[Beowulf] SATA II

Mark Hahn hahn at physics.mcmaster.ca
Fri Dec 8 06:29:06 PST 2006

>> Thanks Peter,
>> But do you mean that SATA is not a suitable choice for a beowulf cluster?
> SATA is fine. You just have to be choosy about the SATA/SAS controller,

it's interesting that SAS advertising has obscured the fact that 
SAS is just a further development of SCSI, and not interchangable
with SATA.  for instance, no SATA controller will support any SAS disk,
and any SAS setup uses a form of encapsulation to communicate with
the foreign SATA protocol.  SAS disks follow the traditional price
formula of SCSI disks (at least 4x more than non-boutique disks),
and I suspect the rest of SAS infrastructure will be in line with that.

> and be mindful of reliability issues with desktop drives.

I would claim that this is basically irrelevant for beowulf.
for small clusters (say, < 100 nodes), you'll be hitting a negligable
number of failures per year.  for larger clusters, you can't afford
any non-ephemeral install on the disks anyway - reboot-with-reimage
should only take a couple minutes more than a "normal" reboot.
and if you take the no-install (NFS root) approach (which I strongly
recommend) the status of a node-local disks can be just a minor node
property to be handled by the scheduler.

by all means, buy only 5-year warranty mass-market drives, since 
there's no longer any premium vs 3 or even 1-year drives.

the failure rate I've seen over the past couple years has been 
quite low - probably around .1-.5% AFR (failures/disk-year).
(that's ignoring infant mortality, of course, and a reasonably
cooled operating environment; expect higher rates if your supply chain 
involves piles of un-padded disks sitting on on some shop's 
counter/shelf/display-case ;)

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