[Beowulf] Software RAID?

Ekechi Nwokah ekechi at alexa.com
Tue Nov 27 18:28:45 EST 2007


 

> -----Original Message-----
> From: Mark Hahn [mailto:hahn at mcmaster.ca] 
> Sent: Monday, November 26, 2007 8:45 PM
> To: Ekechi Nwokah
> Cc: Beowulf Mailing List
> Subject: RE: [Beowulf] Software RAID?
> 
> >> 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.
> 

Something like that ;-).

> > 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.
> 

Agreed. In that case you could buy much cheaper JBOD cards without any
queueing at all. That's the point.

> > 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?
> 

Large scale data mining.

> regards, mark hahn.
> 

Ekechi

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