>>>>> "edm" == Eric D Mudama <edmud...@bounceswoosh.org> writes:
>> Hard drives are comprised of multiple platters, with typically
>> an independently navigated head on each side.
edm> This is a gap in your assumptions I believe.
edm> The headstack is a single physical entity, so all heads move
edm> in unison to the same position on all surfaces at the same
edm> time.
yes but AIUI switching heads requires resettling into the new track.
The cylinders are not really cylindrical, just because of wear or
temperature or whatever, so when switching heads the ``channel'' has
to use data from the head as part of a servo loop to settle on the
other surface's track.
I guess the rules do keep changing though.
edm> I think that what you're looking for, however, is already
edm> happening, with server farms moving to multiple 2.5" drives
yeah but you're reading him wrong. He is saying a failed drive may
still be useful if you just avoid the one failed head.
The problem currently is the LBA's are laced through each cylinder,
which is worth doing so that things like short-stroking make sense to
reduce head movement. If you re-swizzled the LBA's so that instead
they filled each side of each platter in turn, like a dual-layer DVD,
it wouldn't change sequential throughput at all, and would have the
benefit that ZFS's existing tendency to put redundant metadata copies
far apart in LBA would end up getting them on different heads, which
actually *is* helpful given known failure modes tend to be head
crashing, head falling off, u.s.w.
I think the idea is doomed firstly because these days when a single
head goes bad, the drive firmware, host adapter, driver, and even the
zfs maintenance commands, all the way up the storage stack to the
sysadmin's keyboard, all shit their pants and become useless. You
have to find the bad drive, remove it, then move on.
Secondly I'm not sure I buy the USENIX claim that you can limp along
less one head. The last failed drive I took apart, was indeed failed
on just one head, but it had scraped all the rust off the platter
(down to glass! it was really glass!), and the inside of the thing
was filled with microscopic grey facepaint. It had slathered the air
filtering pillow and coated all kinds of other surfaces. so...I would
expect the other recording surfaces were not doing too well either,
but I could be wrong. It does match experience, though, of drives
going from partly-failed to completely-failed in a day or a week.
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