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

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Message-ID: <20151230211544.32bc337e@gmail.com>
Date:	Wed, 30 Dec 2015 21:15:44 -0500
From:	Sanidhya Solanki <jpage.lkml@...il.com>
To:	David Sterba <dsterba@...e.cz>
Cc:	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, 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 there is an error during the completion of the xor (provided by the
patch ), we got to the cleanup function.

Hope that is satisfactory.
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