linux-ext4 - Re: ext3 default journal mode

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Message-ID: <20090721020050.GK2416@mit.edu>
Date:	Mon, 20 Jul 2009 22:00:50 -0400
From:	Theodore Tso <tytso@....edu>
To:	Valerie Aurora <vaurora@...hat.com>
Cc:	Ric Wheeler <rwheeler@...hat.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	"Stephen C. Tweedie" <sct@...hat.com>,
	Eric Sandeen <esandeen@...hat.com>,
	Andreas Dilger <adilger@....com>,
	Chris Mason <chris.mason@...cle.com>,
	Josef Bacik <jbacik@...hat.com>, Mingming Cao <cmm@...ibm.com>
Subject: Re: ext3 default journal mode

On Mon, Jul 20, 2009 at 07:04:03PM -0400, Valerie Aurora wrote:
> On Mon, Jul 20, 2009 at 05:29:04PM -0400, Theodore Tso wrote:
> > Here's a revised proposal for the KCONFIG text.  
> > 
> > Hopefully this is balanced about the two sides of the issue, without
> > explicitly advocating for one choice versus another.
> > 
> > What do people think?
> 
> I think it's extremely accurate and detailed, but too long - people's
> brains turn off after about the 15th line or so.  Here's an attempt to
> distill your description down and refer out to another document (which
> one?) for people who want to learn more.

Fair enough;  I've created an external document here:

http://ext4.wiki.kernel.org/index.php/Ext3_Data%3DOrdered_vs_Data%3DWriteback_mode

Comments?

>  config EXT3_DEFAULTS_TO_ORDERED
>  	bool "Default to 'data=ordered' in ext3"
>  	depends on EXT3_FS
>  	help
> 
>           If the mount options for an ext3 filesystem do not
>           include a journal mode, mount it in "data=ordered" mode.

Let me give a try at this:

           The journal mode options for ext3 have different tradeoffs
           between when data is guaranteed to be on disk and
           performance.  The use of "data=writeback" can cause
           unwritten data to appear in files after an system crash or
           power failure, which can be a security issue.  However,
           "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%3DOrdered_vs_Data%3DWriteback_mode

          If you have been historically happy with ext3's performance,
          data=ordered mode will be a safe choice and you should
          answer "y" here.  If you understand the reliability and data
          privacy issues of data=writeback and are willing to make
          that trade off, answer "n".
	   
						- Ted
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