non-degraded component replacement was Re: Distributed spares

[David Greaves <david@dgreaves.com>]

Billy Crook wrote:
> It would be even nicer if there were a way to hot-transfer one
> raid component to another without setting anything faulty.  I suppose
> you could make all the components of the real array be single disk
> raid1 arrays for that purpose.  Then you could have one extra disk set
> aside for this sort of scrubbing, and never even be down one of your
> parities.  I guess I should add that onto my todo list....

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.

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 md0 or D->E read failures on D 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
* 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 :)

David