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

[Josef Bacik <josef@redhat.com>]

On Mon, May 14, 2012 at 02:48:02PM -0400, Jeff Layton wrote:
> On Mon, 14 May 2012 14:33:17 -0400
> Josef Bacik <josef@redhat.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?
> 

Well sync_it just means do we need to call mark_inode_dirty, not necessarily do
we need to write it to disk, so you are just updating a field in memory.  Now if
your jiffies are coarse enough for you to not notice the ctime update then yes
you are incurring an extra lock/inc/unlock, but this is called in the write path
where you are going to do much more latency inducting operations than locking
and unlocking a generally uncontended spin lock.  Thanks,

Josef