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

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Date:	Mon, 14 May 2012 14:48:02 -0400
From:	Jeff Layton <jlayton@...hat.com>
To:	Josef Bacik <josef@...hat.com>
Cc:	"Ted Ts'o" <tytso@....edu>, Andreas Dilger <adilger@...ger.ca>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	"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 May 2012 14:33:17 -0400
Josef Bacik <josef@...hat.com> wrote:

> On Mon, May 14, 2012 at 01:58:22PM -0400, Ted Ts'o wrote:
> > On Mon, May 14, 2012 at 11:27:42AM -0600, Andreas Dilger wrote:
> > > > And if it at all possible I'd rather have it be something that Just
> > > > Works rather than something that requires extra configuration.
> > > 
> > > Sure, but this is only useful for NFSv4, but costs everyone using
> > > ext4 continuous overhead, so it isn't a clear-cut case to enable
> > > the version just on the thought that NFS might one day be used on
> > > any particular filesystem.
> > 
> > It's not a matter of "NFSv4 might one day be used"; if we don't turn
> > on i_version updates until the file system is actually exported via
> > NFSv4, there would be no deleterious effects.
> > 
> > I always thought that was going to be the plan; that there would be
> > some flag that would be set in struct super_block when the file system
> > was exported that would enable i_version updates.
> > 
> > That way we satisfy the "no extra configuration" needed requirement,
> > which I agree is ideal, but we also don't waste any CPU overhead if
> > the file system is not exported via NFSv4.  I tried to implement
> > anything along these lines because I don't care enough, and I don't
> > use NFSv4 personally....
> > 
> 
> Seems like this is just a bad place to be doing inode_inc_iversion().  If
> MS_IVERSION is set we will update iversion in file_update_time() and then call
> mark_inode_dirty which will jack up the iversion again.  In btrfs we just change
> it wherever we change ctime and that way you don't really notice the extra
> overhead since you are doing it in paths where you are changing a bunch of stuff
> in the inode already, and mostly where you hold the i_mutex so you aren't going
> to be hitting any contention on the i_lock.  Thanks,
> 

Well, you do incur a bit more overhead in btrfs too:

------------------[snip]----------------------
        if (!timespec_equal(&inode->i_mtime, &now))
                sync_it = S_MTIME;

        if (!timespec_equal(&inode->i_ctime, &now))
                sync_it |= S_CTIME;

        if (IS_I_VERSION(inode))
                sync_it |= S_VERSION;
------------------[snip]----------------------

So you'll end up with sync_it being 0 if i_version updates are
disabled, and the mtime/ctime didn't visibly change.

If your jiffies are coarse-grained enough, then you might get "lucky"
rather often, but is that a case worth optimizing for? How often does
it happen that you mark the inode dirty, flush it to disk and then
re-mark it dirty within the same jiffy?

If that's a common occurrence then you might see some performance
impact here from turning i_version on all the time. Otherwise, it seems
like it wouldn't make that big a difference.

-- 
Jeff Layton <jlayton@...hat.com>
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