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

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Date:	Wed, 30 Jan 2008 09:29:21 -0500
From:	Chris Mason <chris.mason@...cle.com>
To:	Al Boldi <a1426z@...ab.com>
Cc:	Jan Kara <jack@...e.cz>, Chris Snook <csnook@...hat.com>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC] ext3: per-process soft-syncing data=ordered mode

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.

-chris
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