Re: [PATCH] ext4: fix how i_version is modified and turn it on by default V2

["J. Bruce Fields" <bfields@fieldses.org>]

On Tue, May 15, 2012 at 04:05:34PM -0400, Josef Bacik wrote:
> On Tue, May 15, 2012 at 01:55:33PM -0600, Andreas Dilger wrote:
> > It should be fairly straight forward to have a flag set in the ext4
> > superblock (s_state flag?) that indicates that the filesystem has
> > been exported via NFS.  There might be other optimizations that can
> > be done based on this (e.g. avoid some of the directory cookie
> > hijinx that are only needed if NFS has exported the filesystem and
> > needs to keep persistent cookies across reboots).
> > 
> > I think that the ext4_mark_inode_dirty() performance problem could
> > be at least partially fixed by deferring the copy of in-core inode
> > to on-disk inode to use a journal commit callback.  This is far more
> > work than just setting a flag in the superblock, but it has the
> > potential to _improve_ performance rather than make it worse.

Could you give any more pointers for an ext4 ignoramus?  (Where *is* the
journal commit code that would need the callback?  And where is the copy
currently done?)

> Yeah Btrfs doesn't have this sort of problem since we delay inode
> updating sinc it is so costly, we simply let it hang around in the
> in-core inode until we feel like updating it at some point down the
> road.  I'll put together a feature flag or something to make it be
> enabled for always if somebody turns it on.

Thanks for looking at this.

A feature flag would be an improvement over a mount option.

If the flag makes a noticeable difference to performance, then it makes
me nervous toggling it automatically.  And what will we do if statx
starts returning i_version to userspace?

--b.