linux-kernel - Re: [PATCHSET block#for-2.6.36-post] block: replace barrier with sequenced flush

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Message-ID: <4C728210.6040605@kernel.org>
Date:	Mon, 23 Aug 2010 16:13:36 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Hellwig <hch@....de>
CC:	Jens Axboe <jaxboe@...ionio.com>,
	Ric Wheeler <rwheeler@...hat.com>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-raid@...r.kernel.org" <linux-raid@...r.kernel.org>,
	"James.Bottomley@...e.de" <James.Bottomley@...e.de>,
	"tytso@....edu" <tytso@....edu>,
	"chris.mason@...cle.com" <chris.mason@...cle.com>,
	"swhiteho@...hat.com" <swhiteho@...hat.com>,
	"konishi.ryusuke@....ntt.co.jp" <konishi.ryusuke@....ntt.co.jp>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	"vst@...b.net" <vst@...b.net>, "jack@...e.cz" <jack@...e.cz>,
	"hare@...e.de" <hare@...e.de>
Subject: Re: [PATCHSET block#for-2.6.36-post] block: replace barrier with
 sequenced flush

Hello,

On 08/23/2010 04:08 PM, Christoph Hellwig wrote:
> On Mon, Aug 23, 2010 at 04:01:15PM +0200, Jens Axboe wrote:
>> The problem purely exists on arrays that report write back cache enabled
>> AND don't implement SYNC_CACHE as a noop. Do any of them exist, or are
>> they purely urban legend?
> 
> I haven't seen it.  I don't care particularly about this case, but once
> it a while people want to disable flushing for testing or because they
> really don't care.
> 
> What about adding a sysfs attribue to every request_queue that allows
> disabling the cache flushing feature?  Compared to the barrier option
> this controls the feature at the right level and makes it available
> to everyone instead of beeing duplicated.  After a while we can then
> simply ignore the barrier/nobarrier options.

Yeah, that sounds reasonable.  blk_queue_flush() can be called anytime
without locking anyway, so it should be really easy to implement too.

Thanks.

-- 
tejun
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