Snapshot File Systems
A snapshot file system is an exact image of a VxFS file system, referred to as the snapped file system, that provides a mechanism for making backups. The snapshot is a consistent view of the file system "snapped" at the point in time the snapshot is made. You can select files to back up from the snapshot (using a standard utility such as cpio or cp), or back up the entire file system image (using the vxdump or fscat utilities).
You use the mount command to create a snapshot file system (the mkfs command is not required). A snapshot file system is always read-only. A snapshot file system exists only as long as it and the snapped file system are mounted and ceases to exist when unmounted. A snapped file system cannot be unmounted until all of its snapshots are unmounted. Although it is possible to have multiple snapshots of a file system made at different times, it is not possible to make a snapshot of a snapshot.
Note
A snapshot file system ceases to exist when unmounted. If mounted again, it is actually a fresh snapshot of the snapped file system.
A snapshot file system must be unmounted before its dependent snapped file system can be unmounted. Neither the fuser command nor the mount command will indicate that a snapped file system cannot be unmounted because a snapshot of it exists.
On cluster file systems, snapshots can be created on any node in the cluster, and backup operations can be performed from that node. The snapshot of a cluster file system is accessible only on the node where it is created, that is, the snapshot file system itself cannot be cluster mounted. See the VERITAS SANPoint Foundation Suite Installation and Configuration Guide for more information on creating snapshots on cluster file systems.
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