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

[Al Boldi <a1426z@gawab.com>]

Chris Mason wrote:
> On Wednesday 30 January 2008, Al Boldi wrote:
> > Jan Kara wrote:
> > > > Chris Snook wrote:
> > > > > Al Boldi wrote:
> > > > > > This RFC proposes to introduce a tunable which allows to disable
> > > > > > fsync and changes ordered into writeback writeout on a
> > > > > > per-process basis like this:
> > > > > >
> > > > > >       echo 1 > /proc/`pidof process`/softsync
> > > > >
> > > > > This is basically a kernel workaround for stupid app behavior.
> > > >
> > > > Exactly right to some extent, but don't forget the underlying
> > > > data=ordered starvation problem, which looks like a genuinely deep
> > > > problem maybe related to blockIO.
> > >
> > >   It is a problem with the way how ext3 does fsync (at least that's
> > > what we ended up with in that konqueror problem)... It has to flush
> > > the current transaction which means that app doing fsync() has to wait
> > > till all dirty data of all files on the filesystem are written (if we
> > > are in ordered mode). And that takes quite some time... There are
> > > possibilities how to avoid that but especially with freshly created
> > > files, it's tough and I don't see a way how to do it without some
> > > fundamental changes to JBD.
> >
> > Ok, but keep in mind that this starvation occurs even in the absence of
> > fsync, as the benchmarks show.
> >
> > 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%.
>
> Do you see this on older kernels as well?  The first thing we need to
> understand is if this particular stall is new.

2.6.24,22,19 and 2.4.32 show the same problem.


Thanks!

--
Al