Solaris ZFS Administration Guide

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:

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:


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
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:

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:

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 

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
tank  listsnapshots  off        default
# zpool set listsnapshots=on tank
# zpool get listsnapshots tank
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
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:
# 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:

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

ProcedureHow to Manage Automatic ZFS Snapshots

  1. 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.

  2. 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.

  3. 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
  4. 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  -
  5. 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
  6. 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
  7. 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
  8. 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
  9. 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

ProcedureHow 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:

  1. 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.

  2. 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:

    • The timeline or date and time that the snapshot was taken

    • The location in the backup timeline of the current snapshot

    • The number of snapshots available for this directory and the space consumed by ZFS snapshots

  3. 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:

    • The file version number, if you browse in the current or latest version of a directory

    • The difference, if any, between the file snapshot and the latest version of the file

  4. Recover files from an automatic snapshot from either of the following methods:

    • Drag and drop a snapshot into another file manager window

    • Right click the mouse and select the restore to Desktop icon. Then, copy and paste the selected snapshot to the present time.