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

Il 15/05/2012 15:28, Josef Bacik ha scritto:

On Mon, May 14, 2012 at 03:27:47PM -0600, Andreas Dilger wrote:

quoted

On 2012-05-14, at 1:05 PM, Josef Bacik wrote:

quoted

On Mon, May 14, 2012 at 02:54:00PM -0400, J. Bruce Fields wrote:

quoted

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.

It shouldn't, let's be honest, most systems aren't going to have such
a coarse jiffie counter that they'll be able to get away with doing
2 calls to write() or ->page_mkwrite() in the same jiffie and skip the
update to mtime/ctime anyway.  If they do they are damned lucky, and
again the amount of overhead added even if they are should be
negligible since 99% of us all incur the overhead from having
to update mtime/ctime anyway.  Thanks,

Seriously?  The whole reason the above checks for timespec_equal()
are there is to avoid calling mark_inode_dirty_sync() thousands of
times per second.  If doing write() calls in the same jiffie were
so rare as you suggest then I don't think such an optimization
would ever have appeared in the first place.

Only a really really stupid question (I don't know NFS protocol well 
enough). In 3.3 kernel, I see that only ext4 uses MS_I_VERSION, so I 
wonder: if i_version change it's needed for exportable fs and so for 
nfs, other exportable fs? Is this only a particular problem for ext4? I 
mean, it doesn't seems a blocking problem (or we could have a lot of 
traffic on fs-devel :) ), it seems a "more compliant behavior". If this 
considerations is right, I think the current behavior of ext4 is ok.

Marco

Marco