Re: [RFC] ext4: Don't send extra barrier during fsync if there areno dirty pages.

[Jan Kara]

   Hi,

> On Wed, Jun 30, 2010 at 09:21:04AM -0400, Ric Wheeler wrote:
> > 
> > The problem with not issuing a cache flush when you have dirty meta
> > data or data is that it does not have any tie to the state of the
> > volatile write cache of the target storage device.
> 
> We track whether or not there is any metadata updates associated with
> the inode already; if it does, we force a journal commit, and this
> implies a barrier operation.
> 
> The case we're talking about here is one where either (a) there is no
> journal, or (b) there have been no metadata updates (I'm simplifying a
> little here; in fact we track whether there have been fdatasync()- vs
> fsync()- worthy metadata updates), and so there hasn't been a journal
> commit to do the cache flush.
> 
> In this case, we want to track when is the last time an fsync() has
> been issued, versus when was the last time data blocks for a
> particular inode have been pushed out to disk.
> 
> To use an example I used as motivation for why we might want an
> fsync2(int fd[], int flags[], int num) syscall, consider the situation
> of:
> 
> 	fsync(control_fd);
> 	fdatasync(data_fd);
> 
> The first fsync() will have executed a cache flush operation.  So when
> we do the fdatasync() (assuming that no metadata needs to be flushed
> out to disk), there is no need for the cache flush operation.
> 
> If we had an enhanced fsync command, we would also be able to
> eliminate a second journal commit in the case where data_fd also had
> some metadata that needed to be flushed out to disk.
  Current implementation already avoids journal commit because of
fdatasync(data_fd). We remeber a transaction ID when inode metadata has
last been updated and do not force a transaction commit if it is already
committed. Thus the first fsync might force a transaction commit but second
fdatasync likely won't.
  We could actually improve the scheme to work for data as well. I wrote
a proof-of-concept patches (attached) and they nicely avoid second barrier
when doing:
echo "aaa" >file1; echo "aaa" >file2; fsync file2; fsync file1

  Ted, would you be interested in something like this?

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SuSE CR Labs