linux-kernel - Re: [RFC][PATCH] md: avoid fullsync if a faulty member missed a dirty transition

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Date:	Tue, 6 May 2008 16:53:58 +1000
From:	Neil Brown <neilb@...e.de>
To:	Mike Snitzer <snitzer@...il.com>
Cc:	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
	paul.clements@...eleye.com
Subject: Re: [RFC][PATCH] md: avoid fullsync if a faulty member missed a dirty transition

On Wednesday April 2, snitzer@...il.com wrote:
> resync via bitmap if faulty's events+1 == bitmap's events_cleared
> 
> For more background please see:
> http://marc.info/?l=linux-raid&m=120703208715865&w=2
> 
> Without this change validate_super() will prevent the previously faulty
> member from recovering via bitmap, e.g.:

I can't help thinking that you are misinterpreting something.  I don't
think there is a clean->dirty transition happening here.
You could confirm this by using --examine on both devices after the
messy shutdown and before re-assembling the array.

Even allowing for that possible confusion, I cannot quite see what is
going on.
It is fairly clear from the event counts that the NBD device is marked
clean, but if this is happening at array-shutdown time, I cannot see
why md would try to write to the NBD device and thereby detect an
error...

Do you have an internal bitmap or a bitmap in an external file?

In general, I would not like to make decisions based on the
oddness/evenness of the event counter.  I consider that to be an
internal implementation detail.  I am happy to make decisions based on
a difference-of-1.  I need to understand the big picture first though.

NeilBrown
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