linux-ext4 - Re: ext3 default journal mode

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Date:	Tue, 21 Jul 2009 13:44:52 -0400
From:	Valerie Aurora <vaurora@...hat.com>
To:	Andreas Dilger <adilger@....com>
Cc:	Theodore Tso <tytso@....edu>, 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>,
	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 05:36:20PM -0600, Andreas Dilger wrote:
> On Jul 20, 2009  19:04 -0400, Valerie Aurora wrote:
> > 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.
> > 
> > (Sorry for the whitespace damage.)
> > 
> > -VAL
> > 
> >  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.
> 
> I would make this a bit more clear:
> 
>             This option sets the default journal mode for ext3 filesystems
> 	    which do not explicitly specify it in /etc/fstab or at mount
> 	    time.  It is always possible to set the journal mode for each
> 	    filesystem independently with "data=writeback", "data=ordered",
> 	    or "data=journal" mount options.
> 
> >           The journal mode options for ext3 have different tradeoffs
> >           between when data is guaranteed to be on disk and
> >           performance.  Many applications assume "data=ordered"
> >           semantics and may lose, destroy, or reveal other user's data
> >           in other journal modes.  However, "data=ordered" mode can
> >           also result in major performance problems, including long
> >           delays before an fsync() call returns.  For details, see:
> 
> I think the "... lose, destroy, ..." part is confusing, as it mentions
> "data=ordered" first and it isn't until the end of the sentence that
> it is clear that "lose, destroy, ..." does not apply to data=ordered.
> Also "data=journal" also does not apply in this case, only "data=writeback"
> so we may as well call that out explicitly.
> 
>             ...  Many applications do not explicitly sync data and assume
> 	    "data=ordered" mode.  Saying 'N' here will use "data=writeback"
> 	    as the default for all ext3 filesystems, and may result in
> 	    files with no data, or garbage data from deleted files,
> 	    which is a security risk on a multi-user system.  However, ...

Good points.  I think Ted's rewrite addresses most of them, what do you think?

-VAL
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