Ed, you're close...

The DSSU will actually duplicate the images to tape at regular intervals 
(scheduled via the STU) or manually.

Then, when a backup needs additional disk space, the previously migrated 
(duplicated) images that already exist on tape will be removed.

If you get into a situation where a backup needs more disk and there 
aren't any available images to remove, you'll run out of disk space and 
the backup will fail with a status 84 (media write error of all things).

Check out the NetBackup 5.0 SAG Vol 1 for more info.

Just to clarify the cleanup process I'm referring to, here's what that
guide says:

When a Stage I (write backup to disk) process detects a full disk staging
storage unit, it pauses the backup, finds the oldest image that has been
successfully copied to the destination storage unit, and expires this
image copy.

HTH
-Tim

On Wed, 28 Apr 2004, Ed Wilts wrote:

> On Wed, Apr 28, 2004 at 10:12:53AM -0500, Simon, John wrote:
> > you could use Vault with 4.5 and your disk storage unit, which is
> > essentially what the disk staging in 5.0 does. It duplicated the disk
> > image to tape then deletes the disk image depending on retention level
> > settings. Course you could do the whole thing yourself with a little
> > bpduplicate action ;)
> 
> 5.0 does a bit more than that.  When a disk staging unit gets full, it
> will suspend the backup, write the disk to tape, and resume the backup.
> It is also smart enough to realize that when you're doing a restore, if
> the data is still on disk (it's not deleted unless it has to be, even if
> the data has been copied to tape), then the restore will take place from
> disk and not tape.
> 
>