304 Administering volume snapshots

Note: 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.

Methods of creating volume snapshots are described in the following sections:

■“Creating instant snapshots” on page 319 describes how to use the vxsnap command to create and administer full-sized and space-optimized instant snapshots.

■“Creating traditional third-mirrorbreak-off snapshots” on page 348 describes how to use the vxassist command to create and administer traditional third-mirror snapshots.

For details of how to use volume snapshots to implement off-host online backup, see “Configuring off-host processing” on page 369.

Note: 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.

Most VxVM commands require superuser or equivalent privileges.

Full details of how to recover from failures of instant snapshot commands may be found in the “Recovery from failure of instant snapshot operations’’ chapter of the Veritas Volume Manager Troubleshooting Guide.