linux-kernel - Re: [PATCH, RFC] ext3: Update Kconfig description of EXT3_DEFAULTS_TO

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Date:	Tue, 11 Aug 2009 14:41:01 +0300
From:	Al Boldi <a1426z@...ab.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Theodore Ts'o <tytso@....edu>, Jan Kara <jack@...e.cz>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH, RFC] ext3: Update Kconfig description of EXT3_DEFAULTS_TO_ORDERED

Jan Kara wrote:
> On Tue 11-08-09 06:49:20, Al Boldi wrote:
> > Theodore Ts'o wrote:
> > > +	  "data=ordered" mode can also result in major performance
> > > +	  problems, including seconds-long delays before an fsync()
> > > +	  call returns.	 For details, see:
> > > +
> > > +	  http://ext4.wiki.kernel.org/index.php/Ext3_data_mode_tradeoffs
> >
> > Why isn't the fsync problem fixable?
>
>   Because it's quite deep in the design of JBD: All the modifications done
> to a filesystem go to one transactions. When the transaction grows big
> enough or old enough, we commit the transaction, which means we write all
> the metadata to the journal and all the ordered data to their final
> location on disk. If you do fsync(), you have to wait for a transaction
> commit with your data to finish, so that you are guaranteed a consistent
> state of metadata is on disk. But when there is heavy background writing,
> it means there's a lot of data you have to write out and wait for... It's
> not easy to work around this - naively, you might want to separate out just
> the writes you care about for fsync() but that's not easily possible
> because bitmaps and group descriptors are modified by other writes as well.

Ok, I remember now, that was the konqueror deadlocks problem.  I think making 
the fsync soft in that case would yield a better result than turning 
ordered-mode off completely.

BTW: did you get around fixing the ordered-mode redundant write out problem?


Thanks!

--
Al

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