Re: size of nfsv4 writes

[Olga Kornievskaia <aglo@citi.umich.edu>]

Trond Myklebust wrote:
> On Wed, 2008-06-04 at 12:40 -0400, Olga Kornievskaia wrote:
>   
>> While testing NFSv4 performance over the 10GE network, we are seeing the 
>> following behavior and would like to know if it is normal or a bug in 
>> the client code.
>>
>> The server offers the max_write of 1M. The client mounts the server with 
>> the "wsize" option of 1M. Yet during the write we are seeing that the 
>> write size is at most 49K. Why does client never come close to 1M limit?
>>     
>
> I have a feeling that is due to some crap in the VM. I'm currently
> investigating a situation where it appears we're sending 1 COMMIT for
> every 1-5 32k WRITEs. This is not a policy that stems from the NFS
> client, so it would appear that the VM is being silly about things.
>
> I'm specially suspicious of the code in get_dirty_limits() that is
> setting a limit to the number of dirty pages based on the number of
> pages a given BDI has written out in the recent past. As far as I can
> see, the intention is to penalise devices that are slow writers, but in
> practice it doesn't do that: it penalises the devices that have the
> least activity.
>
>   
I think we are seeing larger than usual number of COMMIT messages.