linux-kernel - Re: [PATCH 2/5] virtio_blk: implement REQ

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Date:	Tue, 17 Aug 2010 10:17:15 +0200
From:	Tejun Heo <tj@...nel.org>
To:	Christoph Hellwig <hch@....de>
CC:	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,
	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
Subject: Re: [PATCH 2/5] virtio_blk: implement REQ_FLUSH/FUA support

Hello,

On 08/16/2010 08:33 PM, Christoph Hellwig wrote:
> On Mon, Aug 16, 2010 at 06:52:00PM +0200, Tejun Heo wrote:
>> From: Tejun Heo <tj@...nle.org>
>>
>> Remove now unused REQ_HARDBARRIER support and implement REQ_FLUSH/FUA
>> support instead.  A new feature flag VIRTIO_BLK_F_FUA is added to
>> indicate the support for FUA.
> 
> I'm not sure it's worth it.  The pure REQ_FLUSH path works not and is
> well tested with kvm/qemu.   We can still easily add a FUA bit, and
> even a pre-flush bit if the protocol roundtrips matter in real life
> benchmarking.

Hmmm... the underlying storage could be md/dm RAIDs in which case FUA
should be cheaper than FLUSH.

Thanks.

-- 
tejun
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