Re: RAIDframe: what if a disc fails during copyback

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

> In a RAIDframe RAID-1, a disc failed and I reconstructed on a spare.
> Now I want to replace the failed component (actually by the same
> disc, which needed a firmware update) and want to copyback to it.

So, let me make sure I understand you correctly.

So you have drives A, B, and C.  A and B were live.  Let's say B is the
one that failed.  You reconstructed onto C and have been running with A
and C.

Now you have a new B (which in this case is the same hardware with new
firmware) and want to put it back into service.  I'm not sure whether
you want to put it into service in place of A or in place of C.  I'm
going to assume C.

So, you'd pull C, replace it with B, and initiate a reconstruct, which
for RAID 1 means copying from A to B.  Right?

> How will RAIDframe behave if, during the copyback:
> 1. The replaced component fails

Is this B?  Or C?  Because it sounds to me as though C would be out of
service at this point.

> 2. The spare fails

Which is "the spare"?  Are you running with a hot spare?  I think a hot
spare failing means nothing until/unless RAIDframe tries to fall back
on it.

> 3. The other, non-replaced component fails?

That would be A?

> Specifically: Is there any szenario (other than more than one disc
> failing) that will put the RAID into a non-redundant state?  I guess
> 3. may?

For RAID 1 in general, as soon as you have only one non-failed drive,
you have no redundancy.  Based on the assumption that RAIDframe RAID 1
cannot handle more than two drives (always true as far as I know, and
the 9.0 raidctl(8) manpage says it's still true as of 9.0), this means
that....

- If B fails while copying back to it, you are back to non-redundant
   operation on A.

- If A fails while copying back, you have no operational set.  Your
   only real option is to pull A and B, connect C alone, and fall back
   to the state of things as of when you pulled it; then re-add A or B
   and copyback from C.

- If C fails while copying from A to B, nothing in particular happens
   except that you don't have the hot spare you thought you did.

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