Re: [PATCH] ext4: turn on i_version updates by default

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

On Tue, May 15, 2012 at 12:30:21PM +0200, Jan Kara wrote:
> On Mon 14-05-12 19:54:32, J. Bruce Fields wrote:
> > On Mon, May 14, 2012 at 05:33:04PM -0600, Andreas Dilger wrote:
> > > I said as much in another reply - that once i_version is used on
> > > a filesystem, it should be made "sticky" (i.e. permanently enabled
> > > for that filesystem).  However, until that time it shouldn't be
> > > enabled just because it might one day be used.
> > > 
> > > Even better than just blindly bumping the i_version on every change,
> > > it would be better to have users of i_version (i.e. knfsd) flag the
> > > inode with "needs i_version update" then read the version.  When the
> > > filesystem/VFS bumps i_version the next time it can clear this flag
> > > and not update i_version again until after the next time i_version
> > > is actually used.
> > 
> > I really don't want to do anything more complicated than necessary.
> > 
> > What would be the worst-case test for the extra inode dirtying, so we
> > can see what the numbers actually are?
>   Something like:
> 
>   int fd, i;
>   struct timeval tv[2];
> 
>   fd = open("file", O_CREAT | O_RDWR, 0644);
>   if (fd < 0)
>     return 1;
>   for (i = 0; i < 1000000; i++) {
>     gettimeofday(tv);
>     tv[1] = tv[0];
>     if (futimes(fd, tv) < 0)
>       return 1;
>   }
>   return 0;
> 
>   And see how long does it take with and without i_version?

The complaint I hear from Andreas is that we'll cause file_update_time()
to call mark_inode_dirty() more often.

I don't believe futimes() calls file_update_time().

So maybe replace that futimes() by a one-byte write?

--b.