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.
Figure: 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.
Even though the contents of the original volume can change, the snapshot volume preserves 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.
One type of volume snapshot in VxVM is 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.
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.
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 information about how and when to use volume snapshots, see the Veritas Storage Foundation and High Availability Solutions Solutions Guide.
See the vxassist(1M) manual page.
See the vxsnap(1M) manual page.