As discussed in the introduction, instant open systems backup can be much easier to set up. Say you are hosting AIX data on an EMC symmetrix. All you need to do, is split off a BCV for that data, then mount it on a different server. That gives you an instant backup. Restores simply require that files are copied from the BCV back to the original server. Can it really be that simple?
Yes, almost. There are a few problems with the scenario above. If the BCV is mounted on a separate server, then what is to stop users updating it, and corrupting the backups? The BCV needs to be set up as read only. Another issue is that you will certainly want several generations of backups. Can you afford to keep 10 full copies of all your data online? You want to copy that data off to tape each day, so the BCV can be refreshed.
There is another issue, the backups will be recorded for the 'wrong' server. You need to know where the original data came from, so if you need to do a restore from tape, you can restore to the right server.
Lots of backup products use snapshots in the background as a temporary data store, so a backup happens very quickly from the application perspective, while the data movement off to tape or cheap disk happens in the background. Examples include Netbackup and TSM when backing up VM machines.