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|Oracle Solaris ZFS Administration Guide Oracle Solaris 10 8/11 Information Library|
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/matt@friday
You can create snapshots for all descendent file systems by using the -r option. For example:
# zfs snapshot -r tank/home@snap1 # zfs list -t snapshot -r tank/home zfs list -t snapshot -r tank/home NAME USED AVAIL REFER MOUNTPOINT tank/home@snap1 0 - 34K - tank/home/mark@snap1 0 - 2.00G - tank/home/matt@snap1 0 - 1.00G - tank/home/tom@snap1 0 - 2.00G -
Snapshots have no modifiable properties. Nor can dataset properties be applied to a snapshot. For example:
# zfs set compression=on tank/home/matt@friday cannot set property for 'tank/home/matt@friday': this property can not be modified for snapshots
Snapshots are destroyed by using the zfs destroy command. For example:
# zfs destroy tank/home/matt@friday
A dataset cannot be destroyed if snapshots of the dataset exist. For example:
# zfs destroy tank/home/matt cannot destroy 'tank/home/matt': filesystem has children use '-r' to destroy the following datasets: tank/home/matt@tuesday tank/home/matt@wednesday tank/home/matt@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 1 whenever a hold is put on a snapshot and decreases by 1 whenever a hold is released.
In the previous Oracle Solaris release, a snapshot could only be destroyed by using the zfs destroy command if it had no clones. In this Oracle 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/cindy/snap@1:
# zfs hold keep tank/home/cindy@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/cindy@snap1 cannot destroy 'tank/home/cindy@snap1': dataset is busy
To destroy a held snapshot, use the -d option. For example:
# zfs destroy -d tank/home/cindy@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 Fri May 6 06:34:03 2011
# zfs holds -r tank/home@now NAME TAG TIMESTAMP tank/home/cindy@now keep Fri May 6 06:34:03 2011 tank/home/mark@now keep Fri May 6 06:34:03 2011 tank/home/matt@now keep Fri May 6 06:34:03 2011 tank/home/tom@now keep Fri May 6 06:34:03 2011 tank/home@now keep Fri May 6 06:34:03 2011
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/cindy@snap1 tank/home/cindy@today
In addition, the following shortcut syntax is equivalent to the preceding syntax:
# zfs rename tank/home/cindy@snap1 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/cindy@today pool/home/cindy@saturday cannot rename to 'pool/home/cindy@today': snapshots must be part of same dataset
You can recursively rename snapshots by using the zfs rename -r command. For example:
# zfs list -t snapshot -r users/home NAME USED AVAIL REFER MOUNTPOINT users/home@now 23.5K - 35.5K - users/home@yesterday 0 - 38K - users/home/lori@yesterday 0 - 2.00G - users/home/mark@yesterday 0 - 1.00G - users/home/neil@yesterday 0 - 2.00G - # zfs rename -r users/home@yesterday @2daysago # zfs list -t snapshot -r users/home NAME USED AVAIL REFER MOUNTPOINT users/home@now 23.5K - 35.5K - users/home@2daysago 0 - 38K - users/home/lori@2daysago 0 - 2.00G - users/home/mark@2daysago 0 - 1.00G - users/home/neil@2daysago 0 - 2.00G -
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/matt/.zfs/snapshot tuesday wednesday thursday
You can list snapshots as follows:
# zfs list -t snapshot -r tank/home NAME USED AVAIL REFER MOUNTPOINT tank/home/cindy@today 0 - 2.00G - tank/home/mark@today 0 - 2.00G - tank/home/matt@tuesday 20K - 1.00G - tank/home/matt@wednesday 20K - 1.00G - tank/home/matt@thursday 0 - 1.00G -
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/cindy@today Fri May 6 6:32 2011 tank/home/mark@today Fri May 6 6:22 2011 tank/home/matt@tuesday Tue May 3 6:27 2011 tank/home/matt@wednesday Wed May 4 6:28 2011 tank/home/matt@thursday Thu May 5 6:28 2011
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 -r rpool NAME AVAIL USED USEDSNAP USEDDS USEDREFRESERV USEDCHILD rpool 59.1G 7.84G 21K 109K 0 7.84G rpool@snap1 - 21K - - - - rpool/ROOT 59.1G 4.78G 0 31K 0 4.78G rpool/ROOT@snap1 - 0 - - - - rpool/ROOT/zfsBE 59.1G 4.78G 15.6M 4.76G 0 0 rpool/ROOT/zfsBE@snap1 - 15.6M - - - - rpool/dump 59.1G 1.00G 16K 1.00G 0 0 rpool/dump@snap1 - 16K - - - - rpool/export 59.1G 99K 18K 32K 0 49K rpool/export@snap1 - 18K - - - - rpool/export/home 59.1G 49K 18K 31K 0 0 rpool/export/home@snap1 - 18K - - - - rpool/swap 61.2G 2.06G 0 16K 2.06G 0 rpool/swap@snap1 - 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/matt@tuesday cannot rollback to 'tank/home/matt@tuesday': more recent snapshots exist use '-r' to force deletion of the following snapshots: tank/home/matt@wednesday tank/home/matt@thursday # zfs rollback -r tank/home/matt@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/matt NAME CREATION tank/home/matt@tuesday Tue May 3 6:27 2011
You can determine ZFS snapshot differences by using the zfs diff command.
For example, assume that the following two snapshots are created:
$ ls /tank/home/tim fileA $ zfs snapshot tank/home/tim@snap1 $ ls /tank/home/tim fileA fileB $ zfs snapshot tank/home/tim@snap2
For example, to identify the differences between two snapshots, use syntax similar to the following:
$ zfs diff tank/home/tim@snap1 tank/home/timh@snap2 M /tank/home/tim/ + /tank/home/tim/fileB
In the output, the M indicates that the directory has been modified. The + indicates that fileB exists in the later snapshot.
The R in the following output indicates that a file in a snapshot has been renamed.
$ mv /tank/cindy/fileB /tank/cindy/fileC $ zfs snapshot tank/cindy@snap2 $ zfs diff tank/cindy@snap1 tank/cindy@snap2 M /tank/cindy/ R /tank/cindy/fileB -> /tank/cindy/fileC
The following table summarizes the file or directory changes that are identified by the zfs diff command.
For more information, see zfs(1M).