Re: Need clarification on raid1 resync behavior with bitmap support

["Mike Snitzer" <snitzer@gmail.com>]

On 8/3/07, Neil Brown <neilb@suse.de> wrote:
> On Friday August 3, snitzer@gmail.com wrote:
> >
> > I hand-patched your change into a 2.6.20.1 kernel (I'd imagine your
> > patch is against current git).  I didn't see any difference because
> > unfortunately both of my full resync scenarios included stopping a
> > degraded raid after either: 1) having failed but not been removed a
> > member 2) having failed and removed a member.  In both scenarios if I
> > didn't stop the array and I just removed and re-added the faulty drive
> > the array would _not_ do a full resync.
> >
> > My examples clearly conflict with your assertion that: "This only
> > works if the array has not been shut down and restarted."
>
> I think my changelog entry for the patch was poorly written.
> What I meant to say was:
>   *before this patch*  a remove and re-add only does a partial resync
>     if the array has not been shutdown and restarted in the interim.
>   The implication being that *after the patch*, a shutdown and restart
>   will not interfere and a remove followed by a readd will always do a
>   partial resync, even if the array was shutdown and restarted while
>   degraded.

Great, thanks for clarifying.

> > To be explicit: isn't the bitmap still valid on the fresh members?  If
> > so, why is raid1 just disregarding the fresh bitmap?
>
> Yes.  Exactly.  It is my understanding and experience that the patch I
> sent fixes a bug so that it doesn't disregard the fresh bitmap.  It
> should fix it for 2.6.20.1 as well.
>
> Are you saying that you tried the same scenario with the patch applied
> and it still did a full resync?  How do you measure whether it did a
> full resync or a partial resync?

I must not have loaded the patched raid1.ko because after retesting it
is clear that your patch does in fact fix the issue.  FYI, before, I
could just tell a full resync was occurring by looking at /proc/mdstat
and the time that elapsed.

Thanks for your help, any idea when this fix will make it upstream?

regards,
Mike