Veritas Storage Foundation Intelligent Storage Provisioning 5.0 AdministratorÆs Guide, HP-UX 11i v3, First Edition, May 2008

94 Administering instant snapshots
Limitations of volume snapshots
See “Creating instant snapshots” on page 101.
For details of how to use volume snapshots to implement off-host online backup,
see the Veritas Storage Foundation Intelligent Storage Provisioning Solutions
Guide.
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.
Note: Most VxVM commands require superuser or equivalent privileges.
Limitations of volume snapshots
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.