Re: [Veritas-bu] DSU Question.

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.

I still direct the majority of backups straight to tape, and use the
DSSU for the big long slow ones, like the 1.5 TB Windows client which
can only send data to the NBU server at about 15 MB/sec. I send that
to disk, so it doesn't hold a tape drive for a day and a half.

For large Windows file systems, we use FlashBackup. We're pushing the larger file systems at 50-60MB/sec now.

I also use the DSSU for the backups which don't really have a large
amount of data to backup, but need to run and complete fairly quickly,
like Oracle archive logs.

Absolutely - this is a great use of DSSUs.

We'd tried to push everything to DSSUs but this ended up not working for us because of the lousy destage performace we're seeing. We can push a client directly to tape a lot faster than a destage to tape but we don't have enough tape drives to allow the clients to do this within the backup windows.

One of these days we'll figure out how to destage with decent performance but so far we haven't been able to figure it out (and neither has Symantec, and not for lack of trying).

--
Ed Wilts, Mounds View, MN, USA
ewilts AT ewilts DOT org

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