HP StorageWorks 2000 Series Reference Manual page 116

Before deleting modified data you must unmount the snapshot from data
hosts to avoid data corruption.
To delete the modified (write) data from a snapshot:
1. Unmount the snapshot from hosts.
2. Select Manage > Volume Management > Snapshot Services > Delete Modified Data.
For each virtual disk, the virtual disk panel shows a status icon; the name, RAID
level, size, number of disk drives, and number of volumes; and utility status, if any.
3. Select a virtual disk.
The selected virtual disk's volume names, sizes, and types are displayed, and the
amount of free space.
4. Select the snapshot from which you want to delete modified data.
5. Click Delete Modified Data.
When processing is complete, on the Snapshot Overview page the snapshot's
Unique Data field shows zero.
You can roll back (revert) the data in a master volume to the data that existed when
a specified snapshot was created. You also have the option of rolling back to the
modified (write) data on the snapshot.
Before rolling back a master volume you must unmount it from data
hosts to avoid data corruption. If you want to include snapshot modified data in the
rollback, you must also unmount the snapshot. You can remount the master volume
after the rollback has started. You can remount the snapshot when the rollback is
complete.
Whenever you perform a rollback, the data that existed on the master
volume is replaced by the data on the snapshot; that is, all data on the master
volume written since the snapshot was taken is lost. As a precaution, take a snapshot
of the master volume before starting a rollback.