
Noticed the size's increase of ''myfirstpool/mysecondDS@Charlie''? This is mainly due to new files copied in the snasphot: ZFS had to retained the original blocks of data. Now time to roll  this ZFS dataset back to its original state (if some processes would have open files in the dataset to be rolled back, you should terminate them first) :
+
 
+
<console>
+
###i## zfs rollback myfirstpool/mysecondDS@Charlie
+
###i## ls -l /myfirstpool/mysecondDS
+
total 6
+
drwxr-xr-x 164 root root 169 Aug 18 18:25 portage
+
</console>
+
 
+
Again, ZFS handled everything for you and you now have the contents of ''mysecondDS'' exactly as it was at the time the snapshot ''Charlie'' was taken. Not more complicated than that. Not illustrated here but if you look at the output given by '''zfs list -t all''' at this point you will notice that the ''Charlie'' snapshot only eat very little space. This is normal: the modified blocks have been dropped so ''myfirstpool/mysecondDS'' and its ''myfirstpool/mysecondDS@Charlie'' snapshot are the same module some metadata (hence the few kilobytes of space taken).
+
 
+
=== the .zfs pseudo-directory or the secret passage to your snapshots ===
+
 
+
Any directory where  a ZFS dataset is mounted (having snapshots or not) secretly contains a pseudo-directory named '''.zfs''' (dot-ZFS) and you will not see it even with the option ''-a'' given to a '''ls''' command unless you specify it. It is a contradiction to Unix and Unix-like systems' philosophy to not hide anything to the system administrator. It is not a bug of ZFS On Linux implementation and the Solaris implementation of ZFS exposes the exact behavior. So what is inside this little magic box?
+
 
+
<console>
+
###i## cd /myfirstpool/mysecondDS
+
###i## ls -la | grep .zfs       
+
###i## ls -lad .zfs             
+
dr-xr-xr-x 1 root root 0 Mar  2 15:26 .zfs
+
</console>
+
<console>
+
###i## cd .zfs
+
###i## pwd
+
/myfirstpool/mysecondDS/.zfs
+
###i## ls -la
+
total 4
+
dr-xr-xr-x 1 root root  0 Mar  2 15:26 .
+
drwxr-xr-x 3 root root 145 Mar  2 19:29 ..
+
dr-xr-xr-x 2 root root  2 Mar  2 19:47 shares
+
dr-xr-xr-x 2 root root  2 Mar  2 18:46 snapshot
+
</console>
+
 
+
We will focus on the ''snapshot'' directory and since we did not dropped the ''Charlie'' snapshot (yet) let's see what lies there:
+
 
+
<console>
+
###i## cd snapshot
+
###i## ls -l
+
total 0
+
dr-xr-xr-x 1 root root 0 Mar  2 20:16 Charlie
+
</console>
+
 
+
Yes we found Charlie here (also!), the snapshot is seen as regular directory but pay attention to its permissions:
+
* owning user (root) has read+execute
+
* owning group (root) has read+execute
+
* rest of the world has read+execute
+
 
+
Did you notice? Not a single ''write'' permission on this directory, the only action any user can do is to enter in the directory and list its contents. This not a bug but the nature of ZFS snapshots: they are read-only stuff at the basis. Next question is naturally: can we change something in it? For that we have to enter inside the ''Charlie'' directory:
+
 
+
<console>
+
###i## cd Charlie
+
###i## ls -la
+
total 7
+
drwxr-xr-x  3 root root  3 Mar  2 18:22 .
+
dr-xr-xr-x  3 root root  3 Mar  2 18:46 ..
+
drwx------ 170 root root 171 Mar  2 18:36 portage
+
</console>
+
 
+
No surprise here: at the time we took the snapshot, ''myfirstpool/mysecondDS'' held a copy of the portage tree stored in a directory named ''portage''. At first glance this one ''seems'' to be writable for the root user let's try to create a file in it:
+
 
+
<console>
+
###i## cd portage
+
###i## touch test
+
touch: cannot touch ‘test’: Read-only file system
+
</console>
+
 
+
Thing are a bit tricky here: indeed nothing has been mounted (check with the '''mount''' command!), we are walking though a pseudo-directory exposed by ZFS that holds the ''Charlie'' snapshot. ''Pseudo-directory'' because in fact ''.zfs'' had no physical existence even in the ZFS metadata as they exists in the zpool. It is just a convenient way provided by the ZFS kernel modules to walk inside the various snapshots' content. You can see but you cannot touch :-)
+
 
+
=== Backtracking changes between a dataset and its snapshot ===
+
Is it possible to know what is the difference between a a live dataset and its snapshot? Answer to this question is '''yes''' and the '''zfs''' command will help us in this task. Now we rolled back the ''myfirstpool/mysecondDS'' ZFS dataset back to its original state we have to botch it again:
+
<console>
+
###i## cp -a /lib/firmware/radeon/C* /myfirstpool/mysecondDS
+
</console>
+
 
+
Now inspect the difference between the live ZFS dataset ''myfirstpool/mysecondDS'' and its snasphot Charlie, this is done via '''zfs diff''' and by giving only the snapshot's name (you can inspect the difference between snasphot with that command with a slightly change in parameters):
+
 
+
<console>
+
###i## # zfs diff myfirstpool/mysecondDS@Charlie
+
M      /myfirstpool/mysecondDS/
+
+      /myfirstpool/mysecondDS/CAICOS_mc.bin
+
+      /myfirstpool/mysecondDS/CAICOS_me.bin
+
+      /myfirstpool/mysecondDS/CAICOS_pfp.bin
+
+      /myfirstpool/mysecondDS/CAICOS_smc.bin
+
+      /myfirstpool/mysecondDS/CAYMAN_mc.bin
+
+      /myfirstpool/mysecondDS/CAYMAN_me.bin
+
(...)
+
</console>
+
 
+
So do we have here? Two things: First it shows we have changed something in ''/myfirstpool/mysecondDS'' (notice the 'M' for Modified), second it shows the addition of several files (CAICOS_mc.bin, CAICOS_me.bin, CAICOS_pfp.bin...) by putting a plus sign ('+') on their left.
+
 
+
If we botch a bit more ''myfirstpool/mysecondDS'' by removing the file ''/myfirstpool/mysecondDS/portage/sys-libs/glibc/Manifest'' :
+
 
+
<console>
+
###i## rm /myfirstpool/mysecondDS/portage/sys-libs/glibc/Manifest
+
###i## zfs diff myfirstpool/mysecondDS@Charlie
+
M      /myfirstpool/mysecondDS/
+
M      /myfirstpool/mysecondDS/portage/sys-libs/glibc
+
-      /myfirstpool/mysecondDS/portage/sys-libs/glibc/Manifest
+
+      /myfirstpool/mysecondDS/CAICOS_mc.bin
+
+      /myfirstpool/mysecondDS/CAICOS_me.bin
+
+      /myfirstpool/mysecondDS/CAICOS_pfp.bin
+
+      /myfirstpool/mysecondDS/CAICOS_smc.bin
+
+      /myfirstpool/mysecondDS/CAYMAN_mc.bin
+
+      /myfirstpool/mysecondDS/CAYMAN_me.bin
+
(...)
+
</console>
+
 
+
Obviously deleted content is marked by a minus sign ('-').
+
 
+
Now a real butchery:
+
<console>
+
###i## rm -rf /myfirstpool/mysecondDS/portage/sys-devel/gcc
+
###i## zfs diff myfirstpool/mysecondDS@Charlie
+
# zfs diff myfirstpool/mysecondDS@Charlie           
+
M      /myfirstpool/mysecondDS/
+
M      /myfirstpool/mysecondDS/portage/sys-devel
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files/awk
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files/awk/fixlafiles.awk
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files/awk/fixlafiles.awk-no_gcc_la
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files/c89
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files/c99
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files/gcc-4.6.4-fix-libgcc-s-path-with-vsrl.patch
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files/gcc-spec-env.patch
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files/gcc-spec-env-r1.patch
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files/gcc-4.8.2-fix-cache-detection.patch
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files/fix_libtool_files.sh
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files/gcc-configure-texinfo.patch
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files/gcc-4.8.1-bogus-error-with-int.patch
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/gcc-4.3.3-r2.ebuild
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/metadata.xml
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/gcc-4.6.4-r2.ebuild
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/gcc-4.6.4.ebuild
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/gcc-4.8.1-r1.ebuild
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/gcc-4.8.1-r2.ebuild
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/gcc-4.6.2-r1.ebuild
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/gcc-4.8.1-r3.ebuild
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/gcc-4.8.2.ebuild
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/gcc-4.8.1-r4.ebuild
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/Manifest
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/gcc-4.7.3-r1.ebuild
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/gcc-4.8.2-r1.ebuild
+
M      /myfirstpool/mysecondDS/portage/sys-libs/glibc
+
-      /myfirstpool/mysecondDS/portage/sys-libs/glibc/Manifest
+
+      /myfirstpool/mysecondDS/CAICOS_mc.bin
+
+      /myfirstpool/mysecondDS/CAICOS_me.bin
+
+      /myfirstpool/mysecondDS/CAICOS_pfp.bin
+
+      /myfirstpool/mysecondDS/CAICOS_smc.bin
+
+      /myfirstpool/mysecondDS/CAYMAN_mc.bin
+
+      /myfirstpool/mysecondDS/CAYMAN_me.bin
+
(...)
+
</console>
+
 
+
No need to explain that digital mayhem! What happens if, in addition, we change the contents of the file ''/myfirstpool/mysecondDS/portage/sys-devel/autoconf/Manifest''?
+
<console>
+
###i## zfs diff myfirstpool/mysecondDS@Charlie
+
M      /myfirstpool/mysecondDS/
+
M      /myfirstpool/mysecondDS/portage/sys-devel
+
M      /myfirstpool/mysecondDS/portage/sys-devel/autoconf/Manifest
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files/awk
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files/awk/fixlafiles.awk
+
-      /myfirstpool/mysecondDS/portage/sys-devel/gcc/files/awk/fixlafiles.awk-no_gcc_la
+
(...)
+
</console>
+
ZFS shows that the file ''/myfirstpool/mysecondDS/portage/sys-devel/autoconf/Manifest'' has changed. So ZFS can help to track files deletion, creation and modifications. What it does not show is the difference of a file's content between as it exists in a live dataset and this dataset's snapshot. Not a big issue! You can explore a snapshot's content via the ''.zfs'' pseudo-directory and use a command like '''/usr/bin/diff''' to examine the difference with the file as it exists on the corresponding live dataset.
+
 
+
<console>
+
###i## diff -u /myfirstpool/mysecondDS/.zfs/snapshot/Charlie/portage/sys-devel/autoconf/Manifest /myfirstpool/mysecondDS/portage/sys-devel/autoconf/Manifest
+
--- /myfirstpool/mysecondDS/.zfs/snapshot/Charlie/portage/sys-devel/autoconf/Manifest  2013-08-18 08:52:01.742411902 -0400
+
+++ /myfirstpool/mysecondDS/portage/sys-devel/autoconf/Manifest 2014-03-02 21:36:50.582258990 -0500
+
@@ -4,7 +4,4 @@
+
DIST autoconf-2.62.tar.gz 1518427 SHA256 83aa747e6443def0ebd1882509c53f5a2133f50...
+
DIST autoconf-2.63.tar.gz 1562665 SHA256 b05a6cee81657dd2db86194a6232b895b8b2606a...
+
DIST autoconf-2.64.tar.bz2 1313833 SHA256 872f4cadf12e7e7c8a2414e047fdff26b517c7...
+
-DIST autoconf-2.65.tar.bz2 1332522 SHA256 db11944057f3faf229ff5d6ce3fcd819f56545...
+
-DIST autoconf-2.67.tar.bz2 1369605 SHA256 00ded92074999d26a7137d15bd1d51b8a8ae23...
+
-DIST autoconf-2.68.tar.bz2 1381988 SHA256 c491fb273fd6d4ca925e26ceed3d177920233c...
+
DIST autoconf-2.69.tar.xz 1214744 SHA256 64ebcec9f8ac5b2487125a86a7760d2591ac9e1d3...
+
(...)
+
</console>
+
 
+
=== Dropping a snapshot ===
+
A snapshot is no more than a dataset frozen in time and thus can be destroyed in the exact same way seen in the paragraphs before. Now we do not need the ''Charlie'' snapshot we can remove it. Simple:
+
<console>
+
###i## zfs destroy myfirstpool/mysecondDS@Charlie
+
###i## zfs list -t all
+
NAME                    USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool            1.71G  6.10G  850M  /myfirstpool
+
myfirstpool/myfirstDS    30K  6.10G    30K  /myfirstpool/myfirstDS
+
myfirstpool/mysecondDS  903M  6.10G  903M  /myfirstpool/mysecondDS
+
</console>
+
 
+
And Charlie is gone forever ;-)
+
 
+
=== The time travelling machine part 1: examining differences between snapshots ===
+
So far we only used a single snapshot just to keep things simple. However a dataset can hold several snapshots and you can do everything seen so far with them like rolling back, destroying them or examining the difference not only between a snapshot and its corresponding live dataset but also between two snapshots. For this part we will consider the ''myfirstpool/myfirstDS'' dataset which should be empty at this point.
+
 
+
<console>
+
# ls -la /myfirstpool/myfirstDS
+
total 3
+
drwxr-xr-x 2 root root 2 Mar 2 21:14 .
+
drwxr-xr-x 5 root root 6 Mar 2 17:58 ..
+
</console>
+
 
+
Now let's generate some contents, take a snapshot (snapshot-1), add more content, take a snapshot again (snapshot-2), do some modifications again and take a third snapshot (snapshot-3):
+
 
+
<console>
+
###i## echo "Hello, world" > /myfirstpool/myfirstDS/hello.txt
+
###i## cp -R /lib/firmware/radeon /myfirstpool/myfirstDS
+
###i## ls -l /myfirstpool/myfirstDS
+
total 5
+
-rw-r--r-- 1 root root 13 Mar 3 06:41 hello.txt
+
drwxr-xr-x 2 root root 143 Mar 3 06:42 radeon
+
###i## zfs snapshot myfirstpool/myfirstDS@snapshot-1
+
</console>
+
<console>
+
###i## echo "Goodbye, world" > /myfirstpool/myfirstDS/goodbye.txt
+
###i## echo "Are you there?" >> /myfirstpool/myfirstDS/hello.txt
+
###i## cp /proc/config.gz /myfirstpool/myfirstDS
+
###i## rm /myfirstpool/myfirstDS/radeon/CAYMAN_me.bin
+
###i## zfs snapshot myfirstpool/myfirstDS@snapshot-2
+
</console>
+
<console>
+
###i## echo "Still there?" >> /myfirstpool/myfirstDS/goodbye.txt
+
###i## mv /myfirstpool/myfirstDS/hello.txt /myfirstpool/myfirstDS/hello_new.txt
+
###i## cat /proc/version > /myfirstpool/myfirstDS/version.txt
+
###i## zfs snapshot myfirstpool/myfirstDS@snapshot-3
+
</console>
+
<console>
+
###i## zfs list -t all
+
NAME                              USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool                      1.81G  6.00G  850M  /myfirstpool
+
myfirstpool/myfirstDS            3.04M  6.00G  2.97M  /myfirstpool/myfirstDS
+
myfirstpool/myfirstDS@snapshot-1    47K      -  2.96M  -
+
myfirstpool/myfirstDS@snapshot-2    30K      -  2.97M  -
+
myfirstpool/myfirstDS@snapshot-3      0      -  2.97M  -
+
myfirstpool/mysecondDS            1003M  6.00G  1003M  /myfirstpool/mysecondDS
+
</console>
+
 
+
You saw to how use '''zfs diff''' to compare the difference between a snapshot and its corresponding "live" dataset in the above paragraphs. Doing the same exercise with two snapshots is not that much different as you just have to explicitly tell the command what datasets are to be compared against and the command will oputput the result in the exact same manner.So what are the differences between snapshots ''myfirstpool/myfirstDS@snapshot-1'' and ''myfirstpool/myfirstDS@snapshot-2''? Let's make the '''zfs''' command work for us:
+
 
+
<console>
+
###i## zfs diff myfirstpool/myfirstDS@snapshot-1 myfirstpool/myfirstDS@snapshot-2
+
M      /myfirstpool/myfirstDS/
+
M      /myfirstpool/myfirstDS/hello.txt
+
M      /myfirstpool/myfirstDS/radeon
+
-      /myfirstpool/myfirstDS/radeon/CAYMAN_me.bin
+
+      /myfirstpool/myfirstDS/goodbye.txt
+
+      /myfirstpool/myfirstDS/config.gz
+
</console>
+
 
+
Before digging farther, let's think about what we did between the time we created the first snapshot and the second snapshot:
+
* We modified the file /myfirstpool/myfirstDS/hello.txt hence the 'M' shown on left of the second line (thus we changed something under ''/myfirstpool/myfirstDS'' hence a 'M' is also shown on the left of the first line)
+
* We deleted the file ''/myfirstpool/myfirstDS/radeon/CAYMAN_me.bin'' hence the minus sign ('-') shown on the left of the fourth line (and the 'M' shown on left of the third line)
+
* We added two files which were ''/myfirstpool/myfirstDS/goodbye.txt'' and ''/myfirstpool/myfirstDS/config.gz'' hence the plus sign ('+') shown on the left of the fifth and sixth lines (also this is a change happening in ''/myfirstpool/myfirstDS'' hence another reason to show a 'M' on the left of the first line)
+
 
+
Now same exercise this time with snapshots ''myfirstpool/myfirstDS@snapshot-2'' and ''myfirstpool/myfirstDS@snapshot-3'':
+
 
+
<console>
+
###i## zfs diff myfirstpool/myfirstDS@snapshot-2 myfirstpool/myfirstDS@snapshot-3
+
M      /myfirstpool/myfirstDS/
+
R      /myfirstpool/myfirstDS/hello.txt -> /myfirstpool/myfirstDS/hello_new.txt
+
M      /myfirstpool/myfirstDS/goodbye.txt
+
+      /myfirstpool/myfirstDS/version.txt
+
</console>
+
 
+
Try to interpret what you see except for the second line where a "R" (standing for "Rename") is shown. ZFS is smart enough to also show both the old the new names!
+
 
+
Why not push the limit and try a few fancy things. First things first: what happens if we tell to compare two snapshots but in a reverse order?
+
 
+
<console>
+
###i## zfs diff myfirstpool/myfirstDS@snapshot-3 myfirstpool/myfirstDS@snapshot-2
+
Unable to obtain diffs:
+
  Not an earlier snapshot from the same fs
+
</console>
+
 
+
Is ZFS would be a bit more happy if we ask the difference between two snapshots this time with a gap in between (so snapshot 1 with snapshot 3):
+
 
+
<console>
+
###i## zfs diff myfirstpool/myfirstDS@snapshot-1 myfirstpool/myfirstDS@snapshot-3
+
M      /myfirstpool/myfirstDS/
+
R      /myfirstpool/myfirstDS/hello.txt -> /myfirstpool/myfirstDS/hello_new.txt
+
M      /myfirstpool/myfirstDS/radeon
+
-      /myfirstpool/myfirstDS/radeon/CAYMAN_me.bin
+
+      /myfirstpool/myfirstDS/goodbye.txt
+
+      /myfirstpool/myfirstDS/config.gz
+
+      /myfirstpool/myfirstDS/version.txt
+
</console>
+
 
+
Amazing! Here again, take a couple of minutes to think about all operations you did on the dataset between the time you took the first snapshot and the time you took the last snapshot: this summary is the exact reflect of all your previous operations.
+
 
+
Just to put a conclusion on this subject, let's see the differences between the ''myfirstpool/myfirstDS'' dataset and its various snapshots:
+
 
+
<console>
+
###i## zfs diff myfirstpool/myfirstDS@snapshot-1                               
+
M      /myfirstpool/myfirstDS/
+
R      /myfirstpool/myfirstDS/hello.txt -> /myfirstpool/myfirstDS/hello_new.txt
+
M      /myfirstpool/myfirstDS/radeon
+
-      /myfirstpool/myfirstDS/radeon/CAYMAN_me.bin
+
+      /myfirstpool/myfirstDS/goodbye.txt
+
+      /myfirstpool/myfirstDS/config.gz
+
+      /myfirstpool/myfirstDS/version.txt
+
</console>
+
<console>
+
###i## zfs diff myfirstpool/myfirstDS@snapshot-2
+
M      /myfirstpool/myfirstDS/
+
R      /myfirstpool/myfirstDS/hello.txt -> /myfirstpool/myfirstDS/hello_new.txt
+
M      /myfirstpool/myfirstDS/goodbye.txt
+
+      /myfirstpool/myfirstDS/version.txt
+
</console>
+
<console>
+
###i##  zfs diff myfirstpool/myfirstDS@snapshot-3
+
</console>
+
 
+
Having nothing reported for the last '''zfs diff''' is normal as changed in the dataset since the snapshot has been taken.
+
 
+
=== The time travelling machine part 2: rolling back with multiple snapshots ===
+
Examining the differences between the various snapshots of a dataset or the dataset itself would be quite useless if we would not be able to roll the dataset back to one of its previous states. How we have salvaged ''myfirstpool/myfirstDS'' a bit, it would the time to restore it at it was when the first snapshot had been taken:
+
 
+
<console>
+
###i## zfs rollback myfirstpool/myfirstDS@snapshot-1
+
cannot rollback to 'myfirstpool/myfirstDS@snapshot-1': more recent snapshots exist
+
use '-r' to force deletion of the following snapshots:
+
myfirstpool/myfirstDS@snapshot-3
+
myfirstpool/myfirstDS@snapshot-2
+
</console>
+
 
+
Err... Well, ZFS just tells us that several more recent snapshots exists and it refuses to proceed without dropping those latter. Unfortunately for us there is no way to circumvent that: once you jump backward you have no way to move forward again. We could demonstrate the rollback to ''myfirstpool/myfirstDS@snapshot-3'' then ''myfirstpool/myfirstDS@snapshot-2'' then ''myfirstpool/myfirstDS@snapshot-1'' but it would be of very little interest previous sections of this tutorial did that already so second attempt:
+
 
+
<console>
+
###i## zfs rollback -r myfirstpool/myfirstDS@snapshot-1
+
###i## zfs list -t all                                                         
+
NAME                              USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool                      1.81G  6.00G  850M  /myfirstpool
+
myfirstpool/myfirstDS            2.96M  6.00G  2.96M  /myfirstpool/myfirstDS
+
myfirstpool/myfirstDS@snapshot-1    1K      -  2.96M  -
+
myfirstpool/mysecondDS            1003M  6.00G  1003M  /myfirstpool/mysecondDS
+
</console>
+
 
+
''myfirstpool/myfirstDS'' effectively returned to the desired state (notice the size of ''myfirstpool/myfirstDS@snapshot-1'') and the snapshots ''snapshot-2'' and ''snapshot-3'' vanished. Just to convince you:
+
<console>
+
###i## zfs diff myfirstpool/myfirstDS@snapshot-1
+
###i##
+
</console>
+
 
+
No differences at all!
+
 
+
=== Snapshots and clones ===
+
 
+
A clone and a snapshot are two very close things in ZFS:
+
 
+
* A clone appears as mounted dataset (i.e. you can read and write data in it) while a snapshot stays apart and is always read-only
+
* A clone is always spawned from a snapshot
+
 
+
So it is absolutely true to say that a clone is just indeed a writable snapshot. The copy-on-write feature of ZFS plays its role even there: the data blocks hold by the snapshot are only duplicated upon modification. So cloning 20Gb snapshot of data does not lead to an additional 20 Gb of data being eaten from the pool.
+
 
+
How to make a clone? Simple, once again with the '''zfs''' command used like this:
+
 
+
<console>
+
###i## zfs clone myfirstpool/myfirstDS@snapshot-1 myfirstpool/myfirstDS_clone1
+
###i## fs list -t all
+
NAME                              USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool                      1.81G  6.00G  850M  /myfirstpool
+
myfirstpool/myfirstDS            2.96M  6.00G  2.96M  /myfirstpool/myfirstDS
+
myfirstpool/myfirstDS@snapshot-1    1K      -  2.96M  -
+
myfirstpool/myfirstDS_clone1        1K  6.00G  2.96M  /myfirstpool/myfirstDS_clone1
+
myfirstpool/mysecondDS            1003M  6.00G  1003M  /myfirstpool/mysecondDS
+
</console>
+
 
+
Noticed the value of ''MOUNTPOINT'' for ''myfirstpool/myfirstDS_clone1''? No we have a dataset that is mounted! Let's check with the '''mount''' command:
+
 
+
<console>
+
###i## mount | grep clone
+
myfirstpool/myfirstDS_clone1 on /myfirstpool/myfirstDS_clone1 type zfs (rw,xattr)
+
</console>
+
 
+
In theory we can change or write additional data in the clone as it is mounted as being writable (rw). Let it be!
+
 
+
<console>
+
###i## # ls /myfirstpool/myfirstDS_clone1
+
hello.txt  radeon
+
</console>
+
<console>
+
###i## cp /proc/config.gz /myfirstpool/myfirstDS_clone1
+
###i## echo 'This is a clone!' >> /myfirstpool/myfirstDS_clone1/hello.txt
+
</console>
+
<console>
+
###i## ls /myfirstpool/myfirstDS_clone1
+
config.gz  hello.txt  radeon
+
###i## cat /myfirstpool/myfirstDS_clone1/hello.txt                     
+
Hello, world
+
This is a clone!
+
</console>
+
 
+
Unfortunately it is not possible to ask the difference between a clone and a snapshot, '''zfs diff''' expects to see either a snapshot name either two snapshots names. Once spawned, a clone starts its own existence and the clone that served as a seed for it remains attached to its own original dataset.
+
 
+
Because clones are nothing more than a ZFS dataset they can be destroyed just like any ZFS dataset:
+
 
+
<console>
+
###i## zfs destroy myfirstpool/myfirstDS_clone1
+
###i## zfs list -t all                                                       
+
NAME                              USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool                      1.81G  6.00G  850M  /myfirstpool
+
myfirstpool/myfirstDS            2.96M  6.00G  2.96M  /myfirstpool/myfirstDS
+
myfirstpool/myfirstDS@snapshot-1    1K      -  2.96M  -
+
myfirstpool/mysecondDS            1003M  6.00G  1003M  /myfirstpool/mysecondDS
+
</console>
+
 
+
=== Streaming ZFS datasets ===
+
A ZFS snapshot can not only be cloned or explored but also streamed in a local file or even over the network thus allowing to back up or simply an exact bit to bit copy of a ZFS dataset between two machines for example. Snapshots being differential (i.e. incremental) by nature very little network overhead is induced when consecutive snapshots are streamed over the network. A nifty move from the designers was to use ''stdin'' and ''stdout'' as transmission/reception channels thus allowing great a flexibility in processing the ZFS stream. You can envisage, for instance, to compress your stream then crypt it then encode it in base64 then sign it and so on. It sounds a bit overkill but it is possible and in the general case you can use any tool that swallows the data from ''stdin'' and spit it through ''stdout'' in your plumbing.
+
 
+
First things first, just to illustrate some basic concepts here is how to stream a ZFS dataset snapshot to a local file:
+
 
+
<console>
+
###i## zfs send myfirstpool/myfirstDS@snapshot-1 > /tmp/myfirstpool-myfirstDS@snapshot-snap1
+
###i## cat /tmp/myfirstpool-myfirstDS@snapshot-snap1 | zfs receive myfirstpool/myfirstDS@testrecv
+
</console>
+
 
+
Now let's stream it back:
+
 
+
<console>
+
###i## cannot receive new filesystem stream: destination 'myfirstpool/myfirstDS' exists
+
must specify -F to overwrite it
+
</console>
+
 
+
Ouch... ZFS refuses to go any step further because some data would be overwritten.  We do now own any critical data on the dataset so we could destroy it and try again or use a different name nevertheless, just for the sake of the demonstration, let's create another zpool prior restoring the dataset there:
+
 
+
<console>
+
###i## dd if=/dev/zero of=/tmp/zfs-test-disk04.img bs=2G count=1
+
0+1 records in
+
0+1 records out
+
2147479552 bytes (2.1 GB) copied, 6.35547 s, 338 MB/s
+
###i## losetup -f           
+
/dev/loop4
+
###i## losetup /dev/loop4 /tmp/zfs-test-disk04.img
+
###i## zpool create testpool /dev/loop4
+
###i## zpool list
+
NAME          SIZE  ALLOC  FREE    CAP  DEDUP  HEALTH  ALTROOT
+
myfirstpool  7.94G  1.81G  6.12G    22%  1.00x  ONLINE  -
+
testpool    1.98G  89.5K  1.98G    0%  1.00x  ONLINE  -
+
</console>
+
 
+
Take two:
+
 
+
<console>
+
###i## cat /tmp/myfirstpool-myfirstDS@snapshot-snap1 | zfs receive testpool/myfirstDS@testrecv
+
###i## zfs list -t all
+
NAME                              USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool                      1.81G  6.00G  850M  /myfirstpool
+
myfirstpool/myfirstDS            2.96M  6.00G  2.96M  /myfirstpool/myfirstDS
+
myfirstpool/myfirstDS@snapshot-1    1K      -  2.96M  -
+
myfirstpool/mysecondDS            1003M  6.00G  1003M  /myfirstpool/mysecondDS
+
testpool                          3.08M  1.95G    31K  /testpool
+
testpool/myfirstDS                2.96M  1.95G  2.96M  /testpool/myfirstDS
+
testpool/myfirstDS@testrecv          0      -  2.96M  -
+
</console>
+
 
+
Very interesting things happened there! First the data previously stored in the file ''/tmp/myfirstpool-myfirstDS@snapshot-snap1'' been copied as a snapshot in the destination zpool (''testpool'' here) and it has been copied exactly in the same manner given on the command line. Second a clone of this snapshot has been crated for you by ZFS and the snapshot ''myfirstpool/myfirstDS@snapshot-1'' now appears as a live ZFS dataset where data can be read and
+
written! Think two seconds about the error message we got just above, the reason ZFS protested becomes clear now.
+
 
+
An alternative would have been to use the original zpool but this time with a different name for the dataset:
+
 
+
<console>
+
###i## cat /tmp/myfirstpool-myfirstDS@snapshot-snap1 | zfs receive myfirstpool/myfirstDS_copy@testrecv
+
###i## zfs list -t all                                                                               
+
NAME                                  USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool                          1.82G  6.00G  850M  /myfirstpool
+
myfirstpool/myfirstDS                2.96M  6.00G  2.96M  /myfirstpool/myfirstDS
+
myfirstpool/myfirstDS@snapshot-1        1K      -  2.96M  -
+
myfirstpool/myfirstDS_copy          2.96M  6.00G  2.96M  /myfirstpool/myfirstDS_copy
+
myfirstpool/myfirstDS_copy@testrecv      0      -  2.96M  -
+
myfirstpool/mysecondDS              1003M  6.00G  1003M  /myfirstpool/mysecondDS
+
</console>
+
 
+
Now something a bit  more interesting: instead of using a local file, we will stream the dataset to a Solaris 11 machine (OpenIndiana can be used also) over the network using the GNU flavour of '''netcat''' (''net-analyzer/gnu-netcat'') over the port TCP/7000 , in that case the Solaris host is a x86 machine but a SPARC machine would have given the exact same result as ZFS contrary to UFS is platform agnostic.
+
 
+
On the Solaris machine:
+
<console>
+
###i## nc -l -p 7000 | zfs receive nas/zfs-stream-test@s1
+
</console>
+
 
+
On the Linux machine:
+
<console>
+
###i##  zfs send myfirstpool/myfirstDS@snapshot-1 | netcat -c 192.168.1.13 7000
+
</console>
+
 
+
{{Fancywarning|The '''nc''' command coming with the ''net-analyzer/netcat'' package '''does not automatically close the network connection when its input stream is closed''' (i.e. when '''zfs send''' command terminates its job) thus  its Solaris conterpart also waits "forever" at the other end of the "pipe". It is not possible to override this behaviour hence the reason we use its GNU variant (package ''net-analyzer/netcat'').}}
+
 
+
After the dataset has been received on the Solaris machine the ''nas'' zpool now contains the sent snapshot and its corresponding clone, that latter being  automatically created:<console>
+
###i## zfs list -t snapshot
+
NAME                                          USED  AVAIL  REFER  MOUNTPOINT
+
(...)
+
nas/zfs-stream-test                          3.02M  6.17T  3.02M  /nas/zfs-stream-test
+
nas/zfs-stream-test@s1                          0      -  3.02M  -
+
</console>
+
 
+
A quick look in the ''/san/zfs-stream-test'' directory on the same Solaris machine gives:
+
 
+
<console>
+
###i## ls -lR /nas/zfs-stream-test
+
/nas/zfs-stream-test/:
+
total 12
+
-rw-r--r--  1 root    root          13 Mar  3 18:59 hello.txt
+
drwxr-xr-x  2 root    root        143 Mar  3 18:59 radeon
+
 
+
/nas/zfs-stream-test/radeon:
+
total 6144
+
-rw-r--r--  1 root    root        8704 Mar  3 18:59 ARUBA_me.bin
+
-rw-r--r--  1 root    root        8704 Mar  3 18:59 ARUBA_pfp.bin
+
-rw-r--r--  1 root    root        6144 Mar  3 18:59 ARUBA_rlc.bin
+
-rw-r--r--  1 root    root      24096 Mar  3 18:59 BARTS_mc.bin
+
-rw-r--r--  1 root    root        5504 Mar  3 18:59 BARTS_me.bin
+
-rw-r--r--  1 root    root        4480 Mar  3 18:59 BARTS_pfp.bin
+
(...)
+
</console>
+
 
+
The dataset is exactly what it is on the Linux machine!
+
 
+
{{Fancynote|We took only a simple case here: ZFS can is able to handle snapshots is a very flexible way. You can ask, for example, to combine several consecutive snapshots then send them as a single snapshot or you can choose to proceed in incremental steps. A '''man zfs''' will tell you the art of streaming your snapshots.}}
+
 
+
== Govern a dataset by attributes ==
+
In the ZFS world, many aspects are now managed by simply setting/clearing a property attached to a ZFS dataset through the now so well-known command '''zfs'''. You can, for example:
+
* put a size limit on a dataset
+
* control if new files are encrypted and/or compressed
+
* define a quota
+
* control checksum usage  => '''never turn that property off unless having very good reasons you are likely to never have''' (no checksums = no silent data corruption detection)
+
* share a dataset by NFS/CIFS (Samba)
+
* control data deduplication
+
 
+
Not all of a dataset properties are settable, some of them are set and managed by the operating system in the background for you and thus cannot be modified. Like any other action concerning datasets, properties are sets and unset via the '''zfs''' command. Let's start by checking the value of '''all'''  supported attributes for the dataset ''myfirstpool/myfirstDS'':
+
 
+
<console>
+
###i## zfs get all myfirstpool/myfirstDS
+
NAME                  PROPERTY              VALUE                  SOURCE
+
myfirstpool/myfirstDS  type                  filesystem              -
+
myfirstpool/myfirstDS  creation              Sun Mar  2 15:26 2014  -
+
myfirstpool/myfirstDS  used                  2.96M                  -
+
myfirstpool/myfirstDS  available            6.00G                  -
+
myfirstpool/myfirstDS  referenced            2.96M                  -
+
myfirstpool/myfirstDS  compressratio        1.00x                  -
+
myfirstpool/myfirstDS  mounted              yes                    -
+
myfirstpool/myfirstDS  quota                none                    default
+
myfirstpool/myfirstDS  reservation          none                    default
+
myfirstpool/myfirstDS  recordsize            128K                    default
+
myfirstpool/myfirstDS  mountpoint            /myfirstpool/myfirstDS  default
+
myfirstpool/myfirstDS  sharenfs              off                    default
+
myfirstpool/myfirstDS  checksum              on                      default
+
myfirstpool/myfirstDS  compression          off                    default
+
myfirstpool/myfirstDS  atime                on                      default
+
myfirstpool/myfirstDS  devices              on                      default
+
myfirstpool/myfirstDS  exec                  on                      default
+
myfirstpool/myfirstDS  setuid                on                      default
+
myfirstpool/myfirstDS  readonly              off                    default
+
myfirstpool/myfirstDS  zoned                off                    default
+
myfirstpool/myfirstDS  snapdir              hidden                  default
+
myfirstpool/myfirstDS  aclinherit            restricted              default
+
myfirstpool/myfirstDS  canmount              on                      default
+
myfirstpool/myfirstDS  xattr                on                      default
+
myfirstpool/myfirstDS  copies                1                      default
+
myfirstpool/myfirstDS  version              5                      -
+
myfirstpool/myfirstDS  utf8only              off                    -
+
myfirstpool/myfirstDS  normalization        none                    -
+
myfirstpool/myfirstDS  casesensitivity      sensitive              -
+
myfirstpool/myfirstDS  vscan                off                    default
+
myfirstpool/myfirstDS  nbmand                off                    default
+
myfirstpool/myfirstDS  sharesmb              off                    default
+
myfirstpool/myfirstDS  refquota              none                    default
+
myfirstpool/myfirstDS  refreservation        none                    default
+
myfirstpool/myfirstDS  primarycache          all                    default
+
myfirstpool/myfirstDS  secondarycache        all                    default
+
myfirstpool/myfirstDS  usedbysnapshots      1K                      -
+
myfirstpool/myfirstDS  usedbydataset        2.96M                  -
+
myfirstpool/myfirstDS  usedbychildren        0                      -
+
myfirstpool/myfirstDS  usedbyrefreservation  0                      -
+
myfirstpool/myfirstDS  logbias              latency                default
+
myfirstpool/myfirstDS  dedup                off                    default
+
myfirstpool/myfirstDS  mlslabel              none                    default
+
myfirstpool/myfirstDS  sync                  standard                default
+
myfirstpool/myfirstDS  refcompressratio      1.00x                  -
+
myfirstpool/myfirstDS  written              1K                      -
+
myfirstpool/myfirstDS  snapdev              hidden                  default
+
</console>
+
 
+
{{Fancynote|the manual page of the '''zfs''' command gives a list and description of every attributes supported by a dataset.}}
+
 
+
May be something poked your curiosity: "what ''SOURCE'' means?". ''SOURCE'' describes how the property has been determined for the dataset and can have several values:
+
* '''local''': the property has been explicitly set for this dataset
+
* '''default''': a default value has been assigned by the operating system if not explicitely set by the system adminsitrator
+
* '''dash (-)''': immutable property (e.g. dataset creation time, whether the dataset is currently mounted or not...)
+
 
+
Of course you can get the property of a single attribute if you know its name instead of asking for ''all'' properties.
+
 
+
=== Compressing data ===
+
 
+
<console>
+
###i## zfs get compression myfirstpool/myfirstDS
+
NAME                  PROPERTY    VALUE    SOURCE
+
myfirstpool/myfirstDS  compression  off      default
+
</console>
+
 
+
Let's activate the compression on the volume (notice the change in the ''SOURCE'' column). That is being achieved through an attribute simply named ''compression'' which can be changed by running the '''zfs''' command with the '''set''' sub-command followed by the attribute's name (''compression'' here) and value (''on'' here) like this:
+
 
+
<console>
+
###i## zfs set compression=on myfirstpool/myfirstDS
+
###i## zfs get compression myfirstpool/myfirstDS
+
NAME                  PROPERTY    VALUE    SOURCE
+
myfirstpool/myfirstDS  compression  on        local
+
</console>
+
 
+
The attribute's new value becomes immediately effective no need to unmount and remount anything. ''compression'' set to ''on'' will only affect new data and not what already exists on the dataset. For your information, the lzjb compression algorithms is used when ''compression'' is set to ''on'', you can override and use another compression algorithm by explicitly tell your choice. For example if you want to activate LZ4 compression on the dataset:
+
 
+
<console>
+
###i## zfs get compression myfirstpool/myfirstDS
+
NAME                  PROPERTY    VALUE    SOURCE
+
myfirstpool/myfirstDS  compression  off      default
+
</console>
+
<console>
+
###i## zfs set compression=lz4 myfirstpool/myfirstDS
+
###i## zfs get compression myfirstpool/myfirstDS
+
NAME                  PROPERTY    VALUE    SOURCE
+
myfirstpool/myfirstDS  compression  lz4        local
+
</console>
+
Assuming ''myfirstpool/myfirstDS'' is empty with no snapshots:
+
<console>
+
###i## cp -a /usr/src/linux-3.13.5-gentoo /-a /usr/src/linux-3.13.5-gentoo
+
###i## zfs get all myfirstpool/myfirstDS
+
NAME                  PROPERTY              VALUE                  SOURCE
+
myfirstpool/myfirstDS  type                  filesystem              -
+
myfirstpool/myfirstDS  creation              Sun Mar  2 15:26 2014  -
+
myfirstpool/myfirstDS  used                  584M                    -
+
myfirstpool/myfirstDS  available            5.43G                  -
+
myfirstpool/myfirstDS  referenced            584M                    -
+
myfirstpool/myfirstDS  compressratio        1.96x                  -      <<<< Compression ratio
+
myfirstpool/myfirstDS  mounted              yes                    -
+
myfirstpool/myfirstDS  quota                none                    default
+
myfirstpool/myfirstDS  reservation          none                    default
+
myfirstpool/myfirstDS  recordsize            128K                    default
+
myfirstpool/myfirstDS  mountpoint            /myfirstpool/myfirstDS  default
+
myfirstpool/myfirstDS  sharenfs              off                    default
+
myfirstpool/myfirstDS  checksum              on                      default
+
myfirstpool/myfirstDS  compression          on                      local  <<<< LZJB compression active
+
myfirstpool/myfirstDS  atime                on                      default
+
myfirstpool/myfirstDS  devices              on                      default
+
myfirstpool/myfirstDS  exec                  on                      default
+
myfirstpool/myfirstDS  setuid                on                      default
+
myfirstpool/myfirstDS  readonly              off                    default
+
myfirstpool/myfirstDS  zoned                off                    default
+
myfirstpool/myfirstDS  snapdir              hidden                  default
+
myfirstpool/myfirstDS  aclinherit            restricted              default
+
myfirstpool/myfirstDS  canmount              on                      default
+
myfirstpool/myfirstDS  xattr                on                      default
+
myfirstpool/myfirstDS  copies                1                      default
+
myfirstpool/myfirstDS  version              5                      -
+
myfirstpool/myfirstDS  utf8only              off                    -
+
myfirstpool/myfirstDS  normalization        none                    -
+
myfirstpool/myfirstDS  casesensitivity      sensitive              -
+
myfirstpool/myfirstDS  vscan                off                    default
+
myfirstpool/myfirstDS  nbmand                off                    default
+
myfirstpool/myfirstDS  sharesmb              off                    default
+
myfirstpool/myfirstDS  refquota              none                    default
+
myfirstpool/myfirstDS  refreservation        none                    default
+
myfirstpool/myfirstDS  primarycache          all                    default
+
myfirstpool/myfirstDS  secondarycache        all                    default
+
myfirstpool/myfirstDS  usedbysnapshots      0                      -
+
myfirstpool/myfirstDS  usedbydataset        584M                    -
+
myfirstpool/myfirstDS  usedbychildren        0                      -
+
myfirstpool/myfirstDS  usedbyrefreservation  0                      -
+
myfirstpool/myfirstDS  logbias              latency                default
+
myfirstpool/myfirstDS  dedup                off                    default
+
myfirstpool/myfirstDS  mlslabel              none                    default
+
myfirstpool/myfirstDS  sync                  standard                default
+
myfirstpool/myfirstDS  refcompressratio      1.96x                  -
+
myfirstpool/myfirstDS  written              584M                    -
+
myfirstpool/myfirstDS  snapdev              hidden                  default
+
</console>
+
 
+
Notice the value for ''compressionratio'': it no longer shows 1.00x but a shiny 1.96 here (1.96:1 ratio). We have a high compression ratio here because we copied a lot of source code files but if we put a lot of compressed data (images in jpeg or png format for example) the ratio would have decreased a lot.
+
 
+
=== Changing the mountpoint ===
+
 
+
Let's change the mount point of ''myfirstpool/myfirstDS'' to something like ''/mnt/floppy'' instead of ''/myfirstpool/myfirstDS'' for the sake of demonstration purposes. Changing a dataset mountpoint is done via its ''mountpoint'' attribute:
+
 
+
<console>
+
###i## zfs get mountpoint myfirstpool/myfirstDS
+
NAME                  PROPERTY    VALUE                  SOURCE
+
myfirstpool/myfirstDS  mountpoint  /myfirstpool/myfirstDS  default
+
</console>
+
<console>
+
###i## zfs set mountpoint=/mnt/floppy myfirstpool/myfirstDS
+
###i## zfs list 
+
NAME                    USED  AVAIL  REFER  MOUNTPOINT
+
myfirstpool            2.38G  5.43G  850M  /myfirstpool
+
myfirstpool/myfirstDS    584M  5.43G  584M  /mnt/floppy
+
myfirstpool/mysecondDS  1003M  5.43G  1003M  /myfirstpool/mysecondDS
+
</console>
+
<console>
+
###i## mount | grep floppy
+
myfirstpool/myfirstDS on /mnt/floppy type zfs (rw,xattr)
+
</console>
+
 
+
Notice the dataset has been automatically unmounted and remounted at the new location for you and once again the change is effective immediately. If the indicated mountpoint would not be empty ZFS is smart enough to warn you and to not remount it.
+
 
+
=== Sharing a dataset through NFS ===
+
 
+
Now that you are a bit more familiar with ZFS properties you won't be that much surprised to learn that sharing a dataset can be done by setting one of its properties. You can, of course, go the "traditional" way and edit Samba's or NFS related configuration files by hand however why hassle with manual editing since ZFS can do that for you? ZFS On Linux has support for both systems.
+
 
+
Next let's share the ''myfirstpool/myfirstDS'' dataset by NFS to any host within the network 192.168.1.0/24 (read-write access) . An important detail here : '''the zfs command will use NFS v4 by default''' so any options related to NFS v4 can be passed on the command line, refer to options supported by your NFS server documentation for further information on what is supported and how use the feature. To share the dataset by NFS, you must change a property named ''sharenfs'':
+
<console>
+
###i## zfs set sharenfs='rw=@192.168.1.0/24' myfirstpool/myfirstDS
+
</console>
+
 
+
What happened? Simple:
+
 
+
<console>
+
###i## zfs get sharenfs myfirstpool/myfirstDS
+
NAME                  PROPERTY  VALUE              SOURCE
+
myfirstpool/mfirstDS  sharenfs  rw=@192.168.1.0/24  local
+
</console>
+
<console>
+
###i## cat /etc/dfs/sharetab
+
/myfirstpool/myfirstDS  -      nfs    rw=@192.168.1.0/24
+
</console>
+
 
+
{{Fancyimportant|The syntax and behaviour is similar to what is found under Solaris 11: '''zfs share''' reads and updates entries coming from the file ''/etc/dfs/sharetab'' '''(not ''/etc/exports'')'''. This is a Solaris ''touch'': under Solaris 11 the '''zfs'' and '''share''' commands now acts on ''/etc/dfs/sharetab'', ''/etc/dfs/dfstab'' being no longer supported.}}
+
 
+
By a checking with the '''showmount''' command:
+
 
+
<console>
+
###i## showmount -e 
+
Export list for .... :
+
/myfirstpool/myfirstDS 192.168.1.0/24
+
</console>
+
 
+
At this point it should be possible to mount the dataset from another host on the network (here a Solaris 11 machine) and write some data in it:
+
 
+
<console>
+
###i## mkdir -p /mnt/myfirstDS
+
###i## mount 192.168.1.19:/myfirstpool/myfirstDS /mnt/myfirstDS
+
###i##  mount | grep myfirst
+
/mnt/myfirstDS on 192.168.1.19:/myfirstpool/myfirstDS remote/read/write/setuid/devices/rstchown/xattr/dev=89c0002 on Sun Mar  9 14:28:55 2014
+
</console>
+
<console>
+
###i## cp /kernel/amd64/genunix /mnt/myfirstDS
+
</console>
+
 
+
''Et voila!''No sign of protest so the file has been copied. If we check what the ZFS dataset looks like on the Linux host where the ZFS dataset resides,  the copied file (a Solaris kernel image here) is present:
+
 
+
<console>
+
###i## ls -l /myfirstpool/myfirstDS/genunix
+
-rwxr-xr-x 1 root root 5769456 Mar  9 14:32 /myfirstpool/myfirstDS/genunix
+
</console>
+
 
+
$100 question: How to "unshare" the dataset? Simple: just set ''sharenfs'' to ''off''! Be aware that the NFS server will cease to share the dataset no matter if this one is still in use by client machines. Any NFS client still having the dataset mounted at this point will encounter RPC errors whenever an I/O operation is attempted on the share (Solaris NFS client here):
+
<console>
+
###i## ls /mnt/myfirstDS
+
NFS compound failed for server 192.168.1.19: error 7 (RPC: Authentication error)
+
</console>
+
 
+
=== Sharing a dataset through Samba/SMB ===
+
 
+
Let's push the limit a bit and use Samba instead of NFS. ZFS relies on [http:///www.samba.org Samba] (''net-fs/samba'' on Gentoo/Funtoo) to get the job done as it does not implement a SMBFS server on its own. So Samba must be emerged first '''making sure :
+
* it has built-in ACL support''' (''acl'' use flag)
+
* client tools are built (''client'' use flag) as ZoL invokes the '''net''' command behind the scene (i.e. ''net usershare ... '')
+
* usershare must be functional
+
 
+
Quoting the '''zfs''' command's manual page, your Samba server must also be configured like this:
+
 
+
* Samba will need to listen to 'localhost' (127.0.0.1) for the zfs utilities to communicate with samba.  This is the default behaviour for most Linux distributions.
+
* Samba must be able to authenticate a user. This can be done in a number of ways, depending on if using the system password file, LDAP or the Samba specific smbpasswd file. How to do this is outside the scope of this manual. Please refer to the smb.conf(5) manpage for more information.
+
* See the USERSHARE section of the smb.conf(5) man page for all configuration options in case you need to modify any options to the share afterwards. Do note that any changes done with the 'net' command will be undone if the share is every unshared (such as at a reboot etc). In the future, ZoL will be able to set specific options directly using sharesmb=<option>.
+
 
+
What you have to know at this point is that, once emerged on your Funtoo box, Samba has no configuration file thus will refuse to start. You can use the provided  example file ''/etc/samba/smb.conf.example'' as a starting point for ''/etc/samba/smb.conf'', just copy it:
+
 
+
<console>
+
###i## cd /etc/samba
+
###i## cp smb.conf.example smb.conf
+
</console>
+
 
+
Now create the directory ''/var/lib/samba/usershares'' (will host the definitions of all usershares), leaving default permissions (0755) and owner (root:root) untouched for the context of this tutorial, unless you use ZFS delegation, is acceptable.
+
<console>
+
###i## mkdir /var/lib/samba/usersharesusershare
+
</console>
+
 
+
Several important things to know unless you have hours to waste with your friend Google:
+
* When you set the ''sharesmb'' property to ''on'', the '''zfs''' command will invoke Samba's ''net'' command behind the scenes to create a usershare (comment and ACL are values are both specified).  E.g. ''zfs sharesmb=on myfirstpool/myfirstDS'' => ''net usershare add myfirstpool_myfirstDS /myfirstpool/myfirstDS "Comment:/myfirstpool/myfirstDS" "Everyone:F"''
+
* Under which user the ''net usershare'' command will be invoked? Unless ZFS delegation is used, ''root'' will be the owner of the usershare created by ZFS
+
* Unless explicitly overridden in ''/etc/samba/smb.conf'':
+
** ''usershare max shares'' default value is zero so '''no usershare can be created'''. If you forget to set a value greater than zero for ''usershare max shares'' any '''zfs set sharesmb=on''' command will complain with the message  ''cannot share (...) smb add share failed'' (also any ''net usershare add'' command will show the error message ''net usershare: usershares are currently disabled'').
+
** ''usershare path = /var/lib/samba/usershares''
+
** ''usershare owner only'' is set to ''true'' by default so Samba will refuse the share to any remote user not opening a session as ''root'' on the share
+
 
+
So basically a super-minimalistic configuration for Samba would be:
+
 
+
<pre>
+
[global]
+
  workgroup = MYGROUP
+
  server string = Samba Server
+
  security = user
+
  log file = /var/log/samba/log.%m
+
  max log size = 50
+
  # Permits the usershares of being accessed by any other user than 'root' from a remote client machine
+
  usershare owner only = False
+
  # WARNING: default value for usershare max shares is 0 so No usershares possible...
+
  usershare max shares = 10
+
</pre>
+
{{Fancywarning|This configuration is obviously for the sake of demonstration purposes within the scope of this tutorial, '''do not use it for the real world!'''}}
+
 
+
At this point reload or restart Samba if you have altered ''/etc/samba/smb.conf''. Now the usershares are possible, let's share a ZFS dataset over Samba:
+
 
+
<console>
+
###i## zfs set sharesmb=on myfirstpool/myfirstDS
+
###i## zfs get sharesmb myfirstpool/myfirstDS
+
NAME                  PROPERTY  VALUE    SOURCE
+
myfirstpool/myfirstDS  sharesmb  on        local
+
</console>
+
 
+
The command must return without any error message, '''if you have something like ''"cannot share myfirstpool/myfirstDS smb add share failed"'' then usershares are not functional on your machine (see the notes just above)'''. Now a Samba usershare named after the zpool and the dataset names should exist:
+
 
+
<console>
+
###i## net usershare list
+
myfirstpool_myfirstDS
+
</console>
+
<console>
+
###i## net usershare info myfirstpool_myfirstDS
+
[myfirstpool_myfirstDS]
+
path=/myfirstpool/myfirstDS
+
comment=Comment: /myfirstpool/myfirstDS
+
usershare_acl=Everyone:F,
+
guest_ok=n
+
</console>
+
 
+
So far so good! So let's try this on the machine itself:
+
 
+
<console>
+
###i##
+
</console>
+
 
+
= Data redundancy with ZFS =
+
 
+
Nothing is perfect and the storage medium (even in datacenter-class equipment) is prone to failures and fails on a regular basis. Having data redundancy is mandatory to help in preventing single-points of failure (SPoF). Over the past decades, RAID technologies were powerful however their power is precisely their weakness: as operating at the block level, they do not care about what is stored on the data blocks and have no ways to interact with the filesystems stored on them to ensure data integrity is properly handled.
+
 
+
== Some statistics ==
+
 
+
It is not a secret to tell that a general trend in the IT industry is the exponential growth of data quantities. Just thinking about the amount of data Youtube, Google or Facebook generates every day taking the case of the first [http://www.website-monitoring.com/blog/2010/05/17/youtube-facts-and-figures-history-statistics some statistics] gives:
+
* 24 hours of video is generated every ''minute'' in March 2010 (May 2009 - 20h / October 2008 - 15h / May 2008 - 13h)
+
* More than 2 ''billions'' views a day
+
* More video is produced on Youtube every 60 days than 3 major US broadcasting networks did in the last 60 years
+
 
+
Facebook is also impressive (Facebook own stats):
+
 
+
* over 900 million objects that people interact with (pages, groups, events and community pages)
+
* Average user creates 90 pieces of content each month (750 millions users active)
+
* More than 2.5 million websites have integrated with Facebook
+
 
+
What is true with Facebook and Youtube is also true with many other cases (think one minutes about the amount of data stored in iTunes) especially with the growing popularity of cloud computing infrastructures. Despite the progress of the technology a "bottleneck" still exists: the storage reliability is nearly the same over the years. If only one organization in the world generate huge quantities of data it would be the [http://public.web.cern.ch CERN] (''Conseil Européen pour la Recherche Nucléaire'', now officially known as ''European Organization for Nuclear Research'') as their experiments can generate spikes of many terabytes of data within a few seconds. A study done in 2007 quoted by a [http://www.zdnet.com/blog/storage/data-corruption-is-worse-than-you-know/191 ZDNet article] reveals that:
+
 
+
* Even ECC memory cannot be always be helpful: 3 double-bit errors (uncorrectable) occurred in 3 months on 1300 nodes. Bad news: it should be '''zero'''.
+
* RAID systems cannot protect in all cases: monitoring 492 RAID controller for 4 weeks showed an average error rate of 1 per ~10^14 bits, giving roughly 300 errors for every 2.4 petabytes
+
* Magnetic storage is still not reliable even on high-end datacenter class drives: 500 errors found over 100 nodes while writing 2 GB file to 3000+ nodes every 2 hours then read it again and again for 5 weeks.
+
 
+
Overall this means: 22 corrupted files (1 in every 1500 files) for a grand total of 33700 files holding 8.7TB of data. And this study is 5 years old....
+
 
+
== Source of silent data corruption ==
+
 
+
http://www.zdnet.com/blog/storage/50-ways-to-lose-your-data/168
+
 
+
Not an exhaustive list but we can quote:
+
 
+
* Cheap controller or buggy driver that does not reports errors/pre-failure conditions to the operating system;
+
* "bit-leaking": an harddrive consists of many concentric magnetic tracks. When the hard drive magnetic head writes bits on the magnetic surface it generates a very weak magnetic field however sufficient to "leak" on the next track and change some bits. Drives can generally, compensate those situations because they also records some error correction data on the magnetic surface
+
* magnetic surface defects (weak sectors)
+
* Hard drives firmware bugs
+
* Cosmic rays hitting your RAM chips or hard drives cache memory/electronics
+
*
+
 
+
== Building a mirrored pool ==
+
 
+
 
+
== ZFS RAID-Z ==
+
 
+
=== ZFS/RAID-Z vs RAID-5 ===
+
 
+
RAID-5 is very commonly used nowadays because of its simplicity, efficiency and fault-tolerance. Although the technology did its proof over decades, it has a major drawback known as "The RAID-5 write hole". if you are familiar with RAID-5 you already know that is consists of spreading the stripes across all of the disks within the array and interleaving them with a special stripe called the parity. Several schemes of spreading stripes/parity between disks exists in the natures, each one with its own pros and cons, however the "standard" one (also known as ''left-asynchronous'') is:
+
 
+
<pre>
+
Disk_0  | Disk_1  | Disk_2  | Disk_3
+
[D0_S0] | [D0_S1] | [D0_S2] | [D0_P]
+
[D1_S0] | [D1_S1] | [D1_P]  | [D1_S2]
+
[D2_S0] | [D2_P]  | [D2_S1] | [D2_S2]
+
[D2_P]  | [D2_S0] | [D2_S1] | [D2_S2]
+
</pre>
+
 
+
The parity is simply computed by XORing the stripes of the same "row", thus giving the general equation:
+
* [Dn_S0] XOR [Dn_S1] XOR ... XOR [Dn_Sm] XOR [Dn_P] = 0
+
This equation can be rewritten in several ways:
+
* [Dn_S0] XOR [Dn_S1] XOR ... XOR [Dn_Sm] = [Dn_P]
+
* [Dn_S1] XOR [Dn_S2] XOR ... XOR [Dn_Sm] XOR [Dn_P] = [Dn_S0]
+
* [Dn_S0] XOR [Dn_S2] XOR ... XOR [Dn_Sm] XOR [Dn_P] = [Dn_S1]
+
* ...and so on!
+
 
+
Because the equations are a combinations of exclusive-or, it is  possible to easily compute a parameter if it is missing. Let say we have 3 stripes plus one parity composed of 4 bits each but one of them is missing due to a disk failure:
+
 
+
* D0_S0 = 1011
+
* D0_S1 = 0010
+
* D0_S2 = <missing>
+
* D0_P  = 0110
+
 
+
However we know that:
+
* D0_S0 XOR D0_S1 XOR D0_S2 XOR D0_P = 0000 also rewritten as:
+
* D0_S2 = D0_S1 XOR D0_S2 XOR D0_P
+
 
+
Applying boolean algebra it gives:''' D0_S2 = 1011 XOR 0010 XOR 0110 = 1111'''.
+
Proof: '''1011 XOR 0010 XOR 1111 = 0110''' this is the same as '''D0_P'''
+
 
+
''''''So what's the deal?''''''
+
Okay now the funny part, forgot the above hypothesis and imagine we have this:
+
 
+
* D0_S0 = 1011
+
* D0_S1 = 0010
+
* D0_S2 = 1101
+
* D0_P  = 0110
+
 
+
Applying boolean algebra magics gives 1011 XOR 0010 XOR 1101 => 0100. Problem: this is different of D0_P  (0110). Can you tell which one (or which ONES) of the four terms lies? If you find a mathematically acceptable solution, found your company because you have just solved a big computer science problem. If humans can't solve the question, imagine how hard it is for the poor little RAID-5 controller to determine which stripe is right and which one lies and the resulting "datageddon" (i.e. massive data corruption on the RAID-5 array) when the RAID-5 controller detect error and start to rebuild the array.
+
 
+
This is not science fiction, this a pure reality and the weakness stays in the RAID-5 simplicity. Here is how it can happen: an urban legend with RAID-5 arrays is that they update stripes in an atomic transaction (all of the stripes+parity are written or none of them). Too bad, this is just not true, the data is written on the fly and if for a reason or another the machine where the RAID-5 array has a power outage or crash, the RAID-5 controller will simply have no idea about what he was doing and which stripes are up to date which ones are not up to date. Of course, RAID controllers in servers do have a replaceable on-board battery and most of the time the server they reside in is connected to an auxiliary source like a battery-based UPS or a diesel/gas electricity generator. However, Murphy laws or unpredictable hazards can, sometimes, happens....
+
 
+
Another funny scenario: imagine a machine with a RAID-5 array (on UPS this time) but with non ECC memory. the RAID-5 controller splits the data buffer in stripes, computes a data stripe and starts to write them on the different disks of the array. But...but...but... For some odd reason, only one bit in one of the stripes flips (cosmic rays, RFI...) after the parity calculation. Too bad too sad, one of the written stripes contains corrupted data and it is silently written on the array. Datageddon in sight!
+
 
+
Not to make you freaking: storage units have sophisticated error correction capability (a magnetic surface or an optical recording surface is not perfect and reading/writing error occurs) masking most the cases. However, some  established statistics estimates that even with error correction mechanism one bit over 10^16 bits transferred is incorrect. 10^16 is really huge but unfortunately in this beginning of the XXIst century with datacenters brewing massive amounts of data with several hundreds to not say thousands servers this this number starts to give headaches:  '''a big datacenter can face to silent data corruption every 15 minutes''' (Wikepedia). No typo here, a potential disaster may silently appear 5 times an hour for every single day of the year. Detection techniques exists but traditional RAID-5 arrays in them selves can be a problem. Ironic for a so popular and widely used solution :)
+
 
+
If RAID-5 was an acceptable trade-off in the past decades, it simply made its time.  RAID-5 is dead? '''*Horray!*'''
+
 
+
= More advanced topics =
+
 
+
== Z-Volumes (ZVOLs) ==
+
 
+
== ZFS Intention Log (ZIL) ==
+
 
+
== Permission delegation ==
+
 
+
ZFS brings a feature known as delegated administration. Delegated administration enables ordinary users to handle administrative tasks on a dataset without being administrators. '''It is however not a sudo replacement as it covers only ZFS related tasks''' such as sharing/unsharing, disk quota management and so on. Permission delegation shines in flexibility because such delegation can be handled by inheritance though nested datasets. Pewrmission deleguation is handled via '''zfs''' through its '''allow''' and '''disallow''' options.
+
 
+
= Final words and lessons learned =
+
 
+
ZFS on Linux, while still in development, showed strong capabilities and supported many of the features found in the Solaris/OpenIndiana implementation. It also seems to be very stable as no crashes or kernel oops happened while writing this tutorial. Funtoo does not officially support an installations over ZFS datasets however you can always read [[ZFS Install Guide]] to have a Funtoo box relying on ZFS!
+
 
+
= Footnotes & references =
+
Source: [http://docs.huihoo.com/opensolaris/solaris-zfs-administration-guide/html/index.html solaris-zfs-administration-guide]
+
[[Category:Labs]]
+
[[Category:Articles]]
+
[[Category:Filesystems]]
+
 
+
<references/>
+

Latest revision as of 17:53, 25 November 2010

The app-crypt category contains cryptographic (encryption, decryption, steganography and signing) software.