RAID Memory

[camieabz]

I'm thinking of getting a RAID system together.
(Abit BX133)
Now some people say that 128Mb is a minimum and 256Mb should be all you need with standard systems, but with RAID systems do the figures change?
Also, I noticed on another thread someone talking about CAS2, CL2 etc. Now I know about those but what does 2-2-2 5/7 mean?
Last question. I'll put it on this forum to save making several posts. With the board mentioned above, does anyone know if all PIII chips are supported, or is there a limit?

 

[Kelledin]

For RAID arrays:

The absolute minimum needed to do a RAID1 or RAID0 is actually <i>no</i> memory--controllers such as the DPT SmartRAID V Century had no onboard memory and were able to do RAID0 an RAID1 (or even, I believe, a RAID10). Adding cache memory should improve performance, although I've never actually looked into it.

RAID5, however, requires some memory, the more the merrier. The DPT SmartRAID V Decade came with 4MB onboard memory, apparently the bare minimum needed to do RAID5. RAID5 gets major performance benefits out of cache memory because it uses that memory for doing XOR operations (for maintaining/checking data integrity) and also for SCSI command re-ordering (in order to optimize stripe access). (These XOR operations are what makes RAID5 perform worse than RAID0, btw).

I've found 32MB to be a nice amount to go with on the DPT Decade if you only had a 3-drive RAID5. For me, the 32MB memory cache actually gained me a 20% increase in performance over the 4MB cache. To be fair, this probably isn't solely due to the added memory; the SmartRAID V Decade does not do hardware XOR (instead doing it in firmware) until you add the memory expansion. The benefits of more memory increase as you have more drives in a RAID array (especially RAID5). If you're getting an Adaptec 2100S controller (which is actually a DPT SmartRAID VI chipset with a different arrangement of the chips), it comes with 32MB memory already and is upgradeable to at least 64MB (I think 128MB as well). The 3200S and 3400S are similarly equipped. Although I'm told the controllers <i>should</i> work with standard PC100 ECC SDRAM, there are certain modules that Adaptec has specifically approved for use with their RAID controllers. I've seen Mushkin advertising memory upgrade modules for the Adaptec RAID controllers, although I haven't actually looked into them myself.

As for the Abit board, I don't know whether or not it supports 1GHz. It would be sweet if it did; I've found the 440BX chipset to be the best desktop chipset ever made . There should be more info in the specs on <A HREF="http://www.abit-usa.com/" target="_new">Abit's web site</A>.

Kelledin
<A HREF="http://kelledin.tripod.com/scovsms.jpg" target="_new">http://kelledin.tripod.com/scovsms.jpg</A>

 

[Kelledin]

Hmmm I've been a bit asleep at the switch.

I read your post again and realize suddenly that you're probably planning software raid...

A lot of the same info applies to soft raid though. Only detail you want to keep in mind is that RAID5 is only well done in hardware--it will suck your CPU dry. RAID0 and RAID1 can be done with minimal CPU usage and probably won't suck up much memory. RAID2, 3, and 4 are pretty much superseded by RAID5 and are rarely used anymore.

And another bit of info...even though it may mean nothing to you...the Adaptec 3200S and 3400S actually require SO-DIMMs instead of SDRAM DIMMs. The 2100S still takes SDRAM DIMMs. Both require 100MHz ECC memory.

Kelledin
<A HREF="http://kelledin.tripod.com/scovsms.jpg" target="_new">http://kelledin.tripod.com/scovsms.jpg</A>

 

[camieabz]

Ahmm...

I just saw the boards and...well... I realise I left out the fact I was considering RAID 0 / 0 + 1. The system won't be a server.

What I wanted to know was does the recommended min/max amount of memory change greatly?

I'm really sorry, cos you've obviously put a bit of thought into it. Needless to say, I've learned more for my cock-up.

 

[Kelledin]

Heheh no harm done .

The memory requirements don't change greatly, if at all, for RAID0 or 0+1. I just got through benchmarking this sort of setup (in hardware) with different cache amounts a few days ago. I'd say that providing an extra 32MB of memory for a RAID0 or RAID10 would just be silly.

Kelledin
<A HREF="http://kelledin.tripod.com/scovsms.jpg" target="_new">http://kelledin.tripod.com/scovsms.jpg</A>

 

[Guest]

Check this link out to be schooled.

http://www.templeoftech.com/articles.cfm?ArticleID=50&PageID=1