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

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Date:	Tue, 15 May 2012 12:30:21 +0200
From:	Jan Kara <jack@...e.cz>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	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 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?

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