linux-kernel - Re: [PATCH] BTRFS: Runs the xor function if a Block has failed

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Message-ID: <20160105092236.GO4227@twin.jikos.cz>
Date:	Tue, 5 Jan 2016 10:22:36 +0100
From:	David Sterba <dsterba@...e.cz>
To:	Sanidhya Solanki <jpage.lkml@...il.com>
Cc:	David Sterba <dsterba@...e.cz>, clm@...com, jbacik@...com,
	dsterba@...e.com, linux-kernel@...r.kernel.org,
	linux-btrfs@...r.kernel.org
Subject: Re: [PATCH] BTRFS: Runs the xor function if a Block has failed

On Wed, Dec 30, 2015 at 09:15:44PM -0500, Sanidhya Solanki wrote:
> On Wed, 30 Dec 2015 18:18:26 +0100
> David Sterba <dsterba@...e.cz> wrote:
> 
> > That's just the comment copied, the changelog does not explain why
> > it's ok to do just the run_xor there. It does not seem trivial to me.
> > Please describe that the end result after the code change is expected.
> 
> In the RAID 6 case after a failure, we discover that the failure
> affected the entire P stripe, without any bad data occurring. Hence, we
> xor the previously stored parity data to return the data that was lost
> in the P stripe failure.
> 
> The xor-red data is from the parity blocks. Hence, we are left with 
> recovered data belonging to the P stripe.

If the data a rerecovered, why is -EIO still returned? Also, I see some
post-recovery steps eg. for the damaged P stripes (at label pstripes)
and I'd expect something similar for the case you're fixing.

I'm not familiar with the raid56 implementation but the fix looks
suspiciously trivial and I doubt that the xor was omitted out of
laziness.
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