VERITAS File System 4.1 Administrator's Guide

Online Backup Using File System Snapshots
Creating a Snapshot File System
Chapter 6120
Creating a Snapshot File System
You create a snapshot file system by using the -o snapof= option of the mount command.
The -o snapsize= option may also be required if the device you are mounting does not
identify the device size in its disk label, or if you want a size smaller than the entire device.
Use the following syntax to create a snapshot file system:
# mount -F vxfs -o snapof=special,snapsize= snapshot_size \
snapshot_special snapshot_mount_point
You must make the snapshot file system 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 runs out of
blocks to hold copied data, it is disabled and further attempts to access the snapshot file
system fail.
During periods of low activity (such as nights and weekends), a snapshot typically requires
about two to six percent of the blocks of the snapped file system. During a period of high
activity, the snapshot of a typical file system may require 15 percent of the blocks of the
snapped file system. Most file systems do not turn over 15 percent of data in a single day.
These approximate percentages tend to be lower for larger file systems and higher for smaller
file systems. You can allocate blocks to a snapshot based on characteristics such as file system
usage and duration of backups.
CAUTION Any existing data on the device used for the snapshot is overwritten.