Re: [PATCH-v2 0/5] add support for a lazytime mount option

[Theodore Ts'o <tytso@mit.edu>]

On Mon, Nov 24, 2014 at 05:11:45PM -0500, J. Bruce Fields wrote:
> On Mon, Nov 24, 2014 at 06:57:27AM -0500, Theodore Ts'o wrote:
> > If we want to be paranoid, we handle i_version updates non-lazily; I
> > can see arguments in favor of that.
> > 
> > Ext4 only enables MS_I_VERSION if the user asks for it explicitly, so
> > it wouldn't cause me any problems.  However, xfs and btrfs enables it
> > by default, so that means xfs and btrfs wouldn't see the benefits of
> > lazytime (if you're going to have to push I_VERSION to disk, you might
> > as well update the [acm]time while you're at it).  I've always thought
> > that we *should* do is to only enable it if nfsv4 is serving the file
> > system, and not otherwise, though, which would also give us
> > consistency across all the file systems.
> 
> I guess you need to worry about the case where you shutdown nfsd, modify
> a file, then restart nfsd--you don't want a client to miss the
> modification in that case.

Hmmm, is there a way we can determine whether the file system is being
exported via nfsv4 is running, without using MS_I_VERSION?  Maybe a
new flag?  We could just disable lazytime if nfsd is running but
otherwise always increment i_version if lazytime is enabled.  My main
concern with i_version updates was not the in-memry update of
i_version, but rather the unnecessary extra metadata write "tax" that
would be inflicted on all users, including the many that aren't
serving files via NFSv4.

That way, if you shutdown the nfsv4 server, i_version would still be
updated, but we wouldn't be forcing the writes to disk, but then when
nfs v4 server is updated again, the i_version tax would be paid again.

       	      	 	 	    	      - Ted