MD Feature Request: non-degraded component replacement

[David Greaves <david@dgreaves.com>]

Hi Neil

I brought this up in October but got no response - since you seem to be on a
roll I thought I'd try again...

Summary: Add a spare and 'mirror-fail' a device. The spare is synced with the
to-be-removed device and any read errors are corrected from the remaining raid
devices. Once synced, the  to-be-removed device is failed and the spare takes
its place. At no point is the array degraded.

IMHO This one should be high on the todo list. Especially if it's a
pre-requisite for other improvements to resilience.

Right now, if a drive fails or shows signs of going bad then you get into a very
 risky situation.

I'm sure most here know that the risk is because removing the failing drive and
installing a good one to re-sync puts you in a very vulnerable position; if
another drive fails (even one bad block) then you lose data.

The solution involves raid1 - but it needs a twist of raid5/6 and it was
discussed ages ago; see:
  http://arctic.org/~dean/proactive-raid5-disk-replacement.txt


I think this is what was discussed:

Assume md0 has drives A B C D
D is failing
E is new

* add E as spare
* set E to mirror 'failing' drive D (with bitmap?)
* subsequent writes go to both D+E
* recover 99+% of data from D to E by simple mirroring
* any read failures on D when reading from md0 or mirroring D->E are recovered
from reading ABC not E unless E is in sync. D is not failed out. (and it's these
tricks that stops users from doing all this manually)
* any md0 sector read failure on ABC can still (hopefully) be read from D even
if not yet mirrored to E (also not possible if done manually)
* once E is mirrored, D is removed and  the job is done

Personally I think this feature is more important than the reshaping requests;
of course that's just one opinion after replacing about 20 flaky 1Tb drives in
the past 6 months :)

David

-- 
"Don't worry, you'll be fine; I saw it work in a cartoon once..."