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Date: Fri, 15 Apr 2011 13:09:19 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [PATCH] raw-posix: Linearize direct I/O on	Linux
 NFS
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To: Badari Pulavarty <pbadari@us.ibm.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, Stefan Hajnoczi <stefanha@gmail.com>, qemu-devel@nongnu.org, Khoa Huynh <khoa@us.ibm.com>, Christoph Hellwig <hch@lst.de>

On 04/15/2011 11:23 AM, Badari Pulavarty wrote:
> On Fri, 2011-04-15 at 17:34 +0200, Christoph Hellwig wrote:
>> On Fri, Apr 15, 2011 at 04:26:41PM +0100, Stefan Hajnoczi wrote:
>>> On Fri, Apr 15, 2011 at 4:05 PM, Christoph Hellwig<hch@lst.de>  wrote:
>>>> NAK. ?Just wait for the bloody NFS client fix to get in instead of
>>>> adding crap like that.
>>> That's totally fine if NFS client will be fixed in the near future but
>>> this doesn't seem likely:
>>>
>>> http://www.spinics.net/lists/linux-nfs/msg20462.html
>> The code to use preadv/pwritev has been in qemu for over 2 years,
>> and it took people to notice the NFS slowdown until now, so don't
>> expect it to be fixed three days layer.
> True. That brings up a different question - whether we are doing
> enough testing on mainline QEMU :(

The issue here is NFS, not QEMU.  Moreover, the real problem is that 
we're using O_DIRECT.  O_DIRECT seems to result in nothing but problems 
and it never seems to be tested well on any file system.

I think the fundamental problem we keep running into really boils down 
to O_DIRECT being a second class interface within Linux.

Regards,

Anthony Liguori