Re: [Veritas-bu] Architectural question (staging)

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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