[Veritas-bu] Fwd: DSU Question.

On Sun, Mar 29, 2009 at 4:36 AM, Andrew Sydelko <andrew AT sydelko DOT org> wrote:

On Sat, 28 Mar 2009 09:55:04 -0500
Ed Wilts <ewilts AT ewilts DOT org> wrote:

> On Sat, Mar 28, 2009 at 1:07 AM, Dean Allen <dean.deano AT gmail DOT com> wrote:
>
> > Before you get too far into design, you should work out how fast the
> > backups to disk actually are. This can have a big influence on how you
> > use your DSSU. I have a 3TB DSSU, (RAID 6 SATA) and I can't get any
> > better than about 50 MB/sec out of it (usually about 30 MB/sec). I can
> > get the tape drives up to 150 MB/sec. The majority of people seem to
> > think backing up to disk is going to be faster, but that's not
> > necessarily the case.
>
>
> I concur 100% - this has been our experience as well.
>
> SATA drives are cheap, but they're slow. You'll do about half the
> transactional rate of a good fibre channel or SCSI drive.
>
> Many people think that because "it's just backups" that you can use cheaper
> SATA drives. It's actually the opposite - backups are the highest I/O
> generator, by far, in many environments.

This really depends on the controller in use. There are controllers out there that will do 400MB/s or faster. We have 3ware and Areca controllers that do just this. This allows us to use multiple FULL gigabit connections in from clients as well as stage to multiple LTO3 tape drives at the same time using one media server.

This is an HDS AMS 1000. I know the controller is capable of more throughput. The bottleneck seems to be the number of back end fibre loops, of which there are only 2. Another problem is that I am sharing both loops/disk trays with Enterprise Vault for Exchange, which is constantly dribbling at the disk.

If we added more disk trays, the throughput would theoretically increase relative to the number of trays.

To get performance close to a tape drive, I guesstimate I'd need 6 drive trays, which would give me about 50TB of usable space. But I don't need that much - I'd prefer to have 1TB of fast FC drives on the USPV, which is a whole different kettle of fish.

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