Re: [Qemu-devel] [PATCH] raw-posix: Linearize direct I/O on Linux NFS

[Anthony Liguori <aliguori@us.ibm.com>]

On 04/15/2011 05:21 PM, pbadari@linux.vnet.ibm.com wrote:
> On 4/15/2011 10:29 AM, Christoph Hellwig wrote:
>> On Fri, Apr 15, 2011 at 09:23:54AM -0700, Badari Pulavarty wrote:
>>> True. That brings up a different question - whether we are doing
>>> enough testing on mainline QEMU :(
>> It seems you're clearly not doing enough testing on any qemu.  Even
>> the RHEL6 qemu has had preadv/pwritev since the first beta.
>
> Christoph,
>
> When you say "you're" - you really meant RH right ? RH should have 
> caught this in their
> regression year ago as part of their first beta. Correct ?
>
> Unfortunately, you are picking on person who spent time find & 
> analyzing the regression,
> narrowing the problem area and suggesting approaches to address the 
> issue :(

This is a pretty silly discussion to be having.

The facts are:

1) NFS sucks with preadv/pwritev and O_DIRECT -- is anyone really surprised?

2) We could work around this in QEMU by doing something ugly

3) We have no way to detect when we no longer need a work around which 
makes (2) really unappealing.

4) That leaves us with:
     a) waiting for NFS to get fixed properly and just living with worse 
performance on older kernels

     b) having a user-tunable switch to enable bouncing

I really dislike the idea of (b) because we're stuck with it forever and 
it's yet another switch for people to mistakenly depend on.

I'm still waiting to see performance data without O_DIRECT.  I suspect 
that using cache=writethrough will make most of this problem go away in 
which case, we can just recommend that as a work around until NFS is 
properly fixed.

Regards,

Anthony Liguori