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Date:	Thu, 8 Oct 1998 13:12:09 PDT
From:	Steve and Melanie Schiffman <schiffman@ibm.net >
Reply-To:	scoug-general@scoug.com
To:	scoug-general@scoug.com
Subject:	SCOUG-General: RAID i/o speed

Content Type: text/plain

Wirtz, Paul wrote:
>
> That's a whole set of articles to answer that question fully.

I have an article on RAID that I aquired a couple of years ago. If I can
find it again, I will make a copy for you.

>
> The short answer is for a single user instead of a server not much speed
> increase. much less chance of data loss.

I agree with Paul's assesment that RAID is intended to provide data
redundency and therefore avoids data loss when there is a hardware
failure. It does not protect against a software I/O subsystem failure in
all cases. Learned this first hand at my last assignment where a Netware
server software was failing and all 60megs of data had to be restored
from the tape backups.

Peter asks if RAID is faster. If I recall from the material I have seen,
RAID can be slower on writes, while faster on reads. This occurs on RAID
level 1 and 2 because the data has to be written twice. There is more
overhead to doing multiple writes for a data sector whereas a read only
has to be done once and can come from which ever drive can respond the
quickest.

>
> -----Original Message-----
> From: Peter Skye [mailto:pskye@peterskye.com]
>
> I want to know what increase in I/O speed I'll get if I install RAID (I
> know there are various "levels", or kinds, of RAID, and I know they have
> different technologies so their I/O speeds are different).

There is also software RAID vs hardware RAID.

>
> Does anyone have any hands-on experience with any RAID installations,
> and if so can you tell me what speed increase I could expect?
>
Expect a decrease in speed for writes, an increase for reads. You will
not be able to see the difference on a single user system. For a Server
running reasonably moderate loads and above, reads will be improved.

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