Re: [PATCH 0/2] fix nfsd stable write implementation

["J. Bruce Fields" <bfields@fieldses.org>]

On Wed, Oct 31, 2012 at 07:30:27AM +1100, NeilBrown wrote:
> On Tue, 30 Oct 2012 10:07:25 -0400 "J. Bruce Fields" <bfields@fieldses.org>
> wrote:
> 
> > On Tue, Oct 30, 2012 at 10:28:33AM +1100, NeilBrown wrote:
> > > On Fri, 26 Oct 2012 17:06:55 -0400 "J. Bruce Fields" <bfields@redhat.com>
> > > wrote:
> > > 
> > > > From: "J. Bruce Fields" <bfields@redhat.com>
> > > > 
> > > > Peter pointed out to me that the nfs server is implementing stable
> > > > writes by setting the O_SYNC flag.  I can't see why we couldn't write
> > > > and then sync instead, but I don't know this stuff as well as I should;
> > > > does the following look reasonable to people?
> > > 
> > > Bruce changed the code to implement stable writes by calling
> > > vfs_fsync_range().  I can't see why we couldn't use O_SYNC instead.
> > > 
> > > It seems like you are making a change just for the sake of making a change.
> > > Is there some reason that you think a separate 'sync' is more efficient than
> > > using O_SYNC ?
> > 
> > Oh, sorry, see the changelog on the second patch: the problem is that
> > the struct file can be shared across multiple writes in the NFSv4 case,
> > so a single stable write could make all subsequent writes synchronous.
> > 
> > (And some day people would like filehandle caching for v2/v3, in which
> > case we'll run into the same problem.)
> 
> Ahh, I missed that.
> Makes sense then, thanks.
> 
> > 
> > > As a general principle, I think it is best to give the file system as much
> > > information as possible to that it can make whatever optimisation decisions
> > > that it wants to.
> > > 
> > > Setting O_SYNC gives the filesystem more information than not, because it
> > > allows it to change the behaviour of the 'write' request... though I don't
> > > know if any filesystem actually uses the information.
> > 
> > I'm not sure how to figure out if that's a real problem or not.
> 
> Search all filesystems for special handling of O_SYNC or O_DSYNC?
> 
> I strongly suspect that it is not a real problem.

OK, committing for 3.8.

--b.