Good points. However, I have heard disk array sales people try to claim
controller caching improves both read and write performance.  In answer to
your question in your other posting, the AutoRAID cache is 96 MB, pretty
small by today's standards.

-----Original Message-----
From: Steve Dirickson [mailto:[log in to unmask]]
Sent: Thursday, December 09, 1999 8:39 PM
To: [log in to unmask]
Subject: Re: RAID5 Disc's


> Regardless of any theoretical RAID performance characteristics and any
> claims made by any manufacturer, I believe the best advice for someone
> considering RAID is to get a demo unit of whichever product
> and see how it performs on YOUR system with YOUR applications.

Definitely; numbers on a spec sheet don't bring in the dollars--performance
does.

> Disc arrays also promise to
> improve performance through the use of caching.  The caching
> in arrays is
> usually found to be of little or no value in improving overall system
> performance in the case of MPE, because MPE already does an
> excellent job of caching.

I think we're talking about different caching here. The big advantage of
RAID-controller caching is on writes, not reads. An intelligent RAID
controller (with power-fail protection for its cache) can accept a write
from the host and immediately acknowledge it as completed, even though
nothing has actually been sent to any disk. As you mention, intelligent OS
read-ahead caching will always give you more bang for the buck than
controller caching, because the pre-read is done "in context" to what is
actually going on in the application--something the controller simply can't
know about.

Safety note: *never* configure any disk cache setup--RAID or not--to "write
back" unless you have power-loss protection for it!



Steve Dirickson   WestWin Consulting
[log in to unmask]   (360) 598-6111