Veritas-bu

[Veritas-bu] DSSU Storage?

2006-08-20 10:59:20
Subject: [Veritas-bu] DSSU Storage?
From: ewilts at ewilts.org (Ed Wilts)
Date: Sun, 20 Aug 2006 09:59:20 -0500
veritas-bu-bounces at mailman.eng.auburn.edu <> wrote:
> So my question really isn't how much disk storage do I need
> to keep a weeks data around for a week.  That's a fairly easy
> 20 TB or so.  But rather, with High and Low Water Marks (etc)
> if I'm (for example) dumping to an 800GB LTO3 cart, would a
> disk staging storage unit of (say) 1.2 TB be sufficient to
> allow data to continue writing while the first 800GB is "leaked" off
> to tape? 

First issue - the destaging doesn't work properly in 6.0MP3, even with the
current set of patches.  Don't go there yet - we still get error 129s (DSSU
full) on a regular basis.  I'm expecting new patches (bpdm, bptm) for that
in the next couple of days, but as of today, it's still buggy.  The patches
we're expecting will be the first set for our media servers - all of the
other patches have been for the master server.

Secondly, you need to really understand how DSSU destaging works.  It
destages *images*, not tapes.  If you have 1.2TB DSSU and try to write a 2
TB image to DSSU, it will fail.  The size of your DSSUs need to be based on
the images you're putting out there and tape capacity has no factor in the
calculation.  Destaging also can not happen while an image is still being
written.  So if you attempt to write 2 700GB images to your 1.2TB DSSU at
the same time, they will both fail.  You can't destage half an image.

Third, destaging is done not just based on high-water marks, but also based
on time.  That's pretty obvious when you think about it because the goal is
usually to get the stuff to tape and you shouldn't be penalized if your DSSU
is larger than it needs to be.

Best practises also dictate that you use separate DSSUs for fulls vs
incrementals although we haven't done that ourselves simply because some of
the incrementals are larger than some of the fulls.  The best practise is
based on the size of the backups - ideally, you want to destage as few
images as possible when the highwater mark triggers.

You also need to know that no matter what your highwater mark is, a backup
job will only wait so long for destaging to complete.  The job waits and
waits as images are destaged, but if they don't destage fast enough, your
backup job to DSSU will fail with a 129.

Working with DSSUs takes some trial and error and a bunch of knowledge about
your backups and their sizes.  

   .../Ed

Ed Wilts
mailto:ewilts at ewilts.org
Mounds View, MN 55112



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