On 08/05/17 20:52, Nix wrote:
On 8 May 2017, Phil Turmel verbalised:
On 05/08/2017 10:50 AM, Nix wrote:
I wonder... scrubbing is not very useful with md, particularly with RAID
6, because it does no writes unless something mismatches,
This is wrong. The purpose of scrubbing is to expose any sectors that
have degraded (as Wol describes) to the point of generating a read
error. A "check" scrub only writes back to the sectors that report a
URE, giving the drive firmware a chance to fix or relocate the sector.
A check scrub will NOT write on mismatch, just increment the mismatch
counter. This is the recommended regular scrubbing operation. You want
to know when mismatches occur.
And... then what do you do? On RAID-6, it appears the answer is "live
with a high probability of inevitable corruption". That's not very good.
(AIUI, if a check scrub finds a URE, it'll rewrite it, and when in the
common case the drive spares it out and the write succeeds, this will
not be reported as a mismatch: is this right?)
I think you're misunderstanding RAID here. IF the drive says "I can't
read this block", the RAID reconstructs the block, and rewrites it. No
corruption.
If the scrub finds a mismatch, then the drives are reporting
"everything's fine here". Something's gone wrong, but the question is
what? If you've got a four-drive raid that reports a mismatch, how do
you know which of the four drives is corrupt? Doing an auto-correct here
risks doing even more damage. (I think a raid-6 could recover, but
raid-5 is toast ...)
And seeing as drives are pretty much guaranteed (unless something's gone
BADLY wrong) to either (a) accurately return the data written, or (b)
return a read error, that means a data mismatch indicates something is
seriously wrong that is NOTHING to do with the drives.
<snip>
If a sector weakens purely because of neighbouring writes or temperature
or a vibrating housing or something (i.e. not because of actual damage),
so that a rewrite will strengthen it and relocation was never necessary,
surely you've just saved a pointless bit of sector sparing? (I don't
know: I'm not sure what the relative frequency of these things is. Read
and write errors in general are so rare that it's quite possible I'm
worrying about nothing at all. I do know I forgot to scrub my old
hardware RAID array for about three years and nothing bad happened...)
Yes you have saved a sector sparing. Note that a consumer 3TB drive can
return, on average, one error every time it's read from end to end 3
times, and still be considered "within spec" ie "not faulty" by the
manufacturer. And that's a *brand* *new* drive. That's why building a
large array using consumer drives is a stupid idea - 4 x 3TB drives and
a *within* *spec* array must expect to handle at least one error every
scrub.
Okay - most drives are actually way over spec, and could probably be
read end-to-end many times without a single error, but you'd be a fool
to gamble on it.
Cheers,
Wol
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
