Veritas Storage Foundation 5.1 SP1 Advanced Features Administrator"s Guide (5900-1503, April 2011)

You can also take a snapshot of a volume set.
See “Creating instant snapshots” on page 212.
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.
Volume snapshots can also be used to implement off-host online backup.
See “About off-host cluster file system backup” on page 167.
A volume snapshot captures 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 in memory
by the overlying file system, or by applications such as databases that have files
open in the file system. Snapshots are always crash consistent, that is, they can
be put to use by letting the application perform its recovery. This is similar to
how the application recovery occurs after a server crash. If the fsgen volume
usage type is set on a volume that contains a mounted Veritas File System (VxFS),
VxVM coordinates with VxFS to flush data that is in the cache to the volume. For
other file system types, depending on the capabilities of the file system, there
may potentially be inconsistencies between data in memory and in the snapshot.
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:
■ Creating instant snapshots
See “Creating instant snapshots” on page 212.
■ Creating traditional third-mirror break-off snapshots
See “Creating traditional third-mirror break-off snapshots” on page 244.
Snapshot creation using the vxsnap command is the preferred mechanism for
implementing 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.
To recover from the failure of instant snapshot commands, see the Veritas Volume
Manager Troubleshooting Guide.
Administering volume snapshots
About volume snapshots
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