Veritas Storage Foundation 5.1 SP1 Advanced Features Administrator"s Guide (5900-1503, April 2011)

a consistent view of all file system structures on the snapped file system for the
time when the snapshot was created. As data blocks are changed on the snapped
file system, the snapshot gradually fills with data copied from the snapped file
system.
The amount of disk space required for the snapshot depends on the rate of change
of the snapped file system and the amount of time the snapshot is maintained. In
the worst case, the snapped file system is completely full and every file is removed
and rewritten. The snapshot file system would need enough blocks to hold a copy
of every block on the snapped file system, plus additional blocks for the data
structures that make up the snapshot file system. This is approximately 101
percent of the size of the snapped file system. Normally, most file systems do not
undergo changes at this extreme rate. During periods of low activity, the snapshot
should only require two to six percent of the blocks of the snapped file system.
During periods of high activity, the snapshot might require 15 percent of the
blocks of the snapped file system. These percentages tend to be lower for larger
file systems and higher for smaller ones.
Warning: If a snapshot file system runs out of space for changed data blocks, it is
disabled and all further attempts to access it fails. This does not affect the snapped
file system.
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
261Administering snapshot file systems
Snapshot file system backups