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

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Date:	Wed, 30 Jan 2008 09:04:48 +0300
From:	Al Boldi <a1426z@...ab.com>
To:	Jan Kara <jack@...e.cz>
Cc:	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

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%.

Now it turns out that 'echo 3 > /proc/.../drop_caches' has no effect, but 
doing it a few more times makes the hangs go away for while, only to come 
back again and again.


Thanks!

--
Al

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