On Sat, 20 Jul 2013, Steve O'Hara-Smith wrote:
On Sat, 20 Jul 2013 18:14:20 +0100
Frank Leonhardt <fra...@fjl.co.uk> wrote:
It's worth noting, as a warning for anyone who hasn't been there, that
the number of times a second drive in a RAID system fails during a
rebuild is higher than would be expected. During a rebuild the remaining
drives get thrashed, hot, and if they're on the edge, that's when
they're going to go. And at the most inconvenient time. Okay - obvious
when you think about it, but this tends to be too late.
Having the cabinet stuffed full of nominally identical drives
bought at the same time from the same supplier tends to add to the
probability that more than one drive is on the edge when one goes. It's a
pity there are now only two manufacturers of spinning rust.
Often this is presummed to be the reason for double failures close in
time, also common mode failures such as environment, a defective power
supply or excess voltage can be blamed. I have to think that the most
common "cause" for a second failure soon after the first is that a failed
drive often isn't detected until a particular sector is read or written.
Since the resilvering reads and writes every sector on multiple disks,
including unused sectors, it can "detect" latent problems that may have
existed since the drive was new but which haven't been used for data yet,
or have gone bad since the last write, but haven't been read since.
The ZFS scrub processes only sectors with data, so it provides only
partial protection against double failures.
Daniel Feenberg
NBER
--
Steve O'Hara-Smith <st...@sohara.org>
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