[Discuss] Adventures in N40L Land

[bogstad at pobox.com (Bill Bogstad)]

On Tue, Feb 14, 2012 at 11:57 AM, Richard Pieri <richard.pieri at gmail.com> wrote:
> On 2/14/2012 11:32 AM, Bill Bogstad wrote:
>>
>> Reason? ? Also, do you mean actual physical geometry or the lies that
>> all drives seem to give now? ?(Which from what I've seen on a random
>> collection of drives seem to all be the same anyway.)
>
>
> Unbalanced disks generate unbalanced I/O loads which the RAID system may not
> be able handle properly. ?This can cause the RAID controller to fault good
> disks that aren't keeping up with the faster-performing disks in the set.

With in-disk command queueing and the fact that most (all?) RAID5
implementations don't bother to read parity blocks during a read
(unless an error occurs), I would think the head positions would get
out of sync even with identical drives due to the differences in the
stream of READ requests.  The result could be different times for an
operation to the same block location.      And what about the
automatic bad-block sparing that most drives do now (and hide from the
controller unless you explicitly use SMART to find out)?  That is
going to effectively cause "identical" block locations to have
completely different performance characteristics.   Given both of
these, I would think that a sufficiently pathological stream of READ
requests could cause fairly significant differences in performance
even with "identical" drives.

 Have you seen failures caused by this?   Can you provide more details
about the circumstances?

That's not to say that I couldn't see a possible problem with mixing
5400/7200 RPM drives or completely different transfer rates.  But if
all the drives are more or less in the same performance band, is that
going to be enough of a difference to matter?

Thanks,
Bill Bogstad