A snapshot is a read-only copy of a file system or volume. Snapshots can be created almost instantly, and they 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, thus preventing the disk space from being freed.
ZFS snapshots include the following features:
The persist across system reboots.
The theoretical maximum number of snapshots is 264.
Snapshots use no separate backing store. Snapshots consume disk space directly from the same storage pool as the file system or volume from which they were created.
Recursive snapshots are created quickly as one atomic operation. The snapshots are created together (all at once) or not created at all. The benefit of atomic snapshot operations is that the snapshot data is always taken at one consistent time, even across descendent file systems.
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 Sending and Receiving ZFS Data.
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 requirements in ZFS Component Naming Requirements.
# zfs snapshot tank/home/ahrens@friday
You can create snapshots for all descendent file systems by using the -r option. For example:
# zfs snapshot -r tank/home@now # zfs list -t snapshot NAME USED AVAIL REFER MOUNTPOINT rpool/ROOT/zfs2BE@zfs2BE 78.3M - 4.53G - tank/home@now 0 - 26K - tank/home/ahrens@now 0 - 259M - tank/home/anne@now 0 - 156M - tank/home/bob@now 0 - 156M - tank/home/cindys@now 0 - 104M -
Snapshots have no modifiable properties. Nor can dataset properties be applied to a snapshot. For example:
# zfs set compression=on tank/home/ahrens@now cannot set compression property for 'tank/home/ahrens@now': snapshot properties cannot be modified
Snapshots are destroyed by using the zfs destroy command. For example:
# zfs destroy tank/home/ahrens@now
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: tank/home/ahrens@tuesday tank/home/ahrens@wednesday tank/home/ahrens@thursday
For more information about the destroy subcommand, see Destroying a ZFS File System.
If you have different automatic snapshot policies such that older snapshots are being inadvertently destroyed by zfs receive because they no longer exist on the sending side, you might consider using the snapshots hold feature.
Holding a snapshot prevents it from being destroyed. In addition, this feature allows a snapshot with clones to be deleted pending the removal of the last clone by using the zfs destroy -d command. Each snapshot has an associated user-reference count, which is initialized to zero. This count increases by one whenever a hold is put on a snapshot and decreases by one whenever a hold is released.
In the previous Solaris release, a snapshot could only be destroyed by using the zfs destroy command if it had no clones. In this Solaris release, the snapshot must also have a zero user-reference count.
You can hold a snapshot or set of snapshots. For example, the following syntax puts a hold tag, keep, on tank/home/cindys/snap@1.
# zfs hold keep tank/home/cindys@snap1
You can use the -r option to recursively hold the snapshots of all descendent file systems. For example:
# zfs snapshot -r tank/home@now # zfs hold -r keep tank/home@now
This syntax adds a single reference, keep, to the given snapshot or set of snapshots. Each snapshot has its own tag namespace and hold tags must be unique within that space. If a hold exists on a snapshot, attempts to destroy that held snapshot by using the zfs destroy command will fail. For example:
# zfs destroy tank/home/cindys@snap1 cannot destroy 'tank/home/cindys@snap1': dataset is busy
If you want to destroy a held snapshot, use the -d option. For example:
# zfs destroy -d tank/home/cindys@snap1
Use the zfs holds command to display a list of held snapshots. For example:
# zfs holds tank/home@now NAME TAG TIMESTAMP tank/home@now keep Thu Jul 15 11:25:39 2010
# zfs holds -r tank/home@now NAME TAG TIMESTAMP tank/home/cindys@now keep Thu Jul 15 11:25:39 2010 tank/home/mark@now keep Thu Jul 15 11:25:39 2010 tank/home@now keep Thu Jul 15 11:25:39 2010
You can use the zfs release command to release a hold on a snapshot or set of snapshots. For example:
# zfs release -r keep tank/home@now
If the snapshot is released, the snapshot can be destroyed by using the zfs destroy command. For example:
# zfs destroy -r tank/home@now
Two new properties identify snapshot hold information:
The defer_destroy property is on if the snapshot has been marked for deferred destruction by using the zfs destroy -d command. Otherwise, the property is off.
The userrefs property is set to the number of holds on this snapshot, also referred to as the user-reference count.
# zfs rename tank/home/cindys@083006 tank/home/cindys@today
In addition, the following shortcut syntax is equivalent to the preceding syntax:
# zfs rename tank/home/cindys@083006 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 dataset
You can recursively rename snapshots by using the zfs rename -r command. For example:
# zfs list NAME USED AVAIL REFER MOUNTPOINT users 270K 16.5G 22K /users users/home 76K 16.5G 22K /users/home users/home@yesterday 0 - 22K - users/home/markm 18K 16.5G 18K /users/home/markm users/home/markm@yesterday 0 - 18K - users/home/marks 18K 16.5G 18K /users/home/marks users/home/marks@yesterday 0 - 18K - users/home/neil 18K 16.5G 18K /users/home/neil users/home/neil@yesterday 0 - 18K - # zfs rename -r users/home@yesterday @2daysago # zfs list -r users/home NAME USED AVAIL REFER MOUNTPOINT users/home 76K 16.5G 22K /users/home users/home@2daysago 0 - 22K - users/home/markm 18K 16.5G 18K /users/home/markm users/home/markm@2daysago 0 - 18K - users/home/marks 18K 16.5G 18K /users/home/marks users/home/marks@2daysago 0 - 18K - users/home/neil 18K 16.5G 18K /users/home/neil users/home/neil@2daysago 0 - 18K -
You can enable or disable the display of snapshot listings in the zfs list output by using the listsnapshots pool property. This property is enabled by default.
If you disable this property, you can use the zfs list -t snapshot command to display snapshot information. Or, enable the listsnapshots pool property. For example:
# zpool get listsnapshots tank NAME PROPERTY VALUE SOURCE tank listsnapshots on default # zpool set listsnapshots=off tank # zpool get listsnapshots tank NAME PROPERTY VALUE SOURCE tank listsnapshots off local
Snapshots of file systems are accessible in the .zfs/snapshot directory within the root of the 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 /tank/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 tank/home NAME CREATION tank/home@now Wed Jun 30 16:16 2010 tank/home/ahrens@now Wed Jun 30 16:16 2010 tank/home/anne@now Wed Jun 30 16:16 2010 tank/home/bob@now Wed Jun 30 16:16 2010 tank/home/cindys@now Wed Jun 30 16:16 2010
When a snapshot is created, its disk space is initially shared between the snapshot and the file system, and possibly with previous snapshots. As the file system changes, disk 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 disk space unique to (and thus used by) other snapshots.
You can identify additional information about how the values of the used property are consumed. New read-only file system properties describe disk space usage for clones, file systems, and volumes. For example:
$ zfs list -o space # zfs list -ro space tank/home NAME AVAIL USED USEDSNAP USEDDS USEDREFRESERV USEDCHILD tank/home 66.3G 675M 0 26K 0 675M tank/home@now - 0 - - - - tank/home/ahrens 66.3G 259M 0 259M 0 0 tank/home/ahrens@now - 0 - - - - tank/home/anne 66.3G 156M 0 156M 0 0 tank/home/anne@now - 0 - - - - tank/home/bob 66.3G 156M 0 156M 0 0 tank/home/bob@now - 0 - - - - tank/home/cindys 66.3G 104M 0 104M 0 0 tank/home/cindys@now - 0 - - - -
For a description of these properties, see Table 6-1.
You can use the zfs rollback command to discard all changes made to a file system since a specific snapshot was created. 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.
Note - The file system that you want to roll back is 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.
# 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: tank/home/ahrens@wednesday tank/home/ahrens@thursday # zfs rollback -r tank/home/ahrens@tuesday
In this example, the wednesday and thursday snapshots are destroyed because you rolled back to the earlier tuesday snapshot.
# zfs list -r -t snapshot -o name,creation tank/home/ahrens NAME CREATION tank/home/ahrens@now Wed Jun 30 16:16 2010