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About volume snapshots

Veritas Volume Manager (VxVM) provides the capability for taking an image of a volume at a given point in time. Such an image is referred to as a volume snapshot.

See "Volume snapshots" on page 61.

You can also take a snapshot of a volume set.

See "Creating instant snapshots of volume sets" on page 316.

Volume snapshots allow you to make backup copies of your volumes online with minimal interruption to users. You can then use the backup copies to restore data that has been lost due to disk failure, software errors or human mistakes, or to create replica volumes for the purposes of report generation, application development, or testing.

See "Traditional third-mirror break-off snapshots" on page 289.

See "Full-sized instant snapshots" on page 291.

See "Space-optimized instant snapshots" on page 293.

See "Emulation of third-mirror break-off snapshots" on page 294.

See "Linked break-off snapshot volumes" on page 295.

See "Cascaded snapshots" on page 296.

See "Creating multiple snapshots" on page 300.

See "Restoring the original volume from a snapshot" on page 300.

A volume snapshot represents the data that exists in a volume at a given point in time. As such, VxVM does not have any knowledge of data that is cached by the overlying file system, or by applications such as databases that have files open in the file system. If the fsgen volume usage type is set on a volume that contains a Veritas File System (VxFS), intent logging of the file system metadata ensures the internal consistency of the file system that is backed up. For other file system types, depending on the intent logging capabilities of the file system, there may potentially be inconsistencies between data in memory and in the snapshot image.

For databases, a suitable mechanism must additionally be used to ensure the integrity of tablespace data when the volume snapshot is taken. The facility to temporarily suspend file system I/O is provided by most modern database software. For ordinary files in a file system, which may be open to a wide variety of different applications, there may be no way to ensure the complete integrity of the file data other than by shutting down the applications and temporarily unmounting the file system. In many cases, it may only be important to ensure the integrity of file data that is not in active use at the time that you take the snapshot.

There are two alternative methods of creating volume snapshots.

See "Creating instant snapshots" on page 301.

See "Creating traditional third-mirror break-off snapshots" on page 330.

Volume snapshots can be used to implement off-host online backup.

See "Configuring off-host processing" on page 349.

Snapshot creation using the vxsnap command is the preferred mechanism for implementing online and off-host point-in-time copy solutions in VxVM. Support for traditional third-mirror snapshots that are created using the vxassist command may be removed in a future release.

  Note   Most VxVM commands require superuser or equivalent privileges.

There are several ways of recovering from the failure of instant snapshot commands.

See the Veritas Volume Manager Troubleshooting Guide.