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

J. Bruce Fields
2009-06-01T22:30:45Z

On Sat, May 30, 2009 at 11:02:47PM +1000, Greg Banks wrote:
> 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.

Should be a simple nfs-utils patch to change the default.

--b.

>
> > 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.
> --
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