Sun Microsystems StorageTek 6140 Administration Manual page 87

Note: A snapshot of data is not suitable for failure recovery. Continue to use
offline backup methods to create full-volume backup copies.
Snapshots on the array are copy-on-write or dependent copies. In this type of
snapshot, write operations to the primary volume causes the management software
to copy the snapshot metadata and copy-on-write data to the reserve volume.
Because the only blocks that are physically stored in the reserve volume are those
that have changed since the time the snapshot was created, the snapshot uses less
disk space than a full physical copy.
When a write operation occurs on the primary volume to a data block in which the
data has not changed since the snapshot was created, the management software does
the following:
Copies the old data to the reserve volume
Writes the new data to the primary volume
Adds a record to the snapshot bitmap indicating the location of the new data
When a data host sends a read request to the snapshot, the management software
checks whether the requested blocks have changed on the primary volume since the
snapshot was created. If they have changed, the read request is satisfied from the
data stored in the snapshot reserve volume. If blocks have not changed, the read
request is satisfied from the primary volume. Snapshots can also accept write
operations. Write operations to a snapshot are stored in the snapshot reserve
volume.
The management software provides a warning message when the reserve volume
nears the threshold, which is a configurable percentage of the full capacity of the
snapshot reserve (the default is 50 percent). When the reserve volume threshold is
met, the reserve volume's capacity can be expanded using the free capacity on the
virtual disk.
As long as a snapshot is enabled, storage array performance is affected by the copy-
on-write activity to the associated reserve volume. If a snapshot is no longer needed,
you can stop the copy-on-write activity by either disabling or deleting the snapshot.
When a snapshot is disabled, it and its associated reserve volume still exist. When
you need to create a different point-in-time image of the same primary volume, you
can resnap the volume to reuse the disabled snapshot and its associated reserve
volume. This takes less time than creating a new snapshot.
If you do not intend to re-create a snapshot, you can delete the snapshot instead of
disabling it. When you delete a snapshot, the management software also deletes the
associated reserve volume.
To see the current snapshots for a particular volume, go to the Related Information
section of Snapshot Summary page for that volume, as described in "Displaying
Volume Snapshot Information" on page 77.
Chapter 4 Configuring Volume Snapshots 71