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

The ncachemirror attribute specifies the number of mirrors to create in
the cache volume. For backup purposes, the default value of 1 should be
sufficient.
For example, to create the space-optimized instant snapshot, snap4myvol,
of the volume, myvol, in the disk group, mydg, on the disk mydg15, and
which uses a newly allocated cache object that is 1GB in size, but which
can automatically grow in size, use the following command:
# vxsnap -g mydg make source=myvol/new=snap4myvol\
/cachesize=1g/autogrow=yes alloc=mydg15
If a cache is created implicitly by specifying cachesize, and ncachemirror
is specified to be greater than 1, a DCO is attached to the cache volume to
enable dirty region logging (DRL). DRL allows fast recovery of the cache
backing store after a system crash. The DCO is allocated on the same disks
as those that are occupied by the DCO of the source volume. This is done
to allow the cache and the source volume to remain in the same disk group
for disk group move, split and join operations.
2
Use fsck (or some utility appropriate for the application running on the
volume) to clean the temporary volume’s contents. For example, you can use
this command with a VxFS file system:
# fsck -F vxfs /dev/vx/rdsk/diskgroup/snapshot
The specified device must have a valid entry in the /etc/fstab file.
3
To backup the data in the snapshot, use an appropriate utility or operating
system command to copy the contents of the snapshot to tape, or to some
other backup medium.
4
You now have the following options:
■ Refresh the contents of the snapshot. This creates a new point-in-time
image of the original volume ready for another backup. If synchronization
was already in progress on the snapshot, this operation may result in large
portions of the snapshot having to be resynchronized.
See “Refreshing an instant snapshot” on page 233.
■ Restore the contents of the original volume from the snapshot volume.
The space-optimized instant snapshot remains intact at the end of the
operation.
See “Restoring a volume from an instant snapshot” on page 235.
■ Destroy the snapshot.
See “Removing an instant snapshot” on page 236.
Administering volume snapshots
Creating instant snapshots
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