Overview of 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:
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Persist across system reboots.
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The theoretical maximum number of snapshots is 264.
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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.
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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.
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:
filesystem@snapname volume@snapname |
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 |
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 tank/home@now 0 - 29.5K - tank/home/ahrens@now 0 - 2.15M - tank/home/anne@now 0 - 1.89M - tank/home/bob@now 0 - 1.89M - tank/home/cindys@now 0 - 2.15M - |
Snapshots have no modifiable properties. Nor can dataset properties be applied to a snapshot.
# 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:
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 |
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.
Holding ZFS Snapshots
If you have different automatic snapshot policies so 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 the snapshot and decreases by one whenever a hold is released.
In the previous Solaris release, snapshots 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 put a 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 |
The above syntax adds a single reference, keep, to the given snapshot or snapshots. Each snapshot has its own tag namespace and tags must be unique within that space. If a hold exists on a snapshot, attempts to destroy that 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 Fri Oct 2 12:40:12 2009 |
# zfs holds -r tank/home@now NAME TAG TIMESTAMP tank/home/cindys@now keep Fri Oct 2 12:40:12 2009 tank/home/mark@now keep Fri Oct 2 12:40:12 2009 tank/home@now keep Fri Oct 2 12:40:12 2009 |
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 destroy with the zfs destroy command. For example:
# zfs destroy -r tank/home@now |
Two new properties identify snapshot hold information:
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The defer_destroy property is on if the snapshot has been marked for deferred destroy by using the zfs destroy -d command. Otherwise, the property is off.
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The userrefs property is set to the number of holds on this snapshot.
Renaming ZFS Snapshots
You can rename snapshots but they must be renamed within the same pool and dataset from which they were created. For example:
# zfs rename tank/home/cindys@083006 tank/home/cindys@today |
In addition, the following shortcut syntax provides equivalent snapshot renaming syntax as the example above.
# 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 with 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 - |
Displaying and Accessing ZFS Snapshots
By default, snapshots are no longer displayed in the zfs list output. You must 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 off default # zpool set listsnapshots=on tank # zpool get listsnapshots tank NAME PROPERTY VALUE SOURCE tank listsnapshots on local |
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 /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/ahrens@tuesday Mon Aug 31 11:03 2009 tank/home/ahrens@wednesday Mon Aug 31 11:03 2009 tank/home/ahrens@thursday Mon Aug 31 11:03 2009 tank/home/cindys@now Mon Aug 31 11:04 2009 |
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.
You can identify additional information about how the values of the used property are consumed. New read-only file system properties describe space usage for clones, file systems, and volumes. For example:
$ zfs list -o space NAME AVAIL USED USEDSNAP USEDDS USEDREFRESERV USEDCHILD rpool 25.4G 7.79G 0 64K 0 7.79G rpool/ROOT 25.4G 6.29G 0 18K 0 6.29G rpool/ROOT/snv_98 25.4G 6.29G 0 6.29G 0 0 rpool/dump 25.4G 1.00G 0 1.00G 0 0 rpool/export 25.4G 38K 0 20K 0 18K rpool/export/home 25.4G 18K 0 18K 0 0 rpool/swap 25.8G 512M 0 111M 401M 0 |
For a description of these properties, see Table 6–1.
Rolling Back 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.
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.
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: tank/home/ahrens@wednesday tank/home/ahrens@thursday # 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 Wed Aug 27 16:35 2009 |
Managing Automatic ZFS Snapshots
The Time Slider snapshot tool automatically snapshots ZFS file systems and allows you to browse and recover snapshots of file systems. This tool provides the following features:
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SMF snapshot service instances that schedule reoccurring snapshots
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Automatic snapshots are scheduled from the zfssnap crontab
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Older snapshots are removed based on a predefined percentage of file system space used
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Browse and recover files from snapshots by using the GNOME file manager
You will need to add yourself to the zfssnap role to use the GNOME file manager to modify Time Slider behavior.
When the Time Slider tool is enabled, ZFS file system snapshots are created based on the following criteria:
frequent snapshots every 15 mins, keeping 4 snapshots hourly snapshots every hour, keeping 24 snapshots daily snapshots every day, keeping 31 snapshots weekly snapshots every week, keeping 7 snapshots monthly snapshots every month, keeping 12 snapshots |
How to Manage Automatic ZFS Snapshots
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Start a privileged shell, if running an OpenSolaris release. Otherwise, become a privileged user.
user@opensolaris:-$ pfexec bash #
In this example, the bash shell is selected. You can choose to use any shell with the pfexec command. If you support multiple versions of the SolarisTM OS, use the standard su root command. The su command works on all Solaris versions.
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Enable the Time Slider service, which is disabled by default.
You can enable or disable these services from the command line or from the System->Preferences->Time Slider Setup menu. From this menu, you can also customize which ZFS file systems to snapshot and adjust the file system capacity setting for when snapshots are removed.
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Review the default automatic snapshot service instances that are started when the Time Slider service is enabled.
# svcs | grep auto-snapshot online Oct_22 svc:/system/filesystem/zfs/auto-snapshot:frequent online Oct_22 svc:/system/filesystem/zfs/auto-snapshot:hourly online Oct_22 svc:/system/filesystem/zfs/auto-snapshot:weekly online Oct_22 svc:/system/filesystem/zfs/auto-snapshot:monthly online Oct_22 svc:/system/filesystem/zfs/auto-snapshot:daily
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Confirm that automatic snapshots are created.
For example:
# zfs list -t snapshot NAME USED AVAIL REFER MOUNTPOINT rpool@zfs-auto-snap:weekly-2008-11-13-15:39 0 - 46.5K - rpool@zfs-auto-snap:daily-2008-11-13-15:39 0 - 46.5K - rpool@zfs-auto-snap:hourly-2008-11-13-15:39 0 - 46.5K - rpool@zfs-auto-snap:frequent-2008-11-13-15:39 0 - 46.5K - rpool/ROOT@zfs-auto-snap:weekly-2008-11-13-15:39 0 - 18K - rpool/ROOT@zfs-auto-snap:daily-2008-11-13-15:39 0 - 18K - rpool/ROOT@zfs-auto-snap:hourly-2008-11-13-15:39 0 - 18K - rpool/ROOT@zfs-auto-snap:frequent-2008-11-13-15:39 0 - 18K - . . .
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Disable or enable specific automatic snapshot services for the top-level dataset and all descendent datasets from the command line.
For example:
# zfs set com.sun:auto-snapshot=false rpool # zfs set com.sun:auto-snapshot=true rpool/ROOT/opensolaris
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Choose to only take snapshots under a given schedule for a dataset and all direct descendent datasets from the command line.
For example:
# zfs set com.sun:auto-snapshot=false rpool/export # zfs set com.sun:auto-snapshot:weekly=true rpool/export
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Change the frequency of a given snapshot schedule from the command line.
For example:
# svccfg -s svc:/system/filesystem/zfs/auto-snapshot:frequent setprop zfs/period = 30 # svccfg -s svc:/system/filesystem/zfs/auto-snapshot:frequent refresh # svcadm restart svc:/system/filesystem/zfs/auto-snapshot:frequent
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Disable automatic snapshot services for the swap and dump volumes, from the command line, if necessary.
For example:
# zfs set com.sun:auto-snapshot=false rpool/dump # zfs set com.sun:auto-snapshot=false rpool/swap
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Remove a range of unwanted snapshots, from the command line, if necessary.
For example, remove all automatic snapshots in the bash shell, as follows:
for s in `zfs list -H -o name -t snapshot | grep @zfs-auto-snap`; do zfs destroy $s; done
How to Recover Automatic ZFS Snapshots (GNOME File
Manager)
You can browse and recover snapshots from the GNOME desktop file manager, which is accessed as follows:
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Click the Desktop icon under the Places tab
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Click the clock icon with the slider from the Desktop - File Browser
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Browse your snapshots by opening any folder in the GNOME file manager.
If the Restore icon (clock with slider ) is enabled, snapshots of this directory are available.
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Click on the clock icon to access snapshot navigation features.
Directly below the location: URL section is a one line description of the following information:
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The timeline or date and time that the snapshot was taken
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The location in the backup timeline of the current snapshot
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The number of snapshots available for this directory and the space consumed by ZFS snapshots
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Drag the slider into the past to retrieve previous versions of your files.
You can open all your files in read-only mode. Or, you can browse in list view mode (as oppose to icon view) so that a restore information column automatically appears. This column gives you contextual information about either of the following:
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The file version number, if you browse in the current or latest version of a directory
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The difference, if any, between the file snapshot and the latest version of the file
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Recover files from an automatic snapshot from either of the following methods:
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Drag and drop a snapshot into another file manager window
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Right click the mouse and select the restore to Desktop icon. Then, copy and paste the selected snapshot to the present time.
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