ZFS

ZFS is an advanced filesystem created by Sun Microsystems (now owned by Oracle) and released for OpenSolaris in November 2005. Features of ZFS include: pooled storage (integrated volume management -- zpool), Copy-on-write, snapshots, data integrity verification and automatic repair (scrubbing), RAID-Z, and a maximum 16 Exabyte volume size. ZFS is licensed under the Common Development and Distribution License (CDDL).

Described as "The last word in filesystems" ZFS is stable, fast, secure, and future-proof. Being licensed under the GPL incompatible CDDL, it is not possible for ZFS to be distributed along with the Linux Kernel. This requirement, however, does not prevent a native Linux kernel module from being developed and distributed by a third party, as is the case with zfsonlinux.org (ZOL).

ZOL is a project funded by the Lawrence Livermore National Laboratory to develop a native Linux kernel module for its massive storage requirements and super computers.

Playing with ZFS

The rest of this article cover basic setup and usage of ZFS on physical block devices (HDD and SSD for example). Users wishing to experiment with ZFS on virtual block devices (known in ZFS terms as VDEVs) which can be simple files like ~/zfs0.img ~/zfs1.img ~/zfs2.img etc. with no possibility of real data loss are encouraged to see the Playing_with_ZFS article. Common tasks like building a RAIDZ array, purposefully corrupting data and recovering it, etc. are covered.

Installation

The requisite packages are available in the AUR and in an unofficial repo. Details are provided on the ZFS Installation article.

Configuration

ZFS is considered a "zero administration" filesystem by its creators; therefore, configuring ZFS is very straight forward. Configuration is done primarily with two commands: zfs and zpool.

mkinitramfs hook

Users wishing to use the root filesystem on ZFS need to add the ZFS hook to mkinitcpio.conf. Others may omit this step.

Modify the kernel parameters to include the dataset holding the root filesystem by one of these methods:

Use zfs=bootfs to use the ZFS bootfs (set via zpool set bootfs=rpool/ROOT/arch rpool)
Set the kernel parameters to zfs=<pool>/<dataset> to boot directly from a ZFS dataset.

To see all available options for the ZFS hook:

 $ mkinitcpio -H zfs

To use the mkinitcpio hook, user need to add zfs to the HOOKS array in /etc/mkinitcpio.conf:

/etc/mkinitcpio.conf

...
HOOKS="base udev autodetect modconf encrypt zfs filesystems usbinput"
...

Note: It is not necessary to use the "fsck" hook with ZFS. ZFS automatically fixes any errors that occur within the filesystem. However, if the hook is required for another filesystem used on the system, such as ext4, the current ZFS packaging implementation does not yet properly handle fsck requests from mkinitcpio and an error is produced when generating a new ramdisk.

It is important to place this after any hooks which are needed to prepare the drive before it is mounted. For example, if the ZFS volume is encrypted, then place encrypt before the zfs hook to unlock it first.

Recreate the ramdisk:

 # mkinitcpio -p linux

Automatic Start

For ZFS to live by its "zero administration" namesake, the zfs daemon must be loaded at startup. A benefit to this is that it is not necessary to mount the zpool in /etc/fstab; the zfs daemon imports and mounts one zfs pool automatically. The daemon mounts a zfs pool reading the file /etc/zfs/zpool.cache.

Set a pool as to be automatically mounted by the zfs daemon:

# zpool set cachefile=/etc/zfs/zpool.cache <pool>

Systemd

Enable the service so it is automatically started at boot time:

 # systemctl enable zfs.service

To manually start the daemon:

 # systemctl start zfs.service

Create a storage pool

Use # parted --list to see a list of all available drives. It is not necessary nor recommended to partition the drives before creating the zfs filesystem.

Having identified the list of drives, it is now time to get the id's of the drives to add to the zpool. The zfs on Linux developers recommend using device ids when creating ZFS storage pools of less than 10 devices. To find the id's, simply:

 $ ls -lah /dev/disk/by-id/

The ids should look similar to the following:

 lrwxrwxrwx 1 root root  9 Aug 12 16:26 ata-ST3000DM001-9YN166_S1F0JKRR -> ../../sdc
 lrwxrwxrwx 1 root root  9 Aug 12 16:26 ata-ST3000DM001-9YN166_S1F0JTM1 -> ../../sde
 lrwxrwxrwx 1 root root  9 Aug 12 16:26 ata-ST3000DM001-9YN166_S1F0KBP8 -> ../../sdd
 lrwxrwxrwx 1 root root  9 Aug 12 16:26 ata-ST3000DM001-9YN166_S1F0KDGY -> ../../sdb

Now, finally, create the ZFS pool:

 # zpool create -f -m <mount> <pool> raidz <ids>

create: subcommand to create the pool.

-f: Force creating the pool. This is to overcome the "EFI label error". See #does not contain an EFI label.

-m: The mount point of the pool. If this is not specified, than the pool will be mounted to /<pool>.

pool: This is the name of the pool.

raidz: This is the type of virtual device that will be created from the pool of devices. Raidz is a special implementation of raid5. See Jeff Bonwick's Blog -- RAID-Z for more information about raidz.

ids: The names of the drives or partitions that to include into the pool. Get it from /dev/disk/by-id.

Here is an example for the full command:

 # zpool create -f -m /mnt/data bigdata raidz ata-ST3000DM001-9YN166_S1F0KDGY ata-ST3000DM001-9YN166_S1F0JKRR ata-ST3000DM001-9YN166_S1F0KBP8 ata-ST3000DM001-9YN166_S1F0JTM1

In case Advanced Format disks are used which have a native sector size of 4096 bytes instead of 512 bytes, the automated sector size detection algorithm of ZFS might detect 512 bytes because the backwards compatibility with legacy systems. This would result in degraded performance. To make sure a correct sector size is used, the ashift=12 option should be used (See the ZFS on Linux FAQ). The full command would in this case be:

 # zpool create -f -o ashift=12 -m /mnt/data bigdata raidz ata-ST3000DM001-9YN166_S1F0KDGY ata-ST3000DM001-9YN166_S1F0JKRR ata-ST3000DM001-9YN166_S1F0KBP8 ata-ST3000DM001-9YN166_S1F0JTM1

If the command is successful, there will be no output. Using the $ mount command will show that the pool is mounted. Using # zpool status will show that the pool has been created.

# zpool status

  pool: bigdata
 state: ONLINE
 scan: none requested
config:

        NAME                                       STATE     READ WRITE CKSUM
        bigdata                                    ONLINE       0     0     0
          -0                                       ONLINE       0     0     0
            ata-ST3000DM001-9YN166_S1F0KDGY-part1  ONLINE       0     0     0
            ata-ST3000DM001-9YN166_S1F0JKRR-part1  ONLINE       0     0     0
            ata-ST3000DM001-9YN166_S1F0KBP8-part1  ONLINE       0     0     0
            ata-ST3000DM001-9YN166_S1F0JTM1-part1  ONLINE       0     0     0

errors: No known data errors

At this point it would be good to reboot the machine to ensure that the ZFS pool is mounted at boot. It is best to deal with all errors before transferring data.

Tuning

Although many knobs are available on a zfs pool, there are two major ones user can consider:

atime
compression

Atime is enabled by default but for most users, it represents superfluous writes to the zpool and it can be disabled using the zfs command:

# zfs set atime=off <pool>

Compression is just that, transparent compression of data. Consult the man page for various options. A recent advancement is the lz4 algorithm which offers excellent compression and performance. Enable it (or any other) using the zfs command:

# zfs set compression=lz4 <pool>

Other options for zfs can be displayed again, using the zfs command:

# sudo zfs get all <pool>

Usage

To see all the commands available in ZFS, use :

 $ man zfs

or:

 $ man zpool

Scrub

ZFS pools should be scrubbed at least once a week. To scrub the pool:

 # zpool scrub <pool>

To do automatic scrubbing once a week, set the following line in the root crontab:

# crontab -e

...
30 19 * * 5 zpool scrub <pool>
...

Replace <pool> with the name of the ZFS pool.

Check zfs pool status

To print a nice table with statistics about the ZFS pool, including and read/write errors, use

 # zpool status -v

Destroy a storage pool

ZFS makes it easy to destroy a mounted storage pool, removing all metadata about the ZFS device. This command destroys any data contained in the pool:

 # zpool destroy <pool>

And now when checking the status:

# zpool status

no pools available

To find the name of the pool, see #Check zfs pool status.

Export a storage pool

If a storage pool is to be used on another system, it will first need to be exported. It is also necessary to export a pool if it has been imported from the archiso as the hostid is different in the archiso as it is in the booted system. The zpool command will refuse to import any storage pools that have not been exported. It is possible to force the import with the -f argument, but this is considered bad form.

Any attempts made to import an un-exported storage pool will result in an error stating the storage pool is in use by another system. This error can be produced at boot time abruptly abandoning the system in the busybox console and requiring an archiso to do an emergency repair by either exporting the pool, or adding the zfs_force=1 to the kernel boot parameters (which is not ideal). See #On boot the zfs pool does not mount stating: "pool may be in use from other system"

To export a pool,

 # zpool export bigdata

Rename a Zpool

Renaming a zpool that is already created is accomplished in 2 steps:

# zpool export oldname
# zpool import oldname newname

Setting a Different Mount Point

The mount point for a given zpool can be moved at will with one command:

# zfs set mountpoint=/foo/bar poolname

Swap volume

ZFS does not allow to use swapfiles, but users can use a ZFS volume (ZVOL) as swap. It is importart to set the ZVOL block size to match the system page size, which can be obtained by the getconf PAGESIZE command (default on x86_64 is 4KiB). Other options useful for keeping the system running well in low-memory situations are keeping it always synced, turning off compression, and not caching the zvol data.

Create a 8GiB zfs volume:

 # zfs create -V 8G -b $(getconf PAGESIZE) \
              -o compression=off \
              -o primarycache=metadata \
              -o secondarycache=none \
              -o sync=always \
              -o com.sun:auto-snapshot=false <pool>/swap

Prepare it as swap partition:

 # mkswap -f /dev/zvol/<pool>/swap

To make it permanent, edit /etc/fstab. ZVOLs support discard, which can potentially help ZFS's block allocator and reduce fragmentation for all other datasets when/if swap is not full.

Add a line to /etc/fstab:

 /dev/zvol/<pool>/swap none swap discard 0 0

Automatic snapshots

The zfs-auto-snapshot-git^AUR package from AUR provides a shell script to automate the management of snapshots, with each named by date and label (hourly, daily, etc), giving quick and convenient snapshotting of all ZFS datasets. The package also installs cron tasks for quarter-hourly, hourly, daily, weekly, and monthly snapshots. Optionally adjust the --keep parameter from the defaults depending on how far back the snapshots are to go (the monthly script by default keeps data for up to a year).

To prevent a dataset from being snapshotted at all, set com.sun:auto-snapshot=false on it. Likewise, set more fine-grained control as well by label, if, for example, no monthlies are to be kept on a snapshot, for example, set com.sun:auto-snapshot:monthly=false.

Troubleshooting

does not contain an EFI label

The following error will occur when attempting to create a zfs filesystem,

 /dev/disk/by-id/<id> does not contain an EFI label but it may contain partition

The way to overcome this is to use -f with the zfs create command.

No hostid found

An error that occurs at boot with the following lines appearing before initscript output:

 ZFS: No hostid found on kernel command line or /etc/hostid.

This warning occurs because the ZFS module does not have access to the spl hosted. There are two solutions, for this. You can either place your spl hostid in the kernel parameters in your boot loader. For example, adding spl.spl_hostid=0x00bab10c.

The other solution is to make sure that there is a hostid in /etc/hostid, and then regenerate the initramfs image. Which will copy the hostid into the initramfs image.

 # mkinitcpio -p linux

On boot the zfs pool does not mount stating: "pool may be in use from other system"

Unexported pool

If the new installation does not boot because the zpool cannot be imported, you will need to chroot into the installation and properly export the zpool. See ZFS#Emergency chroot repair with archzfs.

Once inside the chroot environment, load the ZFS module and force import the zpool,

# zpool import -a -f

now export the pool:

# zpool export <pool>

To see your available pools, use,

# zpool status

It is necessary to export a pool because of the way ZFS uses the hostid to track the system the zpool was created on. The hostid is generated partly based on your network setup. During the installation in the archiso your network configuration could be different generating a different hostid than the one contained in your new installation. Once the zfs filesystem is exported and then re-imported in the new installation, the hostid is reset. See Re: Howto zpool import/export automatically? - msg#00227.

If ZFS complains about "pool may be in use" after every reboot, you should properly export pool as described above, and then rebuild ramdisk in normally booted system:

# mkinitcpio -p linux

Incorrect hostid

First of all double check you actually exported the pool correctly. Exporting the zpool clears the hostid marking the ownership. So during the first boot the zpool should mount correctly. If it does not there is some other problem.

Reboot again, if the zfs pool refuses to mount it means your hostid is not yet correctly set in the early boot phase and it confuses zfs. So you have to manually tell zfs the correct number, once the hostid is coherent across the reboots the zpool will mount correctly.

Boot using zfs_force and write down your hostid. This one is just an example.

% hostid
0a0af0f8

You can always ignore the check adding zfs_force=1 in your kernel parameters, but it is not advisable as a permanent solution.

Tips and tricks

Embed the archzfs packages into an archiso

It is a good idea make an installation media with the needed software included. Otherwise, you will need the latest archiso installation media burned to a CD or a USB key.

To embed zfs in the archiso, from an existing install, download the archiso package.

# pacman -S archiso

Start the process:

# cp -r /usr/share/archiso/configs/releng /root/media

Edit the packages.x86_64 file adding those lines:

spl-utils
spl
zfs-utils
zfs

Edit the pacman.conf file adding those lines (TODO, correctly embed keys in the installation media?):

[demz-repo-archiso]
SigLevel = Never
Server = http://demizerone.com/$repo/$arch

Add other packages in packages.both, packages.i686, or packages.x86_64 if needed and create the image.

# ./build.sh -v

The image will be in the /root/media/out directory.

More informations about the process can be read in this guide or in the Archiso article.

If you are installing onto a UEFI system, see Unified Extensible Firmware Interface#Create UEFI bootable USB from ISO for creating UEFI compatible installation media.

Emergency chroot repair with archzfs

Here is how to use the archiso to get into your ZFS filesystem for maintenance.

Boot the latest archiso and bring up your network:

   # wifi-menu
   # ip link set eth0 up

Test the network connection:

   # ping google.com

Sync the pacman package database:

   # pacman -Syy

(optional) Install your favorite text editor:

   # pacman -S vim

Add archzfs archiso repository to pacman.conf:

/etc/pacman.conf

[demz-repo-archiso]
Server = http://demizerone.com/$repo/$arch

Sync the pacman package database:

   # pacman -Syy

Install the ZFS package group:

   # pacman -S archzfs

Load the ZFS kernel modules:

   # modprobe zfs

Import your pool:

   # zpool import -a -R /mnt

Mount your boot partitions (if you have them):

   # mount /dev/sda2 /mnt/boot
   # mount /dev/sda1 /mnt/boot/efi

Chroot into your ZFS filesystem:

   # arch-chroot /mnt /bin/bash

Check your kernel version:

   # pacman -Qi linux
   # uname -r

uname will show the kernel version of the archiso. If they are different, you will need to run depmod (in the chroot) with the correct kernel version of your chroot installation:

   # depmod -a 3.6.9-1-ARCH (version gathered from pacman -Qi linux)

This will load the correct kernel modules for the kernel version installed in your chroot installation.

Regenerate your ramdisk:

   # mkinitcpio -p linux

There should be no errors.

ZFS

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