linux-kernel - Re: [PATCH] virtio-blk: set QUEUE_ORDERED

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Message-ID: <20090825141608.GA32658@lst.de>
Date:	Tue, 25 Aug 2009 16:16:08 +0200
From:	Christoph Hellwig <hch@....de>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	Christoph Hellwig <hch@....de>, borntraeger@...ibm.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH] virtio-blk: set QUEUE_ORDERED_DRAIN by default

On Tue, Aug 25, 2009 at 11:41:37PM +0930, Rusty Russell wrote:
> On Fri, 21 Aug 2009 06:26:16 am Christoph Hellwig wrote:
> > Currently virtio-blk doesn't set any QUEUE_ORDERED_ flag by default, which
> > means it does not allow filesystems to use barriers.  But the typical use
> > case for virtio-blk is to use a backed that uses synchronous I/O
> 
> Really?  Does qemu open with O_SYNC?
> 
> I'm definitely no block expert, but this seems strange...
> Rusty.

Qemu can open it various ways, but the only one that is fully safe
is O_SYNC (cache=writethrough).  The O_DIRECT (cache=none) option is also
fully safe with the above patch under some limited circumstances 
(disk write caches off and using a host device or fully allocated file).

Fixing the cache=writeback option and the majority case for cache=none
requires implementing a cache flush command, and for the latter one
also fixes to the host kernel I'm working on.  You will get another
patch to implement the proper cache controls in virtio-blk for me in
a couple of days, too.
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