Chapter 6. Working With ZFS Snapshots and Clones

Table of Contents

ZFS Snapshots
Creating and Destroying ZFS Snapshots
Displaying and Accessing ZFS Snapshots
Rolling Back to a ZFS Snapshot
ZFS Clones
Creating a ZFS Clone
Destroying a ZFS Clone
Saving and Restoring ZFS Data
Saving ZFS Data With Other Backup Products
Saving a ZFS Snapshot
Restoring a ZFS Snapshot
Remote Replication of ZFS Data

ZFS Snapshots

A snapshot is a read-only copy of a file system or volume. Snapshots can be created almost instantly, and initially consume no additional disk space within the pool. However, as data within the active dataset changes, the snapshot consumes disk space by continuing to reference the old data and so prevents the space from being freed.

ZFS snapshots include the following features:

  • Provides persistence across system reboots.

  • The theoretical maximum number of snapshots is 264.

  • Uses no separate backing store. Snapshots consume disk space directly from the same storage pool as the file system from which they were created.

Snapshots of volumes cannot be accessed directly, but they can be cloned, backed up, rolled back to, and so on. For information about backing up a ZFS snapshot, see Saving and Restoring ZFS Data.

Creating and Destroying ZFS Snapshots

Snapshots are created by using the zfs snapshot command, which takes as its only argument the name of the snapshot to create. The snapshot name is specified as follows:


The snapshot name must satisfy the naming conventions defined in ZFS Component Naming Requirements.

In the following example, a snapshot of tank/home/ahrens that is named friday is created.

# zfs snapshot tank/home/ahrens@friday

Snapshots have no modifiable properties. Nor can dataset properties be applied to a snapshot.

# zfs set compression=on tank/home/ahrens@tuesday
cannot set compression property for 'tank/home/ahrens@tuesday': snapshot
properties cannot be modified

Snapshots are destroyed by using the zfs destroy command. For example:

# zfs destroy tank/home/ahrens@friday

A dataset cannot be destroyed if snapshots of the dataset exist. For example:

# zfs destroy tank/home/ahrens
cannot destroy 'tank/home/ahrens': filesystem has children
use '-r' to destroy the following datasets:

In addition, if clones have been created from a snapshot, then they must be destroyed before the snapshot can be destroyed.

For more information about the destroy subcommand, see Destroying a ZFS File System.

Renaming ZFS Snapshots

You can rename snapshots but they must be renamed within the pool and dataset from which they were created. For example:

# zfs rename tank/home/cindys@031306 tank/home/cindys@today

The following snapshot rename operation is not supported because the target pool and file system name are different from the pool and file system where the snapshot was created.

# zfs rename tank/home/cindys@today pool/home/cindys@saturday
cannot rename to 'pool/home/cindys@today': snapshots must be part of same 

Displaying and Accessing ZFS Snapshots

Snapshots of file systems are accessible in the .zfs/snapshot directory within the root of the containing file system. For example, if tank/home/ahrens is mounted on /home/ahrens, then the tank/home/ahrens@thursday snapshot data is accessible in the /home/ahrens/.zfs/snapshot/thursday directory.

# ls /home/ahrens/.zfs/snapshot
tuesday wednesday thursday

You can list snapshots as follows:

# zfs list -t snapshot
NAME                        USED  AVAIL  REFER  MOUNTPOINT
pool/home/anne@monday          0      -   780K  -
pool/home/bob@monday           0      -  1.01M  -
tank/home/ahrens@tuesday   8.50K      -   780K  -
tank/home/ahrens@wednesday 8.50K      -  1.01M  -
tank/home/ahrens@thursday      0      -  1.77M  -
tank/home/cindys@today     8.50K      -   524K  -

You can list snapshots that were created for a particular file system as follows:

# zfs list -r -t snapshot -o name,creation pool/home
NAME                   CREATION
pool/home/anne@monday  Mon Mar 13 11:46 2006
pool/home/bob@monday   Mon Mar 13 11:46 2006

Snapshot Space Accounting

When a snapshot is created, its space is initially shared between the snapshot and the file system, and possibly with previous snapshots. As the file system changes, space that was previously shared becomes unique to the snapshot, and thus is counted in the snapshot's used property. Additionally, deleting snapshots can increase the amount of space unique to (and thus used by) other snapshots.

A snapshot's space referenced property is the same as the file system's was when the snapshot was created.

Rolling Back to a ZFS Snapshot

The zfs rollback command can be used to discard all changes made since a specific snapshot. The file system reverts to its state at the time the snapshot was taken. By default, the command cannot roll back to a snapshot other than the most recent snapshot.

To roll back to an earlier snapshot, all intermediate snapshots must be destroyed. You can destroy earlier snapshots by specifying the r option.

If clones of any intermediate snapshots exist, the R option must be specified to destroy the clones as well.


The file system that you want to roll back must be unmounted and remounted, if it is currently mounted. If the file system cannot be unmounted, the rollback fails. The f option forces the file system to be unmounted, if necessary.

In the following example, the tank/home/ahrens file system is rolled back to the tuesday snapshot:

# zfs rollback tank/home/ahrens@tuesday
cannot rollback to 'tank/home/ahrens@tuesday': more recent snapshots exist
use '-r' to force deletion of the following snapshots:
# zfs rollback -r tank/home/ahrens@tuesday

In the above example, the wednesday and thursday snapshots are removed because you rolled back to the previous tuesday snapshot.

# zfs list -r -t snapshot -o name,creation tank/home/ahrens
NAME                  CREATION
tank/home/ahrens@tuesday  Mon Mar 13 11:05 2006