Re: Read/Write NFS I/O performance degraded by FLUSH_STABLE page flushing

Greg Banks
2009-05-30T13:02:47Z

On Sat, May 30, 2009 at 10:26 PM, Trond Myklebust
<trond.myklebust@fys.uio.no> wrote:
> On Sat, 2009-05-30 at 10:22 +1000, Greg Banks wrote:
>> On Sat, May 30, 2009 at 3:35 AM, Trond Myklebust
>> <trond.myklebust@fys.uio.no> wrote:
>> > On Fri, 2009-05-29 at 13:25 -0400, Brian R Cowan wrote:
>> >>
>>
>
> Firstly, the server only uses O_SYNC if you turn off write gathering
> (a.k.a. the 'wdelay' option). The default behaviour for the Linux nfs
> server is to always try write gathering and hence no O_SYNC.

Well, write gathering is a total crock that AFAICS only helps
single-file writes on NFSv2. For today's workloads all it does is
provide a hotspot on the two global variables that track writes in an
attempt to gather them. Back when I worked on a server product,
no_wdelay was one of the standard options for new exports.

> Secondly, even if it were the case, then this does not justify changing
> the client behaviour.

I totally agree, it was just an observation.

In any case, as Christoph points out, the ext3 performance difference
makes an unstable WRITE+COMMIT slower than a stable WRITE, and you
already assumed that.

--
Greg.