Re: [RFC] ext3: per-process soft-syncing data=ordered mode

[Jan Kara <jack@suse.cz>]

On Thu 31-01-08 11:56:01, Chris Mason wrote:
> On Thursday 31 January 2008, Al Boldi wrote:
> > Andreas Dilger wrote:
> > > On Wednesday 30 January 2008, Al Boldi wrote:
> > > > And, a quick test of successive 1sec delayed syncs shows no hangs until
> > > > about 1 minute (~180mb) of db-writeout activity, when the sync abruptly
> > > > hangs for minutes on end, and io-wait shows almost 100%.
> > >
> > > How large is the journal in this filesystem?  You can check via
> > > "debugfs -R 'stat <8>' /dev/XXX".
> >
> > 32mb.
> >
> > > Is this affected by increasing
> > > the journal size?  You can set the journal size via "mke2fs -J size=400"
> > > at format time, or on an unmounted filesystem by running
> > > "tune2fs -O ^has_journal /dev/XXX" then "tune2fs -J size=400 /dev/XXX".
> >
> > Setting size=400 doesn't help, nor does size=4.
> >
> > > I suspect that the stall is caused by the journal filling up, and then
> > > waiting while the entire journal is checkpointed back to the filesystem
> > > before the next transaction can start.
> > >
> > > It is possible to improve this behaviour in JBD by reducing the amount
> > > of space that is cleared if the journal becomes "full", and also doing
> > > journal checkpointing before it becomes full.  While that may reduce
> > > performance a small amount, it would help avoid such huge latency
> > > problems. I believe we have such a patch in one of the Lustre branches
> > > already, and while I'm not sure what kernel it is for the JBD code rarely
> > > changes much....
> >
> > The big difference between ordered and writeback is that once the slowdown
> > starts, ordered goes into ~100% iowait, whereas writeback continues 100%
> > user.
> 
> Does data=ordered write buffers in the order they were dirtied?  This might 
> explain the extreme problems in transactional workloads.
  Well, it does but we submit them to block layer all at once so elevator
should sort the requests for us...

									Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR