[Beowulf] Consumer vs. Enterprise Hard Drives in Clusters

[Jon Forrest]

Bill Broadley wrote:
> The differences I've seen between "raid edition" drives and regular drives are:

Thanks for the thoughtful comments. I don't claim to be an expert
in any of this. I just hope I'm asking the right questions.

> * Dramatically better vibration resistance.  If you are going to bolt a drive
>   or two into a desktop it doesn't matter so much.  If you are going to plug
>   a drive into a 16 bay enclosure or even into a 1U node with a ton of fast
>   fans you might well see a large performance difference because of the
>   vibration.  The specification sheets do reflect this btw, I assume this
>   is mostly lower density platters and stronger motors for positioning
>   the heads.  This is especially noticeable on the consumer drives with the
>   higher density platters (375-500GB per platters).  I've seen consumer
>   drives that manage 120MB/sec drop to a noisy fluctuation between 18-30
>   MB/sec because of vibration

Wouldn't the effect of vibrations from multiple drives depend
greatly on the mechanical properties of the bay enclosure and
the chassis itself? For example, I have a 16 bay enclosure that's
built like a tank (I know because I dropped it once). I would
think that the vibration of one drive would barely be noticed
by others. Of course this question could be answered by measuring,
assuming the presence of the right instrumentation, which I don't have.

> * Consumer drives (at least the non-media ones) often have occasional
>   thermal recalibrations.   This seems better these days, but last thing
>   you want is a recal triggering a degraded array.

What does the RAID controller and OS see when such a thing happens?

> * Consumer drives will go to heroic efforts to read a bad sector, exactly
>   the opposite of what you want in a RAID drive.  In a RAID it's better to
>   fail and yell bloody murder... especially when the rereading a sector
>   a bunch causes the raid to time out and drop the disk.

But wouldn't failing and yelling bloody murder be treated by
the RAID controller the same as when a drive times out? In either
case, I would expect the RAID controller to see the drive as having
failed. Then, when you replace the drive you'd cause a RAID unit
rebuild which is a very dangerous thing to do these days given how
large drives are and the chances of an I/O error occurring during
the rebuild.

> Of course manufactures claim various things about error rates per billion
> bits, designed duty cycles (40 hours a week vs 24/7), improve temperature
> envelops, and related.  Alas while this is nice to hear I've not seen any
> direct results because of it.

I too agree it would be nice but neither you nor Google appear to be
seeing it (assuming, as one poster said, that we're all using the
same definition of "enterprise drive").

What's surprising to me is that if this were true then I'd expect the
manufacturer's warranty to be different for the two classes of drives.
Maybe this is the reason that Seagate is changing to a 3-year warranty
for their consumer drive (I haven't seen anything about the warranty
for the enterprise drives).

> As an example, 500GB wd caviar $64.99, 500GB WD RE3 $89.99.  IMO if you are
> building a raid or heavily used 1U with a ton of fans the extra $25 is worth it.

If there are real differences between the drives then this would be an
easier decision. There doesn't appear to be a consensus, however, that
the differences in the field are significant.

Cordially,

-- 
Jon Forrest
Research Computing Support
College of Chemistry
173 Tan Hall
University of California Berkeley
Berkeley, CA
94720-1460
510-643-1032
jlforrest at berkeley.edu