Harware vs. Software RAID (WAS: Re: [nycbug-talk] Fwd: Adaptec AAC raid support)

jh jhlists
Mon Mar 21 17:05:50 EST 2005

Jay Savage wrote:

> It depends on how big you need your RAID to be, among other things,
> and what hardware you want to build it with.  On IDE/ATA, you need one
> controller per disk or quickly run out of bandwidth.  So your RAID is
> effectively limited to the number of PCI slots in your box.  

I have some experience with building large soft-raids using PCI IDE 
cards on FreeBSD and Linux.

Some problems with this approach:

* There are either few or no PCI-X IDE controllers out there (unless you 
count crappy firmware raid cards, which could work, I guess - but the 
price advantage of soft raid starts to go away). Hopefully PCIe will 
change this state of affairs. But using PCI only is not going to give 
you great performance - and that may be OK for some applications.

I've found that using hardware RAID cards is essential for performance, 
if for no other reason than they work fine with wide PCI slots. I've 
also used the "many IDE card" approach for situations where "good 
enough" performance was, well, good enough.

* At least one vendor, Promise, puts a "bug" in their firmware that 
limits how many PCI IDE cards can go in one system. I've verified this 
with Promise; their firmware makes any more than two cards simply not 
function. Avoid Promise at all costs. It is also worth mentioning that 
this was something that happened with newer firmware - older cards with 
downgrade firmware do not exhibit this, when flashed with newer 
firmware, they do.

I've found SIIG cards, using a Silicon Image chipset, to be unencumbered 
by such foolishness, and they work fine (at least under Linux, haven't 
use them under FreeBSD as of yet).


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