VERITAS Storage Foundation 4.1 Intelligent Storage Provisioning Administrator's Guide

74 VERITAS Storage Foundation ISP Administrator’s Guide
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 be inconsistencies between in-memory data and the data 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.
Traditional third-mirror break-off snapshots cannot be taken of application volumes that
have been created by ISP.
Most VxVM commands require superuser or equivalent privileges.
For more information about instant volume snapshot features, see the chapter “Understanding
VERITAS Volume Manager” in the VERITAS Volume Manager Administrator’s Guide.
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.