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

[Stefan Hajnoczi]

On Fri, Apr 15, 2011 at 5:10 PM, Anthony Liguori <address@hidden> wrote:
> On 04/15/2011 10:34 AM, 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<address@hidden>  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.
>>
>> I can't event see you in the relevent threads arguing for getting it
>> fixed, so don't complain.
>
> In general, since we are userspace, we should try to run well on whatever
> kernel we're on.
>
> What I don't like about this patch is that likelihood of false positives.
>  We check for Linux and for NFS but that means an old userspace is doing
> unoptimal things on newer kernels.  Even if we had a kernel version check,
> if the fix gets backported to an older kernel, we'll still get a false
> positive.
>
> Ideally, we'd be able to query the kernel to see whether we should bounce or
> not.  But AFAIK there is nothing even close to an interface to do this
> today.

Bah, good point.  I was planning to sneak in a uname() later but that
really doesn't cut it due to backports/downstream.

Stefan