RE: [PATCH] nfs: only do COMMIT for range written with direct I/O

["Myklebust, Trond" <Trond.Myklebust@netapp.com>]

> -----Original Message-----
> From: Jeff Layton [mailto:jlayton@redhat.com]
> Sent: Monday, December 05, 2011 12:11 PM
> To: Jeff Layton
> Cc: Myklebust, Trond; linux-nfs@vger.kernel.org; khoa@us.ibm.com
> Subject: Re: [PATCH] nfs: only do COMMIT for range written with direct
I/O
> 
> On Wed, 30 Nov 2011 09:07:54 -0500
> Jeff Layton <jlayton@redhat.com> wrote:
> 
> > When given a range to write with unstable writes, the current code
> > always does a COMMIT of the entire file afterward. This is
potentially
> > expensive on some servers and unnecessary. Instead, just do a COMMIT
> > for the offset and count that was written.
> >
> > Khoa, who reported this bug, stated that this made a big difference
in
> > performance in their environment, which I believe involves GPFS on
the
> > server. He didn't pass along any hard numbers so I can't quantify
the
> > gain, but it stands to reason that clustered filesystems might
suffer
> > more contention issues when issuing a commit over the whole file.
> >
> > Reported-by: Khoa Huynh <khoa@us.ibm.com>
> > Signed-off-by: Jeff Layton <jlayton@redhat.com>
> 
> Khoa found that he made a mistake when testing this originally, and
any
> benefit that the patch provides seems to be negligible. I still think
it's safe
> and reasonable to only issue a commit for the range that was written,
but
> there doesn't seem to be any compelling need for this patch right now.
> 
> Trond, do you have an opinion here? Should we go ahead and commit this
> patch or something like it, or leave well-enough alone?

I'd prefer to wait until I see a tangible benefit. I know that recent
kernels do have support for a COMMIT range on the Linux kernel server
side, so maybe it is just a question of shooting up an ext4 or XFS based
server and running a few tests with a large O_DIRECT writer on one
client, and a smaller O_DIRECT writer on another...

Cheers
  Trond