Features of ZFS include: pooled storage (integrated volume management – zpool), Copy-on-write, snapshots, data integrity verification and automatic repair (scrubbing), RAID-Z, a maximum 16 Exabyte file size, and a maximum 256 Quadrillion Zettabytes storage with no limit on number of filesystems (datasets) or files. 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 CDDL, and thus incompatible with GPL, 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.
As a result:
- ZFSonLinux project must keep up with Linux kernel versions. After making stable ZFSonLinux release - Arch ZFS maintainers release them.
- This situation sometimes locks down the normal rolling update process by unsatisfied dependencies because the new kernel version, proposed by update, is unsupported by ZFSonLinux.
- 1 Installation
- 2 Experimenting with ZFS
- 3 Configuration
- 4 Creating a storage pool
- 5 Tuning
- 6 Usage
- 6.1 Native encryption
- 6.2 Scrub
- 6.3 Check zfs pool status
- 6.4 Destroy a storage pool
- 6.5 Exporting a storage pool
- 6.6 Renaming a zpool
- 6.7 Setting a different mount point
- 6.8 Access Control Lists
- 6.9 Swap volume
- 6.10 Automatic snapshots
- 7 Troubleshooting
- 7.1 Creating a zpool fails
- 7.2 ZFS is using too much RAM
- 7.3 Does not contain an EFI label
- 7.4 No hostid found
- 7.5 Pool cannot be found while booting from SAS/SCSI devices
- 7.6 On boot the zfs pool does not mount stating: "pool may be in use from other system"
- 7.7 Devices have different sector alignment
- 8 Tips and tricks
- 9 See also
- stable releases. AUR for
- development releases (with support of newer kernel versions). AUR for
- AUR for stable releases for LTS kernels.
- development releases for LTS kernels. AUR for
- AUR for stable releases for hardened kernels.
- development releases for hardened kernels. AUR for
- AUR for stable releases for zen kernels.
- development releases for zen kernels. AUR for
- AUR for versions with dynamic kernel module support.
- development releases for versions with dynamic kernel module support. AUR for
These branches have (according to them) dependencies on the
spl-utils packages. SPL (Solaris Porting Layer) is a Linux Kernel module implementing Solaris APIs for ZFS compatibility.
Test the installation by issuing
zpool status on the command line. If an "insmod" error is produced, try
Root on ZFS
When performing an Arch install on ZFS,AUR or AUR and its dependencies can be installed in the archiso environment as outlined in the previous section.
Users can make use of DKMS Dynamic Kernel Module Support to rebuild the ZFS modules automatically with every kernel upgrade.
InstallAUR or AUR and apply the post-install instructions given by these packages.
IgnorePkgentry to pacman.conf to prevent these packages from upgrading when doing a regular update.
Experimenting with ZFS
Users wishing to experiment with ZFS on virtual block devices (known in ZFS terms as VDEVs) which can be simple files like
~/zfs2.img etc. with no possibility of real data loss are encouraged to see the Experimenting with ZFS article. Common tasks like building a RAIDZ array, purposefully corrupting data and recovering it, snapshotting datasets, etc. are covered.
ZFS is considered a "zero administration" filesystem by its creators; therefore, configuring ZFS is very straight forward. Configuration is done primarily with two commands:
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 can import and mount zfs pools automatically. The daemon mounts the zfs pools reading the file
For each pool you want automatically mounted by the zfs daemon execute:
# zpool set cachefile=/etc/zfs/zpool.cache <pool>
Enable the service so it is automatically started at boot time:
# systemctl enable zfs.target
To manually start the daemon:
# systemctl start zfs.target
See https://github.com/archzfs/archzfs/issues/72 for more information.
In order to mount zfs pools automatically on boot you need to enable the following services and targets:
# systemctl enable zfs-import-cache # systemctl enable zfs-mount # systemctl enable zfs-import.target
# systemctl preset $(tail -n +2 /usr/lib/systemd/system-preset/50-zfs.preset | cut -d ' ' -f 2)
Creating a storage pool
# 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 IDs 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 IDs, simply:
# ls -lh /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
Disk labels and UUID can also be used for ZFS mounts by using GPT partitions. ZFS drives have labels but Linux is unable to read them at boot. Unlike MBR partitions, GPT partitions directly support both UUID and labels independent of the format inside the partition. Partitioning rather than using the whole disk for ZFS offers two additional advantages. The OS does not generate bogus partition numbers from whatever unpredictable data ZFS has written to the partition sector, and if desired, you can easily over provision SSD drives, and slightly over provision spindle drives to ensure that different models with slightly different sector counts can zpool replace into your mirrors. This is a lot of organization and control over ZFS using readily available tools and techniques at almost zero cost.
Use gdisk to partition the all or part of the drive as a single partition. gdisk does not automatically name partitions so if partition labels are desired use gdisk command "c" to label the partitions. Some reasons you might prefer labels over UUID are: labels are easy to control, labels can be titled to make the purpose of each disk in your arrangement readily apparent, and labels are shorter and easier to type. These are all advantages when the server is down and the heat is on. GPT partition labels have plenty of space and can store most international characters wikipedia:GUID_Partition_Table#Partition_entries allowing large data pools to be labeled in an organized fashion.
Drives partitioned with GPT have labels and UUID that look like this.
# ls -l /dev/disk/by-partlabel # lrwxrwxrwx 1 root root 10 Apr 30 01:44 zfsdata1 -> ../../sdd1 # lrwxrwxrwx 1 root root 10 Apr 30 01:44 zfsdata2 -> ../../sdc1 # lrwxrwxrwx 1 root root 10 Apr 30 01:59 zfsl2arc -> ../../sda1
# ls -l /dev/disk/by-partuuid # lrwxrwxrwx 1 root root 10 Apr 30 01:44 148c462c-7819-431a-9aba-5bf42bb5a34e -> ../../sdd1 # lrwxrwxrwx 1 root root 10 Apr 30 01:59 4f95da30-b2fb-412b-9090-fc349993df56 -> ../../sda1 # lrwxrwxrwx 1 root root 10 Apr 30 01:44 e5ccef58-5adf-4094-81a7-3bac846a885f -> ../../sdc1
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, then the pool will be mounted to
- 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. The usage of mirror instead may be better when using RAID-1 .
- ids: The names of the drives or partitions that to include into the pool. Get it from
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
Advanced Format disks
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 of 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
Verifying pool creation
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.
GRUB-compatible pool creation
By default, zpool create enables all features on a pool. If
/boot resides on ZFS when using GRUB you must only enable features supported by GRUB otherwise GRUB will not be able to read the pool. GRUB 2.02 supports the read-write features
large_blocks; this is not suitable for all the features of ZFSonLinux 0.7.1, and must have unsupported features disabled.
You can create a pool with the incompatible features disabled:
# zpool create -o feature@multi_vdev_crash_dump=disabled \ -o feature@large_dnode=disabled \ -o feature@sha512=disabled \ -o feature@skein=disabled \ -o feature@edonr=disabled \ $POOL_NAME $VDEVS
When running the git version of ZFS on Linux, make sure to also add
Importing a pool created by id
Eventually a pool may fail to auto mount and you need to import to bring your pool back. Take care to avoid the most obvious solution.
# ###zpool import zfsdata # Do not do this! Always use -d
This will import your pools using
/dev/sd? which will lead to problems the next time you rearrange your drives. This may be as simple as rebooting with a USB drive left in the machine, which harkens back to a time when PCs would not boot when a floppy disk was left in a machine. Adapt one of the following commands to import your pool so that pool imports retain the persistence they were created with.
# zpool import -d /dev/disk/by-id zfsdata # zpool import -d /dev/disk/by-partlabel zfsdata # zpool import -d /dev/disk/by-partuuid zfsdata
Many parameters are available for zfs file systems, you can view a full list with
zfs get all <pool>. Two common ones to adjust are atime and 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>
As an alternative to turning off atime completely, relatime is available. This brings the default ext4/xfs atime semantics to ZFS, where access time is only updated if the modified time or changed time changes, or if the existing access time has not been updated within the past 24 hours. It is a compromise between atime=off and atime=on. This property only takes effect if atime is on:
# zfs set relatime=on <pool>
Compression is just that, transparent compression of data. ZFS supports a few different algorithms, presently lz4 is the default. gzip is also available for seldom-written yet highly-compressable data; consult the man page for more details. Enable compression using the zfs command:
# zfs set compression=on <pool>
Other options for zfs can be displayed again, using the zfs command:
# zfs get all <pool>
You can also add SSD devices as a write intent log (external ZIL or SLOG) and also as a layer 2 adaptive replacement cache (l2arc). The process to add them is very similar to creating a new VDEV.
All of the below references to device-id are the IDs from
To add a ZIL:
# zpool add <pool> log <device-id>
Or to add a mirrored ZIL:
# zpool add <pool> log mirror <device-id-1> <device-id-2>
To add an l2arc:
# zpool add <pool> cache <device-id>
Or to add a mirrored l2arc:
# zpool add <pool> cache mirror <device-id-1> <device-id-2>
ZFS, unlike most other file systems, has a variable record size, or what is commonly referred to as a block size. By default, the recordsize on ZFS is 128KiB, which means it will dynamically allocate blocks of any size from 512B to 128KiB depending on the size of file being written. This can often help fragmentation and file access, at the cost that ZFS would have to allocate new 128KiB blocks each time only a few bytes are written to.
Most RDBMSes work in 8KiB-sized blocks by default. Although the block size is tunable for MySQL/MariaDB, PostgreSQL, and Oracle, all three of them use an 8KiB block size by default. For both performance concerns and keeping snapshot differences to a minimum (for backup purposes, this is helpful), it is usually desirable to tune ZFS instead to accommodate the databases, using a command such as:
# zfs set recordsize=8K <pool>/postgres
These RDBMSes also tend to implement their own caching algorithm, often similar to ZFS's own ARC. In the interest of saving memory, it is best to simply disable ZFS's caching of the database's file data and let the database do its own job:
# zfs set primarycache=metadata <pool>/postgres
If your pool has no configured log devices, ZFS reserves space on the pool's data disks for its intent log (the ZIL). ZFS uses this for crash recovery, but databases are often syncing their data files to the file system on their own transaction commits anyway. The end result of this is that ZFS will be committing data twice to the data disks, and it can severely impact performance. You can tell ZFS to prefer to not use the ZIL, and in which case, data is only committed to the file system once. Setting this for non-database file systems, or for pools with configured log devices, can actually negatively impact the performance, so beware:
# zfs set logbias=throughput <pool>/postgres
These can also be done at file system creation time, for example:
# zfs create -o recordsize=8K \ -o primarycache=metadata \ -o mountpoint=/var/lib/postgres \ -o logbias=throughput \ <pool>/postgres
Please note: these kinds of tuning parameters are ideal for specialized applications like RDBMSes. You can easily hurt ZFS's performance by setting these on a general-purpose file system such as your /home directory.
If you would like to use ZFS to store your /tmp directory, which may be useful for storing arbitrarily-large sets of files or simply keeping your RAM free of idle data, you can generally improve performance of certain applications writing to /tmp by disabling file system sync. This causes ZFS to ignore an application's sync requests (eg, with
O_SYNC) and return immediately. While this has severe application-side data consistency consequences (never disable sync for a database!), files in /tmp are less likely to be important and affected. Please note this does not affect the integrity of ZFS itself, only the possibility that data an application expects on-disk may not have actually been written out following a crash.
# zfs set sync=disabled <pool>/tmp
Additionally, for security purposes, you may want to disable setuid and devices on the /tmp file system, which prevents some kinds of privilege-escalation attacks or the use of device nodes:
# zfs set setuid=off <pool>/tmp # zfs set devices=off <pool>/tmp
Combining all of these for a create command would be as follows:
# zfs create -o setuid=off -o devices=off -o sync=disabled -o mountpoint=/tmp <pool>/tmp
Please note, also, that if you want /tmp on ZFS, you will need to mask (disable) systemd's automatic tmpfs-backed /tmp, else ZFS will be unable to mount your dataset at boot-time or import-time:
# systemctl mask tmp.mount
ZFS volumes (ZVOLs) can suffer from the same block size-related issues as RDBMSes, but it is worth noting that the default recordsize for ZVOLs is 8KiB already. If possible, it is best to align any partitions contained in a ZVOL to your recordsize (current versions of fdisk and gdisk by default automatically align at 1MiB segments, which works), and file system block sizes to the same size. Other than this, you might tweak the recordsize to accommodate the data inside the ZVOL as necessary (though 8KiB tends to be a good value for most file systems, even when using 4KiB blocks on that level).
RAIDZ and Advanced Format physical disks
Each block of a ZVOL gets its own parity disks, and if you have physical media with logical block sizes of 4096B, 8192B, or so on, the parity needs to be stored in whole physical blocks, and this can drastically increase the space requirements of a ZVOL, requiring 2× or more physical storage capacity than the ZVOL's logical capacity. Setting the recordsize to 16k or 32k can help reduce this footprint drastically.
Users can optionally create a dataset under the zpool as opposed to manually creating directories under the zpool. Datasets allow for an increased level of control (quotas for example) in addition to snapshots. To be able to create and mount a dataset, a directory of the same name must not pre-exist in the zpool. To create a dataset, use:
# zfs create <nameofzpool>/<nameofdataset>
It is then possible to apply ZFS specific attributes to the dataset. For example, one could assign a quota limit to a specific directory within a dataset:
# zfs set quota=20G <nameofzpool>/<nameofdataset>/<directory>
To see all the commands available in ZFS, use :
$ man zfs
$ man zpool
Native ZFS encryption has been made available in 0.7.0.r26 or newer provided by packages likeAUR, AUR or other development builds. Despite the fact that version 0.7 has been released, this feature is still not enabled in the stable version as of 0.7.3, so a development build still needs to be used. An easy way of telling if encryption is available in the version of zfs you have installed is to check for the ZFS_PROP_ENCRYPTION definition in /usr/src/zfs-*/include/sys/fs/zfs.h.
- Supported encryption options:
aes-256-gcm. When encryption is set to
aes-256-ccmwill be used.
- Supported keyformats:
You can also specify iterations of PBKDF2 with
-o pbkdf2iters <n> (it takes time to decrypt the key)
To create a dataset including native encryption with a passphrase, use:
# zfs create -o encryption=on -o keyformat=passphrase <nameofzpool>/<nameofdataset>
To use a key instead of using a passphrase:
# dd if=/dev/urandom of=/path/to/key bs=1 count=32 # zfs create -o encryption=on -o keyformat=raw -o keylocation=file:///path/to/key <nameofzpool>/<nameofdataset>
You can also manually load the keys and then mount the encrypted dataset:
# zfs load-key <nameofzpool>/<nameofdataset> # load key for a specific dataset # zfs load-key -a # load all keys # zfs load-key -r zpool/dataset # load all keys in a dataset
When importing a pool that contains encrypted datasets: ZFS will by default not decrypt these datasets. To do this use
# zpool import -l pool
You can automate this at boot with a custom systemd unit. For example:
[Unit] Description=Load storage encryption keys DefaultDependencies=no Before=systemd-user-sessions.service Before=zfs-mount.service [Service] Type=oneshot RemainAfterExit=yes ExecStart=/usr/bin/bash -c 'systemd-ask-password "Encrypted storage password (%i): " | /usr/bin/zfs load-key zpool/%i' [Install] WantedBy=zfs-mount.service
and enable a service instance for each encrypted volume:
# systemctl enable zfs-key@dataset.
The Before= reference to systemd-user-sessions.service ensures that systemd-ask-password is invoked before the local IO devices are handed over to the system UI
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> ...
<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.
Exporting 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 <pool>
Renaming 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
Access Control Lists
To use ACL on a ZFS pool:
# zfs set acltype=posixacl <nameofzpool>/<nameofdataset> # zfs set xattr=sa <nameofzpool>/<nameofdataset>
xattr is recommended for performance reasons .
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). Another option useful for keeping the system running well in low-memory situations is not caching the ZVOL data.
Create a 8 GiB zfs volume:
# zfs create -V 8G -b $(getconf PAGESIZE) \ -o logbias=throughput -o sync=always\ -o primarycache=metadata \ -o com.sun:auto-snapshot=false <pool>/swap
Prepare it as swap partition:
# mkswap -f /dev/zvol/<pool>/swap # swapon /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
/dev/zvol/<pool>/swap none swap discard 0 0
Keep in mind the Hibernate hook must be loaded before filesystems, so using ZVOL as swap will not allow to use hibernate function. If you need hibernate, keep a partition for it.
ZFS Automatic Snapshot Service for Linux
The 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).AUR package from
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
ZFS Snapshot Manager
The AUR provides a python service that takes daily snapshots from a configurable set of ZFS datasets and cleans them out in a "Grandfather-father-son" scheme. It can be configured to e.g. keep 7 daily, 5 weekly, 3 monthly and 2 yearly snapshots.AUR package from
The package also supports configurable replication to other machines running ZFS by means of
zfs send and
zfs receive. If the destination machine runs this package as well, it could be configured to keep these replicated snapshots for a longer time. This allows a setup where a source machine has only a few daily snapshots locally stored, while on a remote storage server a much longer retention is available.
Creating a zpool fails
If the following error occurs then it can be fixed.
# the kernel failed to rescan the partition table: 16 # cannot label 'sdc': try using parted(8) and then provide a specific slice: -1
One reason this can occur is because ZFS expects pool creation to take less than 1 second. This is a reasonable assumption under ordinary conditions, but in many situations it may take longer. Each drive will need to be cleared again before another attempt can be made.
# parted /dev/sda rm 1 # parted /dev/sda rm 1 # dd if=/dev/zero of=/dev/sdb bs=512 count=1 # zpool labelclear /dev/sda
A brute force creation can be attempted over and over again, and with some luck the ZPool creation will take less than 1 second. Once cause for creation slowdown can be slow burst read writes on a drive. By reading from the disk in parallell to ZPool creation, it may be possible to increase burst speeds.
# dd if=/dev/sda of=/dev/null
This can be done with multiple drives by saving the above command for each drive to a file on separate lines and running
# cat $FILE | parallel
Then run ZPool creation at the same time.
ZFS is using too much RAM
zfs.zfs_arc_max=536870912 # (for 512MB)
For a more detailed description, as well as other configuration options, see gentoo-wiki:zfs#arc.
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. Either place the spl hostid in the kernel parameters in the boot loader. For example, adding
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
Pool cannot be found while booting from SAS/SCSI devices
In case you are booting a SAS/SCSI based, you might occassionally get boot problems where the pool you are trying to boot from cannot be found. A likely reason for this is that your devices are initialized too late into the process. That means that zfs cannot find any devices at the time when it tries to assemble your pool.
In this case you should force the scsi driver to wait for devices to come online before continuing. You can do this by putting this into
options scsi_mod scan=sync
Afterwards, regenerate the initramfs.
This works because the zfs hook will copy the file at
/etc/modprobe.d/zfs.conf into the initcpio which will then be used at build time.
On boot the zfs pool does not mount stating: "pool may be in use from other system"
If the new installation does not boot because the zpool cannot be imported, chroot into the installation and properly export the zpool. See #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 the 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 the network setup. During the installation in the archiso the network configuration could be different generating a different hostid than the one contained in the 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, properly export pool as described above, and then rebuild ramdisk in normally booted system:
# mkinitcpio -p linux
Double check that the pool is properly exported. 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 the hostid is not yet correctly set in the early boot phase and it confuses zfs. 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 the hostid. This one is just an example.
% hostid 0a0af0f8
Users can always ignore the check adding
zfs_force=1 in the kernel parameters, but it is not advisable as a permanent solution.
Devices have different sector alignment
Once a drive has become faulted it should be replaced A.S.A.P. with an identical drive.
# zpool replace bigdata ata-ST3000DM001-9YN166_S1F0KDGY ata-ST3000DM001-1CH166_W1F478BD -f
but in this instance, the following error is produced:
cannot replace ata-ST3000DM001-9YN166_S1F0KDGY with ata-ST3000DM001-1CH166_W1F478BD: devices have different sector alignment
ZFS uses the ashift option to adjust for physical block size. When replacing the faulted disk, ZFS is attempting to use
ashift=12, but the faulted disk is using a different ashift (probably
ashift=9) and this causes the resulting error.
Use zdb to find the ashift of the zpool:
zdb , then use the
-o argument to set the ashift of the replacement drive:
# zpool replace bigdata ata-ST3000DM001-9YN166_S1F0KDGY ata-ST3000DM001-1CH166_W1F478BD -o ashift=9 -f
Check the zpool status for confirmation:
# zpool status -v
pool: bigdata state: DEGRADED status: One or more devices is currently being resilvered. The pool will continue to function, possibly in a degraded state. action: Wait for the resilver to complete. scan: resilver in progress since Mon Jun 16 11:16:28 2014 10.3G scanned out of 5.90T at 81.7M/s, 20h59m to go 2.57G resilvered, 0.17% done config: NAME STATE READ WRITE CKSUM bigdata DEGRADED 0 0 0 raidz1-0 DEGRADED 0 0 0 replacing-0 OFFLINE 0 0 0 ata-ST3000DM001-9YN166_S1F0KDGY OFFLINE 0 0 0 ata-ST3000DM001-1CH166_W1F478BD ONLINE 0 0 0 (resilvering) ata-ST3000DM001-9YN166_S1F0JKRR ONLINE 0 0 0 ata-ST3000DM001-9YN166_S1F0KBP8 ONLINE 0 0 0 ata-ST3000DM001-9YN166_S1F0JTM1 ONLINE 0 0 0 errors: No known data errors
Tips and tricks
Embed the archzfs packages into an archiso
Follow the Archiso steps for creating a fully functional Arch Linux live CD/DVD/USB image.
Enable the archzfs repository:
... [archzfs] Server = http://archzfs.com/$repo/x86_64
archzfs-linux group to the list of packages to be installed (the
archzfs repository provides packages for the x86_64 architecture only).
Complete Build the ISO to finally build the iso.
Encryption in ZFS using dm-crypt
The stable release version of ZFS on Linux does not support encryption directly, but zpools can be created in dm-crypt block devices. Since the zpool is created on the plain-text abstraction, it is possible to have the data encrypted while having all the advantages of ZFS like deduplication, compression, and data robustness.
dm-crypt, possibly via LUKS, creates devices in
/dev/mapper and their name is fixed. So you just need to change
zpool create commands to point to that names. The idea is configuring the system to create the
/dev/mapper block devices and import the zpools from there. Since zpools can be created in multiple devices (raid, mirroring, striping, ...), it is important all the devices are encrypted otherwise the protection might be partially lost.
For example, an encrypted zpool can be created using plain dm-crypt (without LUKS) with:
# cryptsetup --hash=sha512 --cipher=twofish-xts-plain64 --offset=0 \ --key-file=/dev/sdZ --key-size=512 open --type=plain /dev/sdX enc # zpool create zroot /dev/mapper/enc
In the case of a root filesystem pool, the
mkinitcpio.conf HOOKS line will enable the keyboard for the password, create the devices, and load the pools. It will contain something like:
HOOKS="... keyboard encrypt zfs ..."
/dev/mapper/enc name is fixed no import errors will occur.
Creating encrypted zpools works fine. But if you need encrypted directories, for example to protect your users' homes, ZFS loses some functionality.
ZFS will see the encrypted data, not the plain-text abstraction, so compression and deduplication will not work. The reason is that encrypted data has always high entropy making compression ineffective and even from the same input you get different output (thanks to salting) making deduplication impossible. To reduce the unnecessary overhead it is possible to create a sub-filesystem for each encrypted directory and use eCryptfs on it.
For example to have an encrypted home: (the two passwords, encryption and login, must be the same)
# zfs create -o compression=off -o dedup=off -o mountpoint=/home/<username> <zpool>/<username> # useradd -m <username> # passwd <username> # ecryptfs-migrate-home -u <username> <log in user and complete the procedure with ecryptfs-unwrap-passphrase>
Emergency chroot repair with archzfs
To get into the ZFS filesystem from live system for maintenance, there are two options:
- Build custom archiso with ZFS as described in #Embed the archzfs packages into an archiso.
- Boot the latest official archiso and bring up the network. Then enable archzfs repository inside the live system as usual, sync the pacman package database and install the archzfs-archiso-linux package.
To start the recovery, load the ZFS kernel modules:
# modprobe zfs
Import the pool:
# zpool import -a -R /mnt
Mount the boot partitions (if any):
# mount /dev/sda2 /mnt/boot # mount /dev/sda1 /mnt/boot/efi
Chroot into the ZFS filesystem:
# arch-chroot /mnt /bin/bash
Check the kernel version:
# pacman -Qi linux # uname -r
uname will show the kernel version of the archiso. If they are different, run depmod (in the chroot) with the correct kernel version of the chroot installation:
# depmod -a 3.6.9-1-ARCH (version gathered from pacman -Qi linux but using the matching kernel modules directory name under the chroot's /lib/modules)
This will load the correct kernel modules for the kernel version installed in the chroot installation.
Regenerate the ramdisk:
# mkinitcpio -p linux
There should be no errors.
Here a bind mount from /mnt/zfspool to /srv/nfs4/music is created. The configuration ensures that the zfs pool is ready before the bind mount is created.
/mnt/zfspool /srv/nfs4/music none bind,defaults,nofail,x-systemd.requires=zfs-mount.service 0 0
Monitoring / Mailing on Events
See ZED: The ZFS Event Daemon for more information.
At the minimum, an email forwarder, such as msmtp, is required to accomplish this. Make sure it is working correctly.
Uncomment the following in the configuration file:
ZED_EMAIL_ADDR="root" ZED_EMAIL_PROG="mail" ZED_NOTIFY_VERBOSE=0 ZED_EMAIL_OPTS="-s '@SUBJECT@' @ADDRESS@"
Update 'root' in
ZED_EMAIL_ADDR="root" to the email address you want to receive notifications at.
If you want to receive an email no matter the state of your pool, you will want to set
Start and enable
If you set verbose to 1, you can test by running a scrub.
Wrap shell commands in pre & post snapshots
Since it's so cheap to make a snapshot, we can use this as a measure of security for sensitive commands such as system and package upgrades. If we make a snapshot before, and one after, we can later diff these snapshots to find out what changed on the filesystem after the command executed. Furthermore we can also rollback in case the outcome was not desired.
# zfs snapshot -r zroot@pre # pacman -Syyu # dangerous command # zfs snapshot -r zroot@post # zfs diff zroot@pre zroot@post # zfs rollback zroot@pre
A utility that automates the creation of pre and post snapshots around a shell command is znp.
# znp pacman -Syyu # znp find / -name "something*" -delete
and you would get snapshots created before and after the supplied command, and also output of the commands logged to file for future reference so we know what command created the diff seen in a pair of pre/post snapshots.
- Aaron Toponce's 17-part blog on ZFS
- ZFS on Linux
- ZFS on Linux FAQ
- FreeBSD Handbook -- The Z File System
- Oracle Solaris ZFS Administration Guide
- Solaris Internals -- ZFS Troubleshooting Guide[dead link 2017-05-30]
- How Pingdom uses ZFS to back up 5TB of MySQL data every day
- Tutorial on adding the modules to a custom kernel