1. Managing ZFS File Systems in Oracle Solaris 11.4
  2. Working With Oracle Solaris ZFS Snapshots and Clones
  3. Overview of ZFS Snapshots
  4. Creating and Destroying ZFS Snapshots
  5. Holding ZFS Snapshots

Holding ZFS Snapshots

Older snapshots are sometimes inadvertently destroyed due to different automatic snapshot or data retention policies. If a removed snapshot is part of an ongoing ZFS send and receive operation, then the operation might fail. To avoid this scenario, consider placing a hold on a snapshot.

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, you could destroy a snapshot only 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 system1/home/kaydo/snap@1: 

$ zfs hold keep system1/home/kaydo@snap1

To recursively hold the snapshots of all descendant file systems, use the -r option. For example: 

$ zfs snapshot -r system1/home@now
$ zfs hold -r keep system1/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 system1/home/kaydo@snap1
cannot destroy 'system1/home/kaydo@snap1': dataset is busy

To destroy a held snapshot, use the -d option. For example: 

$ zfs destroy -d system1/home/kaydo@snap1

To display a list of held snapshots, use the zfs holds command. For example: 

$ zfs holds system1/home@now
NAME              TAG   TIMESTAMP
system1/home@now  keep  Fri Aug  3 15:15:53 2012  
$ zfs holds -r system1/home@now
NAME                    TAG   TIMESTAMP
system1/home/kaydo@now  keep  Fri Aug  3 15:15:53 2012
system1/home/glori@now  keep  Fri Aug  3 15:15:53 2012
system1/home/hsolo@now  keep  Fri Aug  3 15:15:53 2012
system1/home/cpark@now  keep  Fri Aug  3 15:15:53 2012
system1/home@now        keep  Fri Aug  3 15:15:53 2012

To release a hold on a snapshot or set of snapshots, use the zfs release command. For example: 

$ zfs release -r keep system1/home@now

If the snapshot is released, the snapshot can be destroyed by using the zfs destroy command. For example: 

$ zfs destroy -r system1/home@now

Two properties identify snapshot hold information.

  • The destroyer property is set to on if the snapshot has been marked for deferred destruction by using the zfs destroy -d command. Otherwise, the property is set to off.

  • The userrefs property is set to the number of holds on this snapshot.