VERITAS File System 4.1 Administrator's Guide

Online Backup Using File System Snapshots
Using a Snapshot File System for Backup
Chapter 6 119
Using a Snapshot File System for Backup
After a snapshot file system is created, the snapshot performs 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.
If you create a complete backup of a snapshot file system using a utility such as vxdump and
later restore it, you must run the fsck command on the restored file system because the
snapshot file system is consistent, but not clean. That is, the file system may have some
extended inode operations to complete, but there should be no other changes. Because a
snapshot file system is not writable, it cannot be fully checked, but the fsck -n command can
be used to report any inconsistencies.