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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 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.
Note 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.
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 the 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:
Comparison of snapshot features compares the features that are supported by the 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.