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

[Chris Mason]

On Thursday 31 January 2008, Jan Kara wrote:
> 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...

nr_requests is fairly small, so a long stream of random requests should still 
end up being random IO.

Al, could you please compare the write throughput from vmstat for the 
data=ordered vs data=writeback runs?  I would guess the data=ordered one has 
a lower overall write throughput.

-chris
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