VERITAS File System 4.1 Administrator's Guide

Online Backup Using File System Snapshots
Snapshot File Systems
Chapter 6118
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.