Administering Instant Snapshots

VERITAS Volume Manager (VxVM) provides the capability for taking an image of a volume at a given point in time. Such an image is referred to as a volume snapshot.

Instant 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.

Two kinds of instant volume snapshots are supported for use with application volumes created by VERITAS Intelligent Storage Provisioning (ISP):

Full-sized instant snapshots require an empty volume to be prepared for use as the snapshot volume as described in Creating a Volume for Use as a Full-Sized Instant Snapshot. The length of this volume must be the same as that of the volume whose snapshot is being taken. A snapshot volume can be created in either a data pool or in a clone pool. It must be created in a clone pool if you want to move it to a separate disk group. See Preparing Storage Pools for Full-Sized Instant Snapshots for details.
Space-optimized instant snapshots require less space than a full-sized instant snapshot but they cannot be dissociated from their original volume nor can they be moved into a different disk group. A space-optimized snapshot uses a storage pool or cache, which can be shared by one or more other space-optimized snapshots. You must set up the cache and the space-optimized snapshot volume in advance of creating any space-optimized snapshots, as described in Creating a Shared Cache Volume and Preparing Space-Optimized Snapshots.

The creation and administration of instant snapshots is described in Creating Instant Snapshots and in subsequent sections. For details of how to use volume snapshots to implement off-host online backup, see Implementing Off-Host Processing Solutions.

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.


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Product: Storage Foundation Guides
Manual: Storage Foundation 4.1 Intelligent Storage Provisioning Administrator's Guide
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