linux-kernel - Re: dm-raid: add RAID discard support

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Date:	Thu, 2 Oct 2014 09:52:32 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	NeilBrown <neilb@...e.de>
Cc:	Heinz Mauelshagen <heinzm@...hat.com>,
	device-mapper development <dm-devel@...hat.com>,
	Shaohua Li <shli@...nel.org>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	linux RAID <linux-raid@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: dm-raid: add RAID discard support

On Thu, Oct 02 2014 at 12:04am -0400,
NeilBrown <neilb@...e.de> wrote:

> 
> I plan to submit this to Linus tomorrow, hopefully for 3.7, unless there are
> complaints. It is in my for-next branch now.
> 
> Thanks,
> NeilBrown
> 
> 
> From aec6f821ed92fac5ae4f1db50279a3999de5872a Mon Sep 17 00:00:00 2001
> From: NeilBrown <neilb@...e.de>
> Date: Thu, 2 Oct 2014 13:45:00 +1000
> Subject: [PATCH] md/raid5: disable 'DISCARD' by default due to safety
>  concerns.
> 
> It has come to my attention (thanks Martin) that 'discard_zeroes_data'
> is only a hint.  Some devices in some cases don't do what it
> says on the label.
> 
> The use of DISCARD in RAID5 depends on reads from discarded regions
> being predictably zero.  If a write to a previously discarded region
> performs a read-modify-write cycle it assumes that the parity block
> was consistent with the data blocks.  If all were zero, this would
> be the case.  If some are and some aren't this would not be the case.
> This could lead to data corruption after a device failure when
> data needs to be reconstructed from the parity.
> 
> As we cannot trust 'discard_zeroes_data', ignore it by default
> and so disallow DISCARD on all raid4/5/6 arrays.
> 
> As many devices are trustworthy, and as there are benefits to using
> DISCARD, add a module parameter to over-ride this caution and cause
> DISCARD to work if discard_zeroes_data is set.
> 
> If a site want to enable DISCARD on some arrays but not on others they
> should select DISCARD support at the filesystem level, and set the
> raid456 module parameter.
>     raid456.devices_handle_discard_safely=Y
> 
> As this is a data-safety issue, I believe this patch is suitable for
> -stable.
> DISCARD support for RAID456 was added in 3.7
> 
> Cc: Shaohua Li <shli@...nel.org>
> Cc: "Martin K. Petersen" <martin.petersen@...cle.com>
> Cc: Mike Snitzer <snitzer@...hat.com>
> Cc: Heinz Mauelshagen <heinzm@...hat.com>
> Cc: stable@...r.kernel.org (3.7+)
> Fixes: 620125f2bf8ff0c4969b79653b54d7bcc9d40637
> Signed-off-by: NeilBrown <neilb@...e.de>

Acked-by: Mike Snitzer <snitzer@...hat.com>

Thanks Neil, I'll get something comparable staged for dm-raid for 3.18
(less urgency given that dm-raid doesn't yet support discards).
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