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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.

Volume snapshot as a point-in-time image of a volume shows how a snapshot volume represents a copy of an original volume at a given point in time.

Volume snapshot as a point-in-time image of a volume

Volume snapshot as a point-in-time image of a volume

Click the thumbnail above to view full-sized image.

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.

Warning: If you choose to write to the snapshot volume, it may no longer be suitable for use in restoring the contents of the original volume.

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.

See "Traditional third-mirror break-off snapshots" on page 313.

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.

See "FastResync" on page 66.

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 the third-mirror break-off usage model with full-sized snapshots, where this is necessary for write-intensive applications.

See "Full-sized instant snapshots" on page 315.

See "Space-optimized instant snapshots" on page 317.

See "Emulation of third-mirror break-off snapshots" on page 318.

See "Linked break-off snapshot volumes" on page 319.

See "Comparison of snapshot features" on page 64.

See "Administering volume snapshots" on page 311.

See the vxassist(1M) manual page.

See the vxsnap(1M) manual page.