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

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Date:	Tue, 17 Aug 2010 18:22:38 +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 08/17/2010 03:23 PM, Christoph Hellwig wrote:
>> Hmmm... the underlying storage could be md/dm RAIDs in which case FUA
>> should be cheaper than FLUSH.
> 
> If someone ever wrote a virtio-blk backend that sits directly ontop
> of the Linux block layer that would be true.  Of the five known
> virtio-blk backends all operate on normal files using the Posix I/O
> APIs, or the Linux aio API (optionally in qemu) or in-kernel
> vfs_read/vfs_write (vhost-blk).

Right.

> Given how little testing lguest gets compared to qemu I really don't
> want a protocol addition for it unless it really buys us something.
> Once we're done with this barrier conversion I plan into benchmarking
> FUA and a pre-flush tag on the command for virtio in real life setups,
> and see if it actually buys us anything.

Hmmm... yeah, we can drop it.  Michael, what do you think?

Thanks.

-- 
tejun
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