Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

296 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 310 describes how to use the vxsnap command
to create and administer full-sized and space-optimized instant snapshots.
■ “Creating traditional third-mirror break-off snapshots” on page 337 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 361.
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.