[Beowulf] Software RAID?

Mark Hahn hahn at mcmaster.ca
Mon Nov 26 23:44:54 EST 2007

>> Of course there are a zillion things you didn't mention.  How
>> many drives did you want to use?  What kind? (SAS? SATA?)  If
>> you want 16 drives often you get hardware RAID hardware even
>> if you don't use it.
>> What config did you want?
>> Raid-0? 1? 5? 6? Filesystem?
> So let's say it's 16. But in theory it could be as high as 192. Using

16 drives in a system is reasonable.  for much larger systems, I would go
for more scalable building blocks (network connected, 10GE or IB.)

> multiple JBOD cards that present the drives individually (as separate
> LUNs, for lack of a better term), and use software RAID to do all the
> things that a 3ware/Areca, etc. card would do across the total span of
> drives:
> RAID 0/1/5/6, etc., hotswap, SAS/SATA capability, etc.

software raid on 192 drives in a single system sounds somewhat dubious.
not impossible, by any means, but you'll wind up challenging the fact
that commodity systems don't scale that high.  I'm guessing you'll need
to use a bunch of SAS channels for daisy-chainability.

> Right now, all the hardware cards start to precipitously drop in
> performance under concurrent access, particularly read/write mixes.

I would consider turning off queueing in the card, and leaving all
the request sorting to the kernel.  any ideal how many requests a 
card typically takes upon itself to schedule (afaik, disks themselves 
only ever sort fairly small numbers.)  since you depend strongly on
very effective sorting of requests, I'd lean towards trying to get 
the host to do all the sorting.

> I just haven't seen something like that and I was not aware that md
> could acheive anything close to the performance of a hardware RAID card
> across a reasonable number of drives (12+), let alone provide the
> feature set.

12 disks is enough for MD to need some help - tweaking the stripe cache,
for instance.

can I ask what the actual application is?

regards, mark hahn.
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