linux-ext4 - Re: ext3 default journal mode

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Date:	Mon, 20 Jul 2009 17:36:20 -0600
From:	Andreas Dilger <adilger@....com>
To:	Valerie Aurora <vaurora@...hat.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 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, ...

> 	  XXX some document
> 
> 	  Use "data=ordered" mode unless you know it is causing a
> 	  performance problem for your workload.
> 
>           If you are unsure, say 'Y'.

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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