[TriLUG] How do you know when a RAID array is BROKE?

[David Brain]

> There's been some discussion the last few days about the merits of HW
> vs SW raid on the Backuppc support mailing list.  It seems to be a not
> quite religious issue, maybe not up to the level of vi vs. emacs.

Yes, there certainly are some strong opinions on both sides of the issue.

>
> Just how much performance penalty is there in SW raid?

As a semi-random statistic, I have a db server with a 7 (scsi) disk
array attached, software raid5.  Bonnie tests gave read&write
performance ~= writing to a single local scsci disk (so I suspect the
bottleneck there is the bus itself).  I've seen the raid process eat
5-7% of the cpu on the box during large queries (bearing in mind this
isn't a particularly strong box by current standards - Dell 1650).

Am I prepared to eat ~5% cpu to use software raid on this box - yes -
I'm not going to notice 5%. Also presumably this penalty would
represent an even smaller percentage on a stronger box.

David