On 1/24/2007 2:10 PM, Martin, Jonathan (Contractor) wrote:
> Configuring a few DSSUs on 6.0 MP4 and I wanted to bounce this off 
> someone who has possibly been there before.
> 
> What are the advantages / disadvantages of assigning multiple DSSUs
> to the same volume?

According the Veritas technote on DSSUs, this is *strongly* discouraged.
  Don't do it.

> Example - I've got a 2TB volume on a media server (/array or E:\)
> that has two folders on it, DSSU1 and DSSU2.  Both are configured
> with a 98% High Water Mark and 80% Low Water Mark (defaults.)
> 
> Now it seems to me that if planned carefully, this could work.

Accept that it's not in your control - it's under the control of bpdm.
It decides which images to delete, not you.  It picks the rules, not
you.  This all changes in 6.5, but right now you're at the mercy of bpdm.

> As the disk fills up to 98%, say writing several jobs to DSSU1 the
> system will constantly expire old images until the entire volume is
> at 80%.  Lets say those are backups for week 1.  Week 2 and 3 run
> similarly.  Then comes week 4.  On week 4 I want to write the same
> set of backups to DSSU2, which points to another volume pool because
> these tapes do not go to the same place as DSSU1's tapes.  But there
> is only 20% free disk space.  When I start writing to DSSU2, and it
> gets to 98%, what happens? If I'm reading the documentation
> correctly, both DSSU1 and DSSU2 will start expiring images until the
> volume gets down to 80% free disk space, is that correct?

Nope - you're writing to a dssu that fills the volume, so that dssu will
start to have its images expire.  In this case, it will clean up dssu2
until it gets enough free space.  If it can't delete enough from dssu2
to finish the job, your job will fail with a 129.  It will not try to
delete images from dssu1 since it does not know that doing so will help 
- it only cares about the storage units involved in the active job.

> Would this be any different if I wrote to both DSSUs simultaneously? 
> Once free space was less than 2% wouldn't both DSSUs go into action 
> expiring data?

Nope - just the dssu that it's writing to when the volume fills.

> Comments / suggestions?

Follow the best practises - one DSSU per volume.

        .../Ed

-- 
Ed Wilts, Mounds View, MN, USA
mailto:ewilts at ewilts.org