thanks. exactly what i was looking for.
if, say, i create a new retention policy of 3 days, will the expired hd image itself be overwritten (like it does for tapes) or what will NBU do to it? delete it?
The backup retention will only be applied when all secondary operations (e.g. duplications) have completed.
Until then, retention level on disk is set to Infinity.
When the images expire, the unreferenced blocks will be removed during next cleanup cycle.
Unique blocks referenced by newer backups are not removed.
Please read up how dedupe stores data and how cleanup works in NetBackup Deduplication Guide
PLEASE take note that MSDP or any other kind of dedupe storage is just about worthless if you cannot store a minimum of 2 weeks on ddedupe disk.
You may as well configure the disk as Basic or Advanced where there is a one-to-one relationship between backup image and disk storage.
Or backup straight to tape.
You have previously asked about Oracle backups and we know that you are also backing up SQL.
PLEASE read up in the Appliance (which is also MSDP storage) Performance Tuning Guide on how to best configure Oracle and SQL backups for maximum dedupe rates.
Symantec NetBackup 52xx and 5330 Appliance Capacity Planning and Performance Tuning Guide: DOC8187
(search for Oracle and SQL)
Another option is to investigate which data that you backup are not good dedupe candidates and backup those straight to tape.