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

Table 18-1
Differences between snapshots and Storage Checkpoints (continued)
Storage CheckpointsSnapshots
Are persistentAre transient
Can exist and be mounted on their ownCease to exist after being unmounted
Track changed blocks on each file in the file
system
Track changed blocks on the file system level
Storage Checkpoints also serve as the enabling technology for two other Veritas
features: Block-Level Incremental Backups and Storage Rollback, which are used
extensively for backing up databases.
See “About Storage Checkpoints” on page 267.
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
◆
Mount the file system with the -o snapof= option:
# mount -F vxfs -o snapof=special,snapsize=snapshot_size \
snapshot_special snapshot_mount_point
Administering snapshot file systems
Creating a snapshot file system
264