19.4. zfs Administration

The zfs utility is responsible for creating, destroying, and managing all ZFS datasets that exist within a pool. The pool is managed using zpool.

19.4.1. Creating and Destroying Datasets

Unlike traditional disks and volume managers, space in ZFS is not preallocated. With traditional file systems, after all of the space is partitioned and assigned, there is no way to add an additional file system without adding a new disk. With ZFS, new file systems can be created at any time. Each dataset has properties including features like compression, deduplication, caching, and quotas, as well as other useful properties like readonly, case sensitivity, network file sharing, and a mount point. Datasets can be nested inside each other, and child datasets will inherit properties from their parents. Each dataset can be administered, delegated, replicated, snapshotted, jailed, and destroyed as a unit. There are many advantages to creating a separate dataset for each different type or set of files. The only drawbacks to having an extremely large number of datasets is that some commands like zfs list will be slower, and the mounting of hundreds or even thousands of datasets can slow the FreeBSD boot process.

Create a new dataset and enable LZ4 compression on it:

# zfs list
NAME                  USED  AVAIL  REFER  MOUNTPOINT
mypool                781M  93.2G   144K  none
mypool/ROOT           777M  93.2G   144K  none
mypool/ROOT/default   777M  93.2G   777M  /
mypool/tmp            176K  93.2G   176K  /tmp
mypool/usr            616K  93.2G   144K  /usr
mypool/usr/home       184K  93.2G   184K  /usr/home
mypool/usr/ports      144K  93.2G   144K  /usr/ports
mypool/usr/src        144K  93.2G   144K  /usr/src
mypool/var           1.20M  93.2G   608K  /var
mypool/var/crash      148K  93.2G   148K  /var/crash
mypool/var/log        178K  93.2G   178K  /var/log
mypool/var/mail       144K  93.2G   144K  /var/mail
mypool/var/tmp        152K  93.2G   152K  /var/tmp
# zfs create -o compress=lz4 mypool/usr/mydataset
# zfs list
NAME                   USED  AVAIL  REFER  MOUNTPOINT
mypool                 781M  93.2G   144K  none
mypool/ROOT            777M  93.2G   144K  none
mypool/ROOT/default    777M  93.2G   777M  /
mypool/tmp             176K  93.2G   176K  /tmp
mypool/usr             704K  93.2G   144K  /usr
mypool/usr/home        184K  93.2G   184K  /usr/home
mypool/usr/mydataset  87.5K  93.2G  87.5K  /usr/mydataset
mypool/usr/ports       144K  93.2G   144K  /usr/ports
mypool/usr/src         144K  93.2G   144K  /usr/src
mypool/var            1.20M  93.2G   610K  /var
mypool/var/crash       148K  93.2G   148K  /var/crash
mypool/var/log         178K  93.2G   178K  /var/log
mypool/var/mail        144K  93.2G   144K  /var/mail
mypool/var/tmp         152K  93.2G   152K  /var/tmp

Destroying a dataset is much quicker than deleting all of the files that reside on the dataset, as it does not involve scanning all of the files and updating all of the corresponding metadata.

Destroy the previously-created dataset:

# zfs list
NAME                   USED  AVAIL  REFER  MOUNTPOINT
mypool                 880M  93.1G   144K  none
mypool/ROOT            777M  93.1G   144K  none
mypool/ROOT/default    777M  93.1G   777M  /
mypool/tmp             176K  93.1G   176K  /tmp
mypool/usr             101M  93.1G   144K  /usr
mypool/usr/home        184K  93.1G   184K  /usr/home
mypool/usr/mydataset   100M  93.1G   100M  /usr/mydataset
mypool/usr/ports       144K  93.1G   144K  /usr/ports
mypool/usr/src         144K  93.1G   144K  /usr/src
mypool/var            1.20M  93.1G   610K  /var
mypool/var/crash       148K  93.1G   148K  /var/crash
mypool/var/log         178K  93.1G   178K  /var/log
mypool/var/mail        144K  93.1G   144K  /var/mail
mypool/var/tmp         152K  93.1G   152K  /var/tmp
# zfs destroy mypool/usr/mydataset
# zfs list
NAME                  USED  AVAIL  REFER  MOUNTPOINT
mypool                781M  93.2G   144K  none
mypool/ROOT           777M  93.2G   144K  none
mypool/ROOT/default   777M  93.2G   777M  /
mypool/tmp            176K  93.2G   176K  /tmp
mypool/usr            616K  93.2G   144K  /usr
mypool/usr/home       184K  93.2G   184K  /usr/home
mypool/usr/ports      144K  93.2G   144K  /usr/ports
mypool/usr/src        144K  93.2G   144K  /usr/src
mypool/var           1.21M  93.2G   612K  /var
mypool/var/crash      148K  93.2G   148K  /var/crash
mypool/var/log        178K  93.2G   178K  /var/log
mypool/var/mail       144K  93.2G   144K  /var/mail
mypool/var/tmp        152K  93.2G   152K  /var/tmp

In modern versions of ZFS, zfs destroy is asynchronous, and the free space might take several minutes to appear in the pool. Use zpool get freeing poolname to see the freeing property, indicating how many datasets are having their blocks freed in the background. If there are child datasets, like snapshots or other datasets, then the parent cannot be destroyed. To destroy a dataset and all of its children, use -r to recursively destroy the dataset and all of its children. Use -n -v to list datasets and snapshots that would be destroyed by this operation, but do not actually destroy anything. Space that would be reclaimed by destruction of snapshots is also shown.

19.4.2. Creating and Destroying Volumes

A volume is a special type of dataset. Rather than being mounted as a file system, it is exposed as a block device under /dev/zvol/poolname/dataset. This allows the volume to be used for other file systems, to back the disks of a virtual machine, or to be exported using protocols like iSCSI or HAST.

A volume can be formatted with any file system, or used without a file system to store raw data. To the user, a volume appears to be a regular disk. Putting ordinary file systems on these zvols provides features that ordinary disks or file systems do not normally have. For example, using the compression property on a 250 MB volume allows creation of a compressed FAT file system.

# zfs create -V 250m -o compression=on tank/fat32
# zfs list tank
NAME USED AVAIL REFER MOUNTPOINT
tank 258M  670M   31K /tank
# newfs_msdos -F32 /dev/zvol/tank/fat32
# mount -t msdosfs /dev/zvol/tank/fat32 /mnt
# df -h /mnt | grep fat32
Filesystem           Size Used Avail Capacity Mounted on
/dev/zvol/tank/fat32 249M  24k  249M     0%   /mnt
# mount | grep fat32
/dev/zvol/tank/fat32 on /mnt (msdosfs, local)

Destroying a volume is much the same as destroying a regular file system dataset. The operation is nearly instantaneous, but it may take several minutes for the free space to be reclaimed in the background.

19.4.3. Renaming a Dataset

The name of a dataset can be changed with zfs rename. The parent of a dataset can also be changed with this command. Renaming a dataset to be under a different parent dataset will change the value of those properties that are inherited from the parent dataset. When a dataset is renamed, it is unmounted and then remounted in the new location (which is inherited from the new parent dataset). This behavior can be prevented with -u.

Rename a dataset and move it to be under a different parent dataset:

# zfs list
NAME                   USED  AVAIL  REFER  MOUNTPOINT
mypool                 780M  93.2G   144K  none
mypool/ROOT            777M  93.2G   144K  none
mypool/ROOT/default    777M  93.2G   777M  /
mypool/tmp             176K  93.2G   176K  /tmp
mypool/usr             704K  93.2G   144K  /usr
mypool/usr/home        184K  93.2G   184K  /usr/home
mypool/usr/mydataset  87.5K  93.2G  87.5K  /usr/mydataset
mypool/usr/ports       144K  93.2G   144K  /usr/ports
mypool/usr/src         144K  93.2G   144K  /usr/src
mypool/var            1.21M  93.2G   614K  /var
mypool/var/crash       148K  93.2G   148K  /var/crash
mypool/var/log         178K  93.2G   178K  /var/log
mypool/var/mail        144K  93.2G   144K  /var/mail
mypool/var/tmp         152K  93.2G   152K  /var/tmp
# zfs rename mypool/usr/mydataset mypool/var/newname
# zfs list
NAME                  USED  AVAIL  REFER  MOUNTPOINT
mypool                780M  93.2G   144K  none
mypool/ROOT           777M  93.2G   144K  none
mypool/ROOT/default   777M  93.2G   777M  /
mypool/tmp            176K  93.2G   176K  /tmp
mypool/usr            616K  93.2G   144K  /usr
mypool/usr/home       184K  93.2G   184K  /usr/home
mypool/usr/ports      144K  93.2G   144K  /usr/ports
mypool/usr/src        144K  93.2G   144K  /usr/src
mypool/var           1.29M  93.2G   614K  /var
mypool/var/crash      148K  93.2G   148K  /var/crash
mypool/var/log        178K  93.2G   178K  /var/log
mypool/var/mail       144K  93.2G   144K  /var/mail
mypool/var/newname   87.5K  93.2G  87.5K  /var/newname
mypool/var/tmp        152K  93.2G   152K  /var/tmp

Snapshots can also be renamed like this. Due to the nature of snapshots, they cannot be renamed into a different parent dataset. To rename a recursive snapshot, specify -r, and all snapshots with the same name in child datasets with also be renamed.

# zfs list -t snapshot
NAME                                USED  AVAIL  REFER  MOUNTPOINT
mypool/var/newname@first_snapshot      0      -  87.5K  -
# zfs rename mypool/var/newname@first_snapshot new_snapshot_name
# zfs list -t snapshot
NAME                                   USED  AVAIL  REFER  MOUNTPOINT
mypool/var/newname@new_snapshot_name      0      -  87.5K  -

19.4.4. Setting Dataset Properties

Each ZFS dataset has a number of properties that control its behavior. Most properties are automatically inherited from the parent dataset, but can be overridden locally. Set a property on a dataset with zfs set property=value dataset. Most properties have a limited set of valid values, zfs get will display each possible property and valid values. Most properties can be reverted to their inherited values using zfs inherit.

User-defined properties can also be set. They become part of the dataset configuration and can be used to provide additional information about the dataset or its contents. To distinguish these custom properties from the ones supplied as part of ZFS, a colon (:) is used to create a custom namespace for the property.

# zfs set custom:costcenter=1234 tank
# zfs get custom:costcenter tank
NAME PROPERTY           VALUE SOURCE
tank custom:costcenter  1234  local

To remove a custom property, use zfs inherit with -r. If the custom property is not defined in any of the parent datasets, it will be removed completely (although the changes are still recorded in the pool's history).

# zfs inherit -r custom:costcenter tank
# zfs get custom:costcenter tank
NAME    PROPERTY           VALUE              SOURCE
tank    custom:costcenter  -                  -
# zfs get all tank | grep custom:costcenter
#

19.4.4.1. Getting and Setting Share Properties

Two commonly used and useful dataset properties are the NFS and SMB share options. Setting these define if and how ZFS datasets may be shared on the network. At present, only setting sharing via NFS is supported on FreeBSD. To get the current status of a share, enter:

# zfs get sharenfs mypool/usr/home
NAME             PROPERTY  VALUE    SOURCE
mypool/usr/home  sharenfs  on       local
# zfs get sharesmb mypool/usr/home
NAME             PROPERTY  VALUE    SOURCE
mypool/usr/home  sharesmb  off      local

To enable sharing of a dataset, enter:

#  zfs set sharenfs=on mypool/usr/home

It is also possible to set additional options for sharing datasets through NFS, such as -alldirs, -maproot and -network. To set additional options to a dataset shared through NFS, enter:

#  zfs set sharenfs="-alldirs,-maproot=root,-network=192.168.1.0/24" mypool/usr/home

19.4.5. Managing Snapshots

Snapshots are one of the most powerful features of ZFS. A snapshot provides a read-only, point-in-time copy of the dataset. With Copy-On-Write (COW), snapshots can be created quickly by preserving the older version of the data on disk. If no snapshots exist, space is reclaimed for future use when data is rewritten or deleted. Snapshots preserve disk space by recording only the differences between the current dataset and a previous version. Snapshots are allowed only on whole datasets, not on individual files or directories. When a snapshot is created from a dataset, everything contained in it is duplicated. This includes the file system properties, files, directories, permissions, and so on. Snapshots use no additional space when they are first created, only consuming space as the blocks they reference are changed. Recursive snapshots taken with -r create a snapshot with the same name on the dataset and all of its children, providing a consistent moment-in-time snapshot of all of the file systems. This can be important when an application has files on multiple datasets that are related or dependent upon each other. Without snapshots, a backup would have copies of the files from different points in time.

Snapshots in ZFS provide a variety of features that even other file systems with snapshot functionality lack. A typical example of snapshot use is to have a quick way of backing up the current state of the file system when a risky action like a software installation or a system upgrade is performed. If the action fails, the snapshot can be rolled back and the system has the same state as when the snapshot was created. If the upgrade was successful, the snapshot can be deleted to free up space. Without snapshots, a failed upgrade often requires a restore from backup, which is tedious, time consuming, and may require downtime during which the system cannot be used. Snapshots can be rolled back quickly, even while the system is running in normal operation, with little or no downtime. The time savings are enormous with multi-terabyte storage systems and the time required to copy the data from backup. Snapshots are not a replacement for a complete backup of a pool, but can be used as a quick and easy way to store a copy of the dataset at a specific point in time.

19.4.5.1. Creating Snapshots

Snapshots are created with zfs snapshot dataset@snapshotname. Adding -r creates a snapshot recursively, with the same name on all child datasets.

Create a recursive snapshot of the entire pool:

# zfs list -t all
NAME                                   USED  AVAIL  REFER  MOUNTPOINT
mypool                                 780M  93.2G   144K  none
mypool/ROOT                            777M  93.2G   144K  none
mypool/ROOT/default                    777M  93.2G   777M  /
mypool/tmp                             176K  93.2G   176K  /tmp
mypool/usr                             616K  93.2G   144K  /usr
mypool/usr/home                        184K  93.2G   184K  /usr/home
mypool/usr/ports                       144K  93.2G   144K  /usr/ports
mypool/usr/src                         144K  93.2G   144K  /usr/src
mypool/var                            1.29M  93.2G   616K  /var
mypool/var/crash                       148K  93.2G   148K  /var/crash
mypool/var/log                         178K  93.2G   178K  /var/log
mypool/var/mail                        144K  93.2G   144K  /var/mail
mypool/var/newname                    87.5K  93.2G  87.5K  /var/newname
mypool/var/newname@new_snapshot_name      0      -  87.5K  -
mypool/var/tmp                         152K  93.2G   152K  /var/tmp
# zfs snapshot -r mypool@my_recursive_snapshot
# zfs list -t snapshot
NAME                                        USED  AVAIL  REFER  MOUNTPOINT
mypool@my_recursive_snapshot                   0      -   144K  -
mypool/ROOT@my_recursive_snapshot              0      -   144K  -
mypool/ROOT/default@my_recursive_snapshot      0      -   777M  -
mypool/tmp@my_recursive_snapshot               0      -   176K  -
mypool/usr@my_recursive_snapshot               0      -   144K  -
mypool/usr/home@my_recursive_snapshot          0      -   184K  -
mypool/usr/ports@my_recursive_snapshot         0      -   144K  -
mypool/usr/src@my_recursive_snapshot           0      -   144K  -
mypool/var@my_recursive_snapshot               0      -   616K  -
mypool/var/crash@my_recursive_snapshot         0      -   148K  -
mypool/var/log@my_recursive_snapshot           0      -   178K  -
mypool/var/mail@my_recursive_snapshot          0      -   144K  -
mypool/var/newname@new_snapshot_name           0      -  87.5K  -
mypool/var/newname@my_recursive_snapshot       0      -  87.5K  -
mypool/var/tmp@my_recursive_snapshot           0      -   152K  -

Snapshots are not shown by a normal zfs list operation. To list snapshots, -t snapshot is appended to zfs list. -t all displays both file systems and snapshots.

Snapshots are not mounted directly, so path is shown in the MOUNTPOINT column. There is no mention of available disk space in the AVAIL column, as snapshots cannot be written to after they are created. Compare the snapshot to the original dataset from which it was created:

# zfs list -rt all mypool/usr/home
NAME                                    USED  AVAIL  REFER  MOUNTPOINT
mypool/usr/home                         184K  93.2G   184K  /usr/home
mypool/usr/home@my_recursive_snapshot      0      -   184K  -

Displaying both the dataset and the snapshot together reveals how snapshots work in COW fashion. They save only the changes (delta) that were made and not the complete file system contents all over again. This means that snapshots take little space when few changes are made. Space usage can be made even more apparent by copying a file to the dataset, then making a second snapshot:

# cp /etc/passwd /var/tmp
# zfs snapshot mypool/var/tmp@after_cp
# zfs list -rt all mypool/var/tmp
NAME                                   USED  AVAIL  REFER  MOUNTPOINT
mypool/var/tmp                         206K  93.2G   118K  /var/tmp
mypool/var/tmp@my_recursive_snapshot    88K      -   152K  -
mypool/var/tmp@after_cp                   0      -   118K  -

The second snapshot contains only the changes to the dataset after the copy operation. This yields enormous space savings. Notice that the size of the snapshot mypool/var/tmp@my_recursive_snapshot also changed in the USED column to indicate the changes between itself and the snapshot taken afterwards.

19.4.5.2. Comparing Snapshots

ZFS provides a built-in command to compare the differences in content between two snapshots. This is helpful when many snapshots were taken over time and the user wants to see how the file system has changed over time. For example, zfs diff lets a user find the latest snapshot that still contains a file that was accidentally deleted. Doing this for the two snapshots that were created in the previous section yields this output:

# zfs list -rt all mypool/var/tmp
NAME                                   USED  AVAIL  REFER  MOUNTPOINT
mypool/var/tmp                         206K  93.2G   118K  /var/tmp
mypool/var/tmp@my_recursive_snapshot    88K      -   152K  -
mypool/var/tmp@after_cp                   0      -   118K  -
# zfs diff mypool/var/tmp@my_recursive_snapshot
M       /var/tmp/
+       /var/tmp/passwd

The command lists the changes between the specified snapshot (in this case mypool/var/tmp@my_recursive_snapshot) and the live file system. The first column shows the type of change:

+The path or file was added.
-The path or file was deleted.
MThe path or file was modified.
RThe path or file was renamed.

Comparing the output with the table, it becomes clear that passwd was added after the snapshot mypool/var/tmp@my_recursive_snapshot was created. This also resulted in a modification to the parent directory mounted at /var/tmp.

Comparing two snapshots is helpful when using the ZFS replication feature to transfer a dataset to a different host for backup purposes.

Compare two snapshots by providing the full dataset name and snapshot name of both datasets:

# cp /var/tmp/passwd /var/tmp/passwd.copy
# zfs snapshot mypool/var/tmp@diff_snapshot
# zfs diff mypool/var/tmp@my_recursive_snapshot mypool/var/tmp@diff_snapshot
M       /var/tmp/
+       /var/tmp/passwd
+       /var/tmp/passwd.copy
# zfs diff mypool/var/tmp@my_recursive_snapshot mypool/var/tmp@after_cp
M       /var/tmp/
+       /var/tmp/passwd

A backup administrator can compare two snapshots received from the sending host and determine the actual changes in the dataset. See the Replication section for more information.

19.4.5.3. Snapshot Rollback

When at least one snapshot is available, it can be rolled back to at any time. Most of the time this is the case when the current state of the dataset is no longer required and an older version is preferred. Scenarios such as local development tests have gone wrong, botched system updates hampering the system's overall functionality, or the requirement to restore accidentally deleted files or directories are all too common occurrences. Luckily, rolling back a snapshot is just as easy as typing zfs rollback snapshotname. Depending on how many changes are involved, the operation will finish in a certain amount of time. During that time, the dataset always remains in a consistent state, much like a database that conforms to ACID principles is performing a rollback. This is happening while the dataset is live and accessible without requiring a downtime. Once the snapshot has been rolled back, the dataset has the same state as it had when the snapshot was originally taken. All other data in that dataset that was not part of the snapshot is discarded. Taking a snapshot of the current state of the dataset before rolling back to a previous one is a good idea when some data is required later. This way, the user can roll back and forth between snapshots without losing data that is still valuable.

In the first example, a snapshot is rolled back because of a careless rm operation that removes too much data than was intended.

# zfs list -rt all mypool/var/tmp
NAME                                   USED  AVAIL  REFER  MOUNTPOINT
mypool/var/tmp                         262K  93.2G   120K  /var/tmp
mypool/var/tmp@my_recursive_snapshot    88K      -   152K  -
mypool/var/tmp@after_cp               53.5K      -   118K  -
mypool/var/tmp@diff_snapshot              0      -   120K  -
# ls /var/tmp
passwd          passwd.copy     vi.recover
# rm /var/tmp/passwd*
# ls /var/tmp
vi.recover

At this point, the user realized that too many files were deleted and wants them back. ZFS provides an easy way to get them back using rollbacks, but only when snapshots of important data are performed on a regular basis. To get the files back and start over from the last snapshot, issue the command:

# zfs rollback mypool/var/tmp@diff_snapshot
# ls /var/tmp
passwd          passwd.copy     vi.recover

The rollback operation restored the dataset to the state of the last snapshot. It is also possible to roll back to a snapshot that was taken much earlier and has other snapshots that were created after it. When trying to do this, ZFS will issue this warning:

# zfs list -rt snapshot mypool/var/tmp
AME                                   USED  AVAIL  REFER  MOUNTPOINT
mypool/var/tmp@my_recursive_snapshot    88K      -   152K  -
mypool/var/tmp@after_cp               53.5K      -   118K  -
mypool/var/tmp@diff_snapshot              0      -   120K  -
# zfs rollback mypool/var/tmp@my_recursive_snapshot
cannot rollback to 'mypool/var/tmp@my_recursive_snapshot': more recent snapshots exist
use '-r' to force deletion of the following snapshots:
mypool/var/tmp@after_cp
mypool/var/tmp@diff_snapshot

This warning means that snapshots exist between the current state of the dataset and the snapshot to which the user wants to roll back. To complete the rollback, these snapshots must be deleted. ZFS cannot track all the changes between different states of the dataset, because snapshots are read-only. ZFS will not delete the affected snapshots unless the user specifies -r to indicate that this is the desired action. If that is the intention, and the consequences of losing all intermediate snapshots is understood, the command can be issued:

# zfs rollback -r mypool/var/tmp@my_recursive_snapshot
# zfs list -rt snapshot mypool/var/tmp
NAME                                   USED  AVAIL  REFER  MOUNTPOINT
mypool/var/tmp@my_recursive_snapshot     8K      -   152K  -
# ls /var/tmp
vi.recover

The output from zfs list -t snapshot confirms that the intermediate snapshots were removed as a result of zfs rollback -r.

19.4.5.4. Restoring Individual Files from Snapshots

Snapshots are mounted in a hidden directory under the parent dataset: .zfs/snapshots/snapshotname. By default, these directories will not be displayed even when a standard ls -a is issued. Although the directory is not displayed, it is there nevertheless and can be accessed like any normal directory. The property named snapdir controls whether these hidden directories show up in a directory listing. Setting the property to visible allows them to appear in the output of ls and other commands that deal with directory contents.

# zfs get snapdir mypool/var/tmp
NAME            PROPERTY  VALUE    SOURCE
mypool/var/tmp  snapdir   hidden   default
# ls -a /var/tmp
.               ..              passwd          vi.recover
# zfs set snapdir=visible mypool/var/tmp
# ls -a /var/tmp
.               ..              .zfs            passwd          vi.recover

Individual files can easily be restored to a previous state by copying them from the snapshot back to the parent dataset. The directory structure below .zfs/snapshot has a directory named exactly like the snapshots taken earlier to make it easier to identify them. In the next example, it is assumed that a file is to be restored from the hidden .zfs directory by copying it from the snapshot that contained the latest version of the file:

# rm /var/tmp/passwd
# ls -a /var/tmp
.               ..              .zfs            vi.recover
# ls /var/tmp/.zfs/snapshot
after_cp                my_recursive_snapshot
# ls /var/tmp/.zfs/snapshot/after_cp
passwd          vi.recover
# cp /var/tmp/.zfs/snapshot/after_cp/passwd /var/tmp

When ls .zfs/snapshot was issued, the snapdir property might have been set to hidden, but it would still be possible to list the contents of that directory. It is up to the administrator to decide whether these directories will be displayed. It is possible to display these for certain datasets and prevent it for others. Copying files or directories from this hidden .zfs/snapshot is simple enough. Trying it the other way around results in this error:

# cp /etc/rc.conf /var/tmp/.zfs/snapshot/after_cp/
cp: /var/tmp/.zfs/snapshot/after_cp/rc.conf: Read-only file system

The error reminds the user that snapshots are read-only and cannot be changed after creation. Files cannot be copied into or removed from snapshot directories because that would change the state of the dataset they represent.

Snapshots consume space based on how much the parent file system has changed since the time of the snapshot. The written property of a snapshot tracks how much space is being used by the snapshot.

Snapshots are destroyed and the space reclaimed with zfs destroy dataset@snapshot. Adding -r recursively removes all snapshots with the same name under the parent dataset. Adding -n -v to the command displays a list of the snapshots that would be deleted and an estimate of how much space would be reclaimed without performing the actual destroy operation.

19.4.6. Managing Clones

A clone is a copy of a snapshot that is treated more like a regular dataset. Unlike a snapshot, a clone is not read only, is mounted, and can have its own properties. Once a clone has been created using zfs clone, the snapshot it was created from cannot be destroyed. The child/parent relationship between the clone and the snapshot can be reversed using zfs promote. After a clone has been promoted, the snapshot becomes a child of the clone, rather than of the original parent dataset. This will change how the space is accounted, but not actually change the amount of space consumed. The clone can be mounted at any point within the ZFS file system hierarchy, not just below the original location of the snapshot.

To demonstrate the clone feature, this example dataset is used:

# zfs list -rt all camino/home/joe
NAME                    USED  AVAIL  REFER  MOUNTPOINT
camino/home/joe         108K   1.3G    87K  /usr/home/joe
camino/home/joe@plans    21K      -  85.5K  -
camino/home/joe@backup    0K      -    87K  -

A typical use for clones is to experiment with a specific dataset while keeping the snapshot around to fall back to in case something goes wrong. Since snapshots cannot be changed, a read/write clone of a snapshot is created. After the desired result is achieved in the clone, the clone can be promoted to a dataset and the old file system removed. This is not strictly necessary, as the clone and dataset can coexist without problems.

# zfs clone camino/home/joe@backup camino/home/joenew
# ls /usr/home/joe*
/usr/home/joe:
backup.txz     plans.txt

/usr/home/joenew:
backup.txz     plans.txt
# df -h /usr/home
Filesystem          Size    Used   Avail Capacity  Mounted on
usr/home/joe        1.3G     31k    1.3G     0%    /usr/home/joe
usr/home/joenew     1.3G     31k    1.3G     0%    /usr/home/joenew

After a clone is created it is an exact copy of the state the dataset was in when the snapshot was taken. The clone can now be changed independently from its originating dataset. The only connection between the two is the snapshot. ZFS records this connection in the property origin. Once the dependency between the snapshot and the clone has been removed by promoting the clone using zfs promote, the origin of the clone is removed as it is now an independent dataset. This example demonstrates it:

# zfs get origin camino/home/joenew
NAME                  PROPERTY  VALUE                     SOURCE
camino/home/joenew    origin    camino/home/joe@backup    -
# zfs promote camino/home/joenew
# zfs get origin camino/home/joenew
NAME                  PROPERTY  VALUE   SOURCE
camino/home/joenew    origin    -       -

After making some changes like copying loader.conf to the promoted clone, for example, the old directory becomes obsolete in this case. Instead, the promoted clone can replace it. This can be achieved by two consecutive commands: zfs destroy on the old dataset and zfs rename on the clone to name it like the old dataset (it could also get an entirely different name).

# cp /boot/defaults/loader.conf /usr/home/joenew
# zfs destroy -f camino/home/joe
# zfs rename camino/home/joenew camino/home/joe
# ls /usr/home/joe
backup.txz     loader.conf     plans.txt
# df -h /usr/home
Filesystem          Size    Used   Avail Capacity  Mounted on
usr/home/joe        1.3G    128k    1.3G     0%    /usr/home/joe

The cloned snapshot is now handled like an ordinary dataset. It contains all the data from the original snapshot plus the files that were added to it like loader.conf. Clones can be used in different scenarios to provide useful features to ZFS users. For example, jails could be provided as snapshots containing different sets of installed applications. Users can clone these snapshots and add their own applications as they see fit. Once they are satisfied with the changes, the clones can be promoted to full datasets and provided to end users to work with like they would with a real dataset. This saves time and administrative overhead when providing these jails.

19.4.7. Replication

Keeping data on a single pool in one location exposes it to risks like theft and natural or human disasters. Making regular backups of the entire pool is vital. ZFS provides a built-in serialization feature that can send a stream representation of the data to standard output. Using this technique, it is possible to not only store the data on another pool connected to the local system, but also to send it over a network to another system. Snapshots are the basis for this replication (see the section on ZFS snapshots). The commands used for replicating data are zfs send and zfs receive.

These examples demonstrate ZFS replication with these two pools:

# zpool list
NAME    SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
backup  960M    77K   896M     0%  1.00x  ONLINE  -
mypool  984M  43.7M   940M     4%  1.00x  ONLINE  -

The pool named mypool is the primary pool where data is written to and read from on a regular basis. A second pool, backup is used as a standby in case the primary pool becomes unavailable. Note that this fail-over is not done automatically by ZFS, but must be manually done by a system administrator when needed. A snapshot is used to provide a consistent version of the file system to be replicated. Once a snapshot of mypool has been created, it can be copied to the backup pool. Only snapshots can be replicated. Changes made since the most recent snapshot will not be included.

# zfs snapshot mypool@backup1
# zfs list -t snapshot
NAME                    USED  AVAIL  REFER  MOUNTPOINT
mypool@backup1             0      -  43.6M  -

Now that a snapshot exists, zfs send can be used to create a stream representing the contents of the snapshot. This stream can be stored as a file or received by another pool. The stream is written to standard output, but must be redirected to a file or pipe or an error is produced:

# zfs send mypool@backup1
Error: Stream can not be written to a terminal.
You must redirect standard output.

To back up a dataset with zfs send, redirect to a file located on the mounted backup pool. Ensure that the pool has enough free space to accommodate the size of the snapshot being sent, which means all of the data contained in the snapshot, not just the changes from the previous snapshot.

# zfs send mypool@backup1 > /backup/backup1
# zpool list
NAME    SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
backup  960M  63.7M   896M     6%  1.00x  ONLINE  -
mypool  984M  43.7M   940M     4%  1.00x  ONLINE  -

The zfs send transferred all the data in the snapshot called backup1 to the pool named backup. Creating and sending these snapshots can be done automatically with a cron(8) job.

Instead of storing the backups as archive files, ZFS can receive them as a live file system, allowing the backed up data to be accessed directly. To get to the actual data contained in those streams, zfs receive is used to transform the streams back into files and directories. The example below combines zfs send and zfs receive using a pipe to copy the data from one pool to another. The data can be used directly on the receiving pool after the transfer is complete. A dataset can only be replicated to an empty dataset.

# zfs snapshot mypool@replica1
# zfs send -v mypool@replica1 | zfs receive backup/mypool
send from @ to mypool@replica1 estimated size is 50.1M
total estimated size is 50.1M
TIME        SENT   SNAPSHOT

# zpool list
NAME    SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
backup  960M  63.7M   896M     6%  1.00x  ONLINE  -
mypool  984M  43.7M   940M     4%  1.00x  ONLINE  -

19.4.7.1. Incremental Backups

zfs send can also determine the difference between two snapshots and send only the differences between the two. This saves disk space and transfer time. For example:

# zfs snapshot mypool@replica2
# zfs list -t snapshot
NAME                    USED  AVAIL  REFER  MOUNTPOINT
mypool@replica1         5.72M      -  43.6M  -
mypool@replica2             0      -  44.1M  -
# zpool list
NAME    SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
backup  960M  61.7M   898M     6%  1.00x  ONLINE  -
mypool  960M  50.2M   910M     5%  1.00x  ONLINE  -

A second snapshot called replica2 was created. This second snapshot contains only the changes that were made to the file system between now and the previous snapshot, replica1. Using zfs send -i and indicating the pair of snapshots generates an incremental replica stream containing only the data that has changed. This can only succeed if the initial snapshot already exists on the receiving side.

# zfs send -v -i mypool@replica1 mypool@replica2 | zfs receive /backup/mypool
send from @replica1 to mypool@replica2 estimated size is 5.02M
total estimated size is 5.02M
TIME        SENT   SNAPSHOT

# zpool list
NAME    SIZE  ALLOC   FREE    CAP  DEDUP  HEALTH  ALTROOT
backup  960M  80.8M   879M     8%  1.00x  ONLINE  -
mypool  960M  50.2M   910M     5%  1.00x  ONLINE  -

# zfs list
NAME                         USED  AVAIL  REFER  MOUNTPOINT
backup                      55.4M   240G   152K  /backup
backup/mypool               55.3M   240G  55.2M  /backup/mypool
mypool                      55.6M  11.6G  55.0M  /mypool

# zfs list -t snapshot
NAME                                         USED  AVAIL  REFER  MOUNTPOINT
backup/mypool@replica1                       104K      -  50.2M  -
backup/mypool@replica2                          0      -  55.2M  -
mypool@replica1                             29.9K      -  50.0M  -
mypool@replica2                                 0      -  55.0M  -

The incremental stream was successfully transferred. Only the data that had changed was replicated, rather than the entirety of replica1. Only the differences were sent, which took much less time to transfer and saved disk space by not copying the complete pool each time. This is useful when having to rely on slow networks or when costs per transferred byte must be considered.

A new file system, backup/mypool, is available with all of the files and data from the pool mypool. If -P is specified, the properties of the dataset will be copied, including compression settings, quotas, and mount points. When -R is specified, all child datasets of the indicated dataset will be copied, along with all of their properties. Sending and receiving can be automated so that regular backups are created on the second pool.

19.4.7.2. Sending Encrypted Backups over SSH

Sending streams over the network is a good way to keep a remote backup, but it does come with a drawback. Data sent over the network link is not encrypted, allowing anyone to intercept and transform the streams back into data without the knowledge of the sending user. This is undesirable, especially when sending the streams over the internet to a remote host. SSH can be used to securely encrypt data send over a network connection. Since ZFS only requires the stream to be redirected from standard output, it is relatively easy to pipe it through SSH. To keep the contents of the file system encrypted in transit and on the remote system, consider using PEFS.

A few settings and security precautions must be completed first. Only the necessary steps required for the zfs send operation are shown here. For more information on SSH, see Section 13.8, “OpenSSH”.

This configuration is required:

  • Passwordless SSH access between sending and receiving host using SSH keys

  • Normally, the privileges of the root user are needed to send and receive streams. This requires logging in to the receiving system as root. However, logging in as root is disabled by default for security reasons. The ZFS Delegation system can be used to allow a non-root user on each system to perform the respective send and receive operations.

  • On the sending system:

    # zfs allow -u someuser send,snapshot mypool
  • To mount the pool, the unprivileged user must own the directory, and regular users must be allowed to mount file systems. On the receiving system:

    # sysctl vfs.usermount=1
    vfs.usermount: 0 -> 1
    # sysrc -f /etc/sysctl.conf vfs.usermount=1
    # zfs create recvpool/backup
    # zfs allow -u someuser create,mount,receive recvpool/backup
    # chown someuser /recvpool/backup

The unprivileged user now has the ability to receive and mount datasets, and the home dataset can be replicated to the remote system:

% zfs snapshot -r mypool/home@monday
% zfs send -R mypool/home@monday | ssh someuser@backuphost zfs recv -dvu recvpool/backup

A recursive snapshot called monday is made of the file system dataset home that resides on the pool mypool. Then it is sent with zfs send -R to include the dataset, all child datasets, snapshots, clones, and settings in the stream. The output is piped to the waiting zfs receive on the remote host backuphost through SSH. Using a fully qualified domain name or IP address is recommended. The receiving machine writes the data to the backup dataset on the recvpool pool. Adding -d to zfs recv overwrites the name of the pool on the receiving side with the name of the snapshot. -u causes the file systems to not be mounted on the receiving side. When -v is included, more detail about the transfer is shown, including elapsed time and the amount of data transferred.

19.4.8. Dataset, User, and Group Quotas

Dataset quotas are used to restrict the amount of space that can be consumed by a particular dataset. Reference Quotas work in very much the same way, but only count the space used by the dataset itself, excluding snapshots and child datasets. Similarly, user and group quotas can be used to prevent users or groups from using all of the space in the pool or dataset.

To enforce a dataset quota of 10 GB for storage/home/bob:

# zfs set quota=10G storage/home/bob

To enforce a reference quota of 10 GB for storage/home/bob:

# zfs set refquota=10G storage/home/bob

To remove a quota of 10 GB for storage/home/bob:

# zfs set quota=none storage/home/bob

The general format is userquota@user=size, and the user's name must be in one of these formats:

  • POSIX compatible name such as joe.

  • POSIX numeric ID such as 789.

  • SID name such as joe.bloggs@example.com.

  • SID numeric ID such as S-1-123-456-789.

For example, to enforce a user quota of 50 GB for the user named joe:

# zfs set userquota@joe=50G

To remove any quota:

# zfs set userquota@joe=none

Note:

User quota properties are not displayed by zfs get all. Non-root users can only see their own quotas unless they have been granted the userquota privilege. Users with this privilege are able to view and set everyone's quota.

The general format for setting a group quota is: groupquota@group=size.

To set the quota for the group firstgroup to 50 GB, use:

# zfs set groupquota@firstgroup=50G

To remove the quota for the group firstgroup, or to make sure that one is not set, instead use:

# zfs set groupquota@firstgroup=none

As with the user quota property, non-root users can only see the quotas associated with the groups to which they belong. However, root or a user with the groupquota privilege can view and set all quotas for all groups.

To display the amount of space used by each user on a file system or snapshot along with any quotas, use zfs userspace. For group information, use zfs groupspace. For more information about supported options or how to display only specific options, refer to zfs(1).

Users with sufficient privileges, and root, can list the quota for storage/home/bob using:

# zfs get quota storage/home/bob

19.4.9. Reservations

Reservations guarantee a minimum amount of space will always be available on a dataset. The reserved space will not be available to any other dataset. This feature can be especially useful to ensure that free space is available for an important dataset or log files.

The general format of the reservation property is reservation=size, so to set a reservation of 10 GB on storage/home/bob, use:

# zfs set reservation=10G storage/home/bob

To clear any reservation:

# zfs set reservation=none storage/home/bob

The same principle can be applied to the refreservation property for setting a Reference Reservation, with the general format refreservation=size.

This command shows any reservations or refreservations that exist on storage/home/bob:

# zfs get reservation storage/home/bob
# zfs get refreservation storage/home/bob

19.4.10. Compression

ZFS provides transparent compression. Compressing data at the block level as it is written not only saves space, but can also increase disk throughput. If data is compressed by 25%, but the compressed data is written to the disk at the same rate as the uncompressed version, resulting in an effective write speed of 125%. Compression can also be a great alternative to Deduplication because it does not require additional memory.

ZFS offers several different compression algorithms, each with different trade-offs. With the introduction of LZ4 compression in ZFS v5000, it is possible to enable compression for the entire pool without the large performance trade-off of other algorithms. The biggest advantage to LZ4 is the early abort feature. If LZ4 does not achieve at least 12.5% compression in the first part of the data, the block is written uncompressed to avoid wasting CPU cycles trying to compress data that is either already compressed or uncompressible. For details about the different compression algorithms available in ZFS, see the Compression entry in the terminology section.

The administrator can monitor the effectiveness of compression using a number of dataset properties.

# zfs get used,compressratio,compression,logicalused mypool/compressed_dataset
NAME        PROPERTY          VALUE     SOURCE
mypool/compressed_dataset  used              449G      -
mypool/compressed_dataset  compressratio     1.11x     -
mypool/compressed_dataset  compression       lz4       local
mypool/compressed_dataset  logicalused       496G      -

The dataset is currently using 449 GB of space (the used property). Without compression, it would have taken 496 GB of space (the logicalused property). This results in the 1.11:1 compression ratio.

Compression can have an unexpected side effect when combined with User Quotas. User quotas restrict how much space a user can consume on a dataset, but the measurements are based on how much space is used after compression. So if a user has a quota of 10 GB, and writes 10 GB of compressible data, they will still be able to store additional data. If they later update a file, say a database, with more or less compressible data, the amount of space available to them will change. This can result in the odd situation where a user did not increase the actual amount of data (the logicalused property), but the change in compression caused them to reach their quota limit.

Compression can have a similar unexpected interaction with backups. Quotas are often used to limit how much data can be stored to ensure there is sufficient backup space available. However since quotas do not consider compression, more data may be written than would fit with uncompressed backups.

19.4.11. Deduplication

When enabled, deduplication uses the checksum of each block to detect duplicate blocks. When a new block is a duplicate of an existing block, ZFS writes an additional reference to the existing data instead of the whole duplicate block. Tremendous space savings are possible if the data contains many duplicated files or repeated information. Be warned: deduplication requires an extremely large amount of memory, and most of the space savings can be had without the extra cost by enabling compression instead.

To activate deduplication, set the dedup property on the target pool:

# zfs set dedup=on pool

Only new data being written to the pool will be deduplicated. Data that has already been written to the pool will not be deduplicated merely by activating this option. A pool with a freshly activated deduplication property will look like this example:

# zpool list
NAME  SIZE ALLOC  FREE CAP DEDUP HEALTH ALTROOT
pool 2.84G 2.19M 2.83G  0% 1.00x ONLINE -

The DEDUP column shows the actual rate of deduplication for the pool. A value of 1.00x shows that data has not been deduplicated yet. In the next example, the ports tree is copied three times into different directories on the deduplicated pool created above.

# for d in dir1 dir2 dir3; do
> mkdir $d && cp -R /usr/ports $d &
> done

Redundant data is detected and deduplicated:

# zpool list
NAME SIZE  ALLOC FREE CAP DEDUP HEALTH ALTROOT
pool 2.84G 20.9M 2.82G 0% 3.00x ONLINE -

The DEDUP column shows a factor of 3.00x. Multiple copies of the ports tree data was detected and deduplicated, using only a third of the space. The potential for space savings can be enormous, but comes at the cost of having enough memory to keep track of the deduplicated blocks.

Deduplication is not always beneficial, especially when the data on a pool is not redundant. ZFS can show potential space savings by simulating deduplication on an existing pool:

# zdb -S pool
Simulated DDT histogram:

bucket              allocated                       referenced
______   ______________________________   ______________________________
refcnt   blocks   LSIZE   PSIZE   DSIZE   blocks   LSIZE   PSIZE   DSIZE
------   ------   -----   -----   -----   ------   -----   -----   -----
     1    2.58M    289G    264G    264G    2.58M    289G    264G    264G
     2     206K   12.6G   10.4G   10.4G     430K   26.4G   21.6G   21.6G
     4    37.6K    692M    276M    276M     170K   3.04G   1.26G   1.26G
     8    2.18K   45.2M   19.4M   19.4M    20.0K    425M    176M    176M
    16      174   2.83M   1.20M   1.20M    3.33K   48.4M   20.4M   20.4M
    32       40   2.17M    222K    222K    1.70K   97.2M   9.91M   9.91M
    64        9     56K   10.5K   10.5K      865   4.96M    948K    948K
   128        2   9.50K      2K      2K      419   2.11M    438K    438K
   256        5   61.5K     12K     12K    1.90K   23.0M   4.47M   4.47M
    1K        2      1K      1K      1K    2.98K   1.49M   1.49M   1.49M
 Total    2.82M    303G    275G    275G    3.20M    319G    287G    287G

dedup = 1.05, compress = 1.11, copies = 1.00, dedup * compress / copies = 1.16

After zdb -S finishes analyzing the pool, it shows the space reduction ratio that would be achieved by activating deduplication. In this case, 1.16 is a very poor space saving ratio that is mostly provided by compression. Activating deduplication on this pool would not save any significant amount of space, and is not worth the amount of memory required to enable deduplication. Using the formula ratio = dedup * compress / copies, system administrators can plan the storage allocation, deciding whether the workload will contain enough duplicate blocks to justify the memory requirements. If the data is reasonably compressible, the space savings may be very good. Enabling compression first is recommended, and compression can also provide greatly increased performance. Only enable deduplication in cases where the additional savings will be considerable and there is sufficient memory for the DDT.

19.4.12. ZFS and Jails

zfs jail and the corresponding jailed property are used to delegate a ZFS dataset to a Jail. zfs jail jailid attaches a dataset to the specified jail, and zfs unjail detaches it. For the dataset to be controlled from within a jail, the jailed property must be set. Once a dataset is jailed, it can no longer be mounted on the host because it may have mount points that would compromise the security of the host.

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