HP JFS 3.3 and HP OnLineJFS 3.3 VERITAS File System 3.3 System Administrator's Guide

88 Chapter4
Online Backup
Using a Snapshot File System for Backup
device being mounted does not identify the device size in its disk label, or
if a size smaller than the entire device is desired. Use the following
syntax to create a snapshot file system:
mount -F vxfs -o
snapof= special | mount_point ,snapsize= snapshot_size \
snapshot_special snapshot_mount_point
The snapshot file system must be created large enough to hold any
blocks on the snapped file system that may be written to while the
snapshot file system exists. If a snapshot file system runs out of blocks to
hold copied data, it will be disabled and all further access to the snapshot
file system will fail.
During a period of low activity when the system is relatively inactive (for
example, on nights and weekends), the snapshot only needs to contain
two to six percent of the blocks of the snapped file system. During a
period of higher activity, the snapshot of an “average” file system might
require 15 percent of the blocks of the snapped file system, though most
file systems do not experience this much turnover of data over an entire
day. These percentages tend to be lower for largerfile systems and higher
for smaller ones. You should manage the blocks allocated to the snapshot
based on such things as file system usage and duration of backups.
NOTE A snapshot file system ceases to exist when unmounted. If remounted, it
will be a fresh snapshot of the snapped file system. A snapshot file
system must be unmounted before the corresponding snapped file system
can be unmounted. If you try to unmount a file system that has an
existing snapshot, umount will fail with a “Device Busy” message, and
fuser will not indicate the problem.
CAUTION Any existing data on the disk used for the snapshot is overwritten and
lost.
Making a Backup
Here are some typical examples of making a backup of a 300,000 block
file system named /home (which exists on disk /dev/dsk/c0t0d0) using
a snapshot file system on /dev/dsk/c0t1d0 with a snapshot mount