Re: Maximizing failed disk replacement on a RAID5 array

Hello John,

On Fri, Jun 10, 2011 at 7:25 AM, John Robinson
[off-list ref] wrote:

On 07/06/2011 09:52, John Robinson wrote:

quoted

On 06/06/2011 19:06, Durval Menezes wrote:
[...]

quoted

It would be great to have a
"duplicate-this-bad-old-disk-into-this-shiny-new-disk" functionality,
as it would enable an almost-no-downtime disk replacement with
minimum risk, but it seems we can't have everything... :-0 Maybe it's
something for the wishlist?

It's already on the wishlist, described as a hot replace.

Actually I've been thinking about this. I think I'd rather the hot replace
functionality did a normal rebuild from the still-good drives, and only if
it came across a read error from those would it attempt to refer to the
contents of the known-to-be-failing drive (and then also attempt to repair
the read error on the supposedly-still-good drive that gave a read error, as
already happens).

This looks like a very good idea. The old (failing) drive would be
kept "on reserve", ready to be accessed for eventual failed sectors on
the other old (good) drives...

My rationale for this is as follows: if we want to hot-replace a drive
that's known to be failing, we should trust it less than the remaining
still-good drives, and treat it with kid gloves. It may be suffering from
bit-rot. We'd rather not hit all the bad sectors on the failing drive,
because each time we do that we send the drive into 7 seconds (or more, for
cheap drives without TLER) of re-reading, plus any Linux-level re-reading
there might be. Further, making the known-to-be-failing drive work extra
hard (doing the equivalent of dd'ing from it while also still using it to
serve its contents as an array member) might make it die completely before
we've finished.

I agree completely.

What will this do for rebuild time? Well, I don't think it'll be any slower.

I think it will actually be faster.

On the one hand, you'd think that copying from one drive to another would be
faster than a rebuild, because you're only reading 1 drive instead of N-1,
but on the other, your array is going to run slowly (pretty much degraded
speed) anyway because you're keeping one drive in constant use reading from
it, and you risk it becoming much, much slower if you do run in to hundreds
or thousands of read errors on the failing drive.

So overall I think hot-replace should be a normal replace with a possible
second source of data/parity.

Your reasoning sounds good to me.

Thoughts?

Only sadness that it's not implemented yet... :-)

Yes, I know, -ENOPATCH

Exactly :-)

Cheers,
-- 
   Durval Menezes.

Cheers,

John.

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