Have to agree with Ed here. I spent many, many months mucking around
with SAN Media Servers, and configuring Disk Staging Units, that then
write to tape!
I came across 2 major problems:
1) Performance was slower than LTO3 drives (FC attached, with SSO)
2) Writing from disk to tape (using the DSSU config) was quick, but and
this is an annoying BUT, it would randomly fail to write to tape!! 
3) The time difference between writing to Disk and Tape was 2 mins. This
difference was mainly where the Robot was picking the tape and mounting
it !
I have stopped going down this route, but although I have my DSSU's
ready and a good 4TB storage available, if the drives went pearshaped, I
could still do the backups to disk, short term !


From: veritas-bu-boun...@mailman.eng.auburn.edu
[mailto:veritas-bu-boun...@mailman.eng.auburn.edu] On Behalf Of Ed Wilts
Sent: Sunday, April 25, 2010 8:45 PM
To: Victor Engle
Cc: veritas-bu@mailman.eng.auburn.edu
Subject: Re: [Veritas-bu] Architectural question (staging)

On Sun, Apr 25, 2010 at 2:14 PM, Victor Engle <victor.en...@gmail.com>

        Just wanted to get some opinions about whether disk staging
units are
        worthwhile. My backup server has two BasicDisk staging units
with the
        storage units configured such that the data goes to disk and is
        moved to tape. I have a tape library with four LTO-3 drives
        via FC. So what I'm wondering is, since the LTO drives are
        fast, and since I'm writing the data ultimately to tape anyway,
        it be better to just write directly to tape. The disk is just
        fashioned spinning disk with no de-duplication so there are
        operational costs for the disks. All tape and disk storage units
        local to the backup server. I'm thinking it would be better to
add LTO
        drives and eliminate the disk for now and maybe later add a
        de-duplicating disk unit.

We worked with disk staging units for at least a year before we mostly
gave up.  The biggest challenge we ran into was that destaging was too
slow. Even though we proved to Symantec that we could read from those
disk drives at over 100MB/sec, we could never destage even half that
fast.  We had an open case with Symantec for a VERY long time before we
agreed that it wasn't going to get fixed.

        Under what circumstances does it make sense to stage data on
disk. I
        would appreciate hearing what your thoughts and experiences are
        regard to disk staging.

There are times when DSSUs make sense.  1.  If you don't have a tape
drive free but want to do a backup anyway - we still use DSSUs for
things like small backups of Oracle archive logs. 2.  If you need to
throttle your backups, especially across things like a bunch of virtual
servers on the same physical server.  NBU only allows you to set the
maximum jobs per client name, not per client.  DSSUs make an acceptable
choke point for clusters.  3.  If you have small backups, but don't have
a lot of them at once, multiplexing may not buy you enough performance
boosts.  Use DSSUs to write those little jobs to disk and then destage
them at once.

If you currently multiplex, realize that your restores are going to be
slower than if you don't multiplex.  All tapes created from a DSSU
destage are non-multiplexed so your restores can go faster.

DSSUs also give you a staging area for restores.  If your tapes go
offsite, you may still be able to do a restore from the staging unit the
next day (or longer) depending on how big your stagig units are.  NBU is
smart enough to realize that if the same data is on both disk and tape
and you kick off a restore, the restore will automatically come from

In general, I'd say that there is a place for DSSUs but it's not the
great benefit we thought it was going to be.


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