Veritas™ File System 5.0.1 Administrator's Guide

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.
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, the snapshot 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.
Warning: Any existing data on the device used for the snapshot is overwritten.
To create a snapshot file system
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Mount the file system with the -o snapof= option:
# mount -F vxfs -o snapof=special,snapsize=snapshot_size \
snapshot_special snapshot_mount_point
Backup examples
In the following examples, the vxdump utility is used to ascertain whether
/dev/vx/dsk/fsvol/vol1 is a snapshot mounted as /backup/home and does the
appropriate work to get the snapshot data through the mount point.
These are typical examples of making a backup of a 300,000 block file system
named /home using a snapshot file system on /dev/vx/dsk/fsvol/vol1 with a
snapshot mount point of /backup/home.
77Online backup using file system snapshots
Creating a snapshot file system