linux-ext4 - Re: ext3 default journal mode

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Message-ID: <4A64E28E.2000907@redhat.com>
Date:	Mon, 20 Jul 2009 17:33:02 -0400
From:	Ric Wheeler <rwheeler@...hat.com>
To:	Theodore Tso <tytso@....edu>
CC:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Valerie Aurora <vaurora@...hat.com>,
	"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 07/20/2009 05:29 PM, 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?
>
> 						- Ted


Hi Ted,

I think that this is a huge improvement - thanks!

Ric

>
> P.S.  Note that date=writeback does not make the filesystem more
> "prone to corruption after crashes".
>
>
> config EXT3_DEFAULTS_TO_ORDERED
> 	bool "Default to 'data=ordered' in ext3"
> 	depends on EXT3_FS
> 	help
> 	  If a filesystem does not explicitly specify a data ordering
> 	  mode, and the journal capability allowed it, ext3 used to
> 	  historically default to 'data=ordered'.
>
>            Data=ordered mode is the mode used by most distributions, but can
>            introduce latency problems in some workloads, especially if there
> 	  is a combination of high bandwidth background writes and foreground
> 	  processes calling fsync() and waiting for the result.   In worst
> 	  case scenarios, the fsync() call can 500ms to multiple seconds
> 	  to return.
>
> 	  The problem with using a default of data=writeback, however,
> 	  is that is that after a system crash or a power failure,
> 	  files that were written right before the system went down
> 	  could contain previously written data or other garbage.
> 	  With data=ordered mode, any blocks in the file will have
> 	  been data written by the application, avoiding a possibility
> 	  of a security breach, which is especially problematic on a
> 	  multi-user system.  Note, however, that data=ordered does
> 	  not guarantee that the file will be consistent at an
> 	  application level; the application must use fsync() at
> 	  appropriate commit points in order to guarantee
> 	  application-level consistency.
>
>            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".
>

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