Re: jbd/jbd2 performance improvements

[Ric Wheeler <rwheeler@redhat.com>]

Theodore Ts'o wrote:
> As I mentioned on the ext4 call yesterday, there was an interesting
> thread on LKML that wasn't cc'ed onto the linux-ext4 mailing list.  So
> in case folks missed it, it might be worth taking a look at this mail
> thread:
>
> 	[PATCH] Give kjournald a IOPRIO_CLASS_RT io priority
>
> 	http://lkml.org/lkml/2008/10/1/405
>
> The main issue that got discussed was the age-old "entaglement" problem.
> The jbd/jbd2 layer is supposed to avoid this by not blocking the
> "current" transaction why the blocks from the previous "committing"
> transaction are still being written out to disk.  Apparently this was
> broken sometime in the 2.5 time-frame:
>
> 	http://lkml.org/lkml/2008/10/2/41
> 	http://lkml.org/lkml/2008/10/2/322
>
> Later in the thread, a major contention point in do_get_write_access()
> was identified as the problem:
>
> 	http://lkml.org/lkml/2008/10/3/7
>
> ... and then andrew produced the following "hacky" fix:
>
> 	http://lkml.org/lkml/2008/10/3/22
>
> If someone has time to runs some benchmarks to see how this improves
> things, especially on a workload that has plenty of "engtanglements",
> that would be great.  (I bet Ric's fs_mark run should do a good job;
> fsyncs to creates lots of commits and the need to modify blocks that had
> been modified in the previous tansactions.)
>
> If we can get some quick testing done, and it shows really good results,
> this could be something that could try fast-tracking into the 2.6.28
> merge window.
>
> 						- Ted
>   

We are going to try and poke at this - do you suspect a single or 
multi-threaded test would work best?

Ric