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

["J. Bruce Fields" <bfields-uC3wQj2KruNg9hUCZPvPmw@public.gmane.org>]

On Mon, May 14, 2012 at 02:33:17PM -0400, Josef Bacik 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.

Agreed, that's weird.

> 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,

I don't think they're worried about the inode_inc_iversion() calls
themselves, but the behavior of file_update_time():

        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;

        if (!sync_it)
                return;
	...
	mark_inode_dirty_sync(inode);

So now mark_inode_dirty_sync() is called on every update, instead of
merely on every update that sees a time change (so at most once a
jiffy).

So mark_inode_dirty_sync (and hence ->dirty_inode = ext4_dirty_inode)
may get called more often if you're writing very frequently.

I'm a bit surprised that's expected to add significant overhead to the
write.

I guess I should stare at the code and try to follow Andreas's
explanation....

--b.
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