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

[Al Boldi <a1426z@gawab.com>]

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.


Thanks!

--
Al