linux-kernel - Re: [PATCH 5/5] dm: implement REQ

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Message-ID: <20100817131309.GA2963@lst.de>
Date:	Tue, 17 Aug 2010 15:13:09 +0200
From:	Christoph Hellwig <hch@....de>
To:	Tejun Heo <tj@...nel.org>
Cc:	Mike Snitzer <snitzer@...hat.com>, jaxboe@...ionio.com,
	linux-fsdevel@...r.kernel.org, linux-scsi@...r.kernel.org,
	linux-ide@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-raid@...r.kernel.org, hch@....de, James.Bottomley@...e.de,
	tytso@....edu, chris.mason@...cle.com, swhiteho@...hat.com,
	konishi.ryusuke@....ntt.co.jp, dm-devel@...hat.com, vst@...b.net,
	jack@...e.cz, rwheeler@...hat.com, hare@...e.de, neilb@...e.de,
	rusty@...tcorp.com.au, mst@...hat.com, Tejun Heo <tj@...nle.org>
Subject: Re: [PATCH 5/5] dm: implement REQ_FLUSH/FUA support

On Tue, Aug 17, 2010 at 11:33:52AM +0200, Tejun Heo wrote:
> > That logic wasn't just about retries (at least not in the latest
> > kernel).  With commit 708e929513502fb0 the -EOPNOTSUPP checking also
> > serves to optimize the barrier+discard case (when discards aren't
> > supported).
> 
> With the patch applied, there's no second flush.  Those requests would
> now be REQ_FLUSH + REQ_DISCARD.  The first can't be avoided anyway and
> there won't be the second flush to begin with, so I don't think this
> worsens anything.

In fact the pre-flush is completely superflous for discards, but that's a
separate discussion and should not be changed as part of this patchset but
rather explicitly.

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