Veritas™ File System 5.0.1 Administrator's Guide

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 Storage Foundation Cluster File System Administrator's Guide.
Snapshot file system backups
After a snapshot file system is created, the snapshot maintains a consistent backup
of data in the snapped file system.
Backup programs, such as cpio, that back up a standard file system tree can be
used without modification on a snapshot file system because the snapshot presents
the same data as the snapped file system. Backup programs, such as vxdump, that
access the disk structures of a file system require some modifications to handle
a snapshot file system.
VxFS utilities recognize snapshot file systems and modify their behavior so that
they operate the same way on snapshots as they do on standard file systems.
Other backup programs that typically read the raw disk image cannot work on
snapshots without altering the backup procedure.
These other backup programs can use the fscat command to obtain a raw image
of the entire file system that is identical to an image obtainable by running a dd
command on the disk device containing the snapped file system at the exact
moment the snapshot was created. The snapread ioctl takes arguments similar
to those of the read system call and returns the same results that are obtainable
by performing a read on the disk device containing the snapped file system at the
exact time the snapshot was created. In both cases, however, the snapshot file
system provides a consistent image of the snapped file system with all activity
complete—it is an instantaneous read of the entire file system. This is much
different than the results that would be obtained by a dd or read command on the
disk device of an active file system.
Online backup using file system snapshots
Snapshot file system backups
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