Volume Snapshots

VERITAS Volume Manager 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. Such snapshots should not be confused with file system snapshots, which are point-in-time images of a VERITAS File System.

The figure, Volume Snapshot as a Point-In-Time Image of a Volume, illustrates how a snapshot volume represents a copy of an original volume at a given point in time. Even though the contents of the original volume can change, the snapshot volume can be used to preserve the contents of the original volume as they existed at an earlier time.

The snapshot volume provides a stable and independent base for making backups of the contents of the original volume, or for other applications such as decision support. In the figure, the contents of the snapshot volume are eventually resynchronized with the original volume at a later point in time.

Another possibility is to use the snapshot volume to restore the contents of the original volume. This may be useful if the contents of the original volume have become corrupted in some way.

Volume Snapshot as a Point-In-Time Image of a Volume

Click the thumbnail above to view full-sized image.

The traditional type of volume snapshot in VxVM is of the third-mirror break-off type. This name comes from its implementation where a snapshot plex (or third mirror) is added to a mirrored volume. The contents of the snapshot plex are then synchronized from the original plexes of the volume. When this synchronization is complete, the snapshot plex can be detached as a snapshot volume for use in backup or decision support applications. At a later time, the snapshot plex can be reattached to the original volume, requiring a full resynchronization of the snapshot plex's contents. For more information about this type of snapshot, see Traditional Third-Mirror Break-Off Snapshots.

The FastResync feature was introduced to track writes to the original volume. This tracking means that only a partial, and therefore much faster, resynchronization is required on reattaching the snapshot plex. In later releases, the snapshot model was enhanced to allow snapshot volumes to contain more than a single plex, reattachment of a subset of a snapshot volume's plexes, and persistence of FastResync across system reboots or cluster restarts.

For more information about FastResync, see FastResync.

Release 4.0 of VxVM introduced full-sized instant snapshots and space-optimized instant snapshots, which offer advantages over traditional third-mirror snapshots such as immediate availability and easier configuration and administration. You can also use a third-mirror break-off usage model with full-sized snapshots, where this is necessary for write-intensive applications.

For more information, see the following sections:

• Full-Sized Instant Snapshots.
• Space-Optimized Instant Snapshots.
• Emulation of Third-Mirror Break-Off Snapshots.

Comparison of Snapshot Features compares the features that are supported by the three different types of snapshot.

For more information about taking snapshots of a volume, see Administering Volume Snapshots, and the vxsnap(1M) and vxassist(1M) manual pages.

Comparison of Snapshot Features

The table, Comparison of Snapshot Features for Supported Snapshot Types, compares the features of the various types of snapshots that are supported in VxVM.

Full-sized instant snapshots are easier to configure and offer more flexibility of use than do traditional third-mirror break-off snapshots. For preference, new volumes should be configured to use snapshots that have been created using the vxsnap command rather than using the vxassist command. Legacy volumes can also be reconfigured to use vxsnap snapshots, but this requires rewriting of administration scripts that assume the vxassist snapshot model.

If storage space is at a premium, space-optimized instant snapshots can be configured with some reduction of supported functionality. For example, space-optimized snapshots cannot be turned into independent volumes, nor can they be moved into a separate disk group for off-host processing.

Comparison of Snapshot Features for Supported Snapshot Types

^{1 If vxsnap is used, the snapshot can be selectively removed following restoration of the original volume.}

^{2 Restoration does not remove the snapshot.}

^{3 Synchronization of the snapshot volume must be complete.}

^{4 Persistent FastResync must have been enabled.}