Thanks Martin. Good info. What criteria do you use to determine the
number of concurrent jobs for a DSSU? Is it reasonable to determine
the concurrent DSSU sessions based on the speed of the clients? For
example, If I have a disk to which I can write 100MB/s and clients
that can write 5MB/s then I would use 20 concurrent sessions?

Thanks all for your responses!

Vic


On Mon, Apr 26, 2010 at 10:14 AM, Martin, Jonathan
<JMARTI05 AT intersil DOT com> wrote:
> We use Disk Stage Storage Units (DSSU) almost exclusively for our backups.
> As someone has already mentioned, you can stream many slow clients to a DSSU
> without impacting your speed significantly. To do this with tape you would
> have to use multiplexing, which is a real performance killer come restore
> time.  The DSSUs essentially allow you to “multiplex” to disk, then single
> stream to tape. Regarding Ed’s speed issue below, I’ve got data here that
> directly correlates number of concurrent streams written to a DSSU with the
> performance to tape.  You’ll need to balance this when setting this on the
> DSSU, but we get away with 8-12 streams on 14x500GB SATA Disks in a Raid-5
> and still drive LTO3.
>
>
>
> Here we were also able to purchase a much smaller tape library.  I replaced
> a library with 8 x SDLT220 drives with a library with 2 x LTO3 drives (and
> disk storage.)  The disk storage is far more reliable than tapes and
> libraries. As Ed noted below you also benefit from having your backups on
> disk which makes restores very snappy.
>
>
>
> -Jonathan
>
>
>
> From: veritas-bu-bounces AT mailman.eng.auburn DOT edu
> [mailto:veritas-bu-bounces AT mailman.eng.auburn DOT edu] On Behalf Of Ed 
> Wilts
> Sent: Sunday, April 25, 2010 3:45 PM
> To: Victor Engle
> Cc: veritas-bu AT mailman.eng.auburn DOT edu
> Subject: Re: [Veritas-bu] Architectural question (staging)
>
>
>
> On Sun, Apr 25, 2010 at 2:14 PM, Victor Engle <victor.engle AT gmail DOT com>
> wrote:
>
> 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 then
> moved to tape. I have a tape library with four LTO-3 drives connected
> via FC. So what I'm wondering is, since the LTO drives are reasonably
> fast, and since I'm writing the data ultimately to tape anyway, would
> it be better to just write directly to tape. The disk is just old
> fashioned spinning disk with no de-duplication so there are
> operational costs for the disks. All tape and disk storage units are
> 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 with
> 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 disk.
>
> 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.
>
>    .../Ed
>
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