Veritas-bu

[Veritas-bu] DSSU Storage?

2006-08-21 09:10:25
Subject: [Veritas-bu] DSSU Storage?
From: JMARTI05 at intersil.com (Martin, Jonathan (Contractor))
Date: Mon, 21 Aug 2006 09:10:25 -0400
>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.

Can you tell me exactly what an "image" is?  When I write backups
directly to tape now, I'm pretty sure we create LOTS of images on tape,
through multistreaming, and checkpoint restart.  Are "images" some how
different from fragments.  I think my terminology is a little screwed
up, but what I think you mean is, you can't destage backups from the
DSSU until the backup is 100% finished writing?  So when I get to the
HWM its only going to purge the backups on the DSSU that are complete,
correct?

>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.

I'm trying to get about 15TB of storage which should cover a full
weekend's backups, but I'm a bit concerned I'll only be getting half
that.  Stupid budget.  I have to say while it should work in theory, I'm
a little worried about making it work after reading your post.

>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.

How long are we talking?  I'm either going LTO3 or DLT-S4 both easily
capable of writing 90mb/sec which is about 5GB a minute.  I should be
able to write a terabyte in 3 hours (give or take.)  If a DSSU is 1TB
(assuming I have multiple DSSUs) is 3 hours enough to "wait" for that
DSSU to finish dumping data?

New Questions:

Can I continue to write to a DSSU while its dumping data to tape?  I'm
making sure this is possible from a performance standpoint, but I'm
wondering how Netbackup reacts?

Are the tapes eventually written considered duplicates?  That is, is a
backup goes to DSSU then to tape, is the primary copy now the tape?  Or
that considered a duplicate and the original gets "expired" from the
DSSU?





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