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

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On Tue, 30 Oct 2012 10:07:25 -0400 "J. Bruce Fields" <bfields@xxxxxxxxxxxx>
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@xxxxxxxxxx>
> > wrote:
> > 
> > > From: "J. Bruce Fields" <bfields@xxxxxxxxxx>
> > > 
> > > 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.

Thanks,
NeilBrown

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