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

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Message-ID: <20120515144334.GA26579@quack.suse.cz>
Date:	Tue, 15 May 2012 16:43:34 +0200
From:	Jan Kara <jack@...e.cz>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Jan Kara <jack@...e.cz>, Andreas Dilger <adilger@...ger.ca>,
	"Myklebust, Trond" <Trond.Myklebust@...app.com>,
	Theodore Ts'o <tytso@....edu>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] ext4: turn on i_version updates by default

On Tue 15-05-12 08:35:50, J. Bruce Fields wrote:
> 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().
  Yeah, right. I didn't check and I was wrong...

> So maybe replace that futimes() by a one-byte write?
  Yes, "pwrite(fd, "data", 1, 0);" should be then a proper test instead of
futimes().

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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