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

[Chris Mason <chris.mason@oracle.com>]

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.

-chris