Re: [PATCH] Documentation/filesystems/ext3.txt

[Andrew Clayton <andrew@digital-domain.net>]

On Tue, 19 May 2009 09:35:16 -0500, Eric Sandeen wrote:

> Andrew Clayton wrote:
> > Update the ext3 document with the fact that data=writeback is now
> > the default journaling mode.
> > 
> > 
> > Signed-off-by: Andrew Clayton <andrew@digital-domain.net>
> > 
> > --- linux-2.6/Documentation/filesystems/ext3.txt.orig
> > 2009-05-18 20:48:27.084220753 +0100 +++
> > linux-2.6/Documentation/filesystems/ext3.txt	2009-05-18
> > 20:49:01.023225288 +0100 @@ -39,11 +39,11 @@
> > data=journal		All data are committed into the journal
> > prior to being written into the main file system. 
> > -data=ordered	(*)	All data are forced directly out to
> > the main file +data=ordered		All data are forced
> > directly out to the main file system prior to its metadata being
> > committed to the journal.
> >  
> > -data=writeback		Data ordering is not preserved, data
> > may be written +data=writeback	(*)	Data ordering is
> > not preserved, data may be written into the main file system after
> > its metadata has been committed to the journal.
> 
> If we're updating this documentation, while we're at it we should add
> that the default is actually controlled by a new kernel config option,
> and also something along the lines of "this mode will produce file
> corruption on a power loss or unclean shutdown, and may lead to
> unintentional information disclosure."

Sure, the data problem is actually eluded to further down the document
where I've expanded it slightly and chat about the default writeback vs
ordered mode.

Hows the below?

> -Eric

Update the ext3 document with the fact that data=writeback is now
 the default journaling mode and mention that the default can be turned 
back to ordered mode via CONFIG_EXT3_DEFAULTS_TO_ORDERED

Also slightly expand upon the problems with writeback mode and possible
file corruption and information disclosures as noted by Eric Sandeen

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>

--- ext3.txt.orig	2009-05-19 16:31:17.000000000 +0100
+++ ext3.txt	2009-05-19 16:59:35.000000000 +0100
@@ -39,11 +39,11 @@
 data=journal		All data are committed into the journal prior to being
 			written into the main file system.
 
-data=ordered	(*)	All data are forced directly out to the main file
+data=ordered		All data are forced directly out to the main file
 			system prior to its metadata being committed to the
 			journal.
 
-data=writeback		Data ordering is not preserved, data may be written
+data=writeback	(*)	Data ordering is not preserved, data may be written
 			into the main file system after its metadata has been
 			committed to the journal.
 
@@ -160,16 +160,19 @@
 There are 3 different data modes:
 
 * writeback mode
-In data=writeback mode, ext3 does not journal data at all.  This mode provides
-a similar level of journaling as that of XFS, JFS, and ReiserFS in its default
-mode - metadata journaling.  A crash+recovery can cause incorrect data to
-appear in files which were written shortly before the crash.  This mode will
-typically provide the best ext3 performance.
+If no mode is explicitly set then this is the default mode.  In data=writeback 
+mode, ext3 does not journal data at all.  This mode provides a similar level of
+journaling as that of XFS, JFS, and ReiserFS in its default mode - metadata 
+journaling.  A crash+recovery can cause file corruption and may lead to 
+sensitve data to appear in files which were written shortly before the crash.  
+This mode will typically provide the best ext3 performance.
 
 * ordered mode
-In data=ordered mode, ext3 only officially journals metadata, but it logically
-groups metadata and data blocks into a single unit called a transaction.  When
-it's time to write the new metadata out to disk, the associated data blocks
+This mode can be made the default via the kernel config option
+CONFIG_EXT3_DEFAULTS_TO_ORDERED.  In data=ordered mode, ext3 only officially 
+journals metadata, but it logically groups metadata and data blocks into a 
+single unit called a transaction.  
+When it's time to write the new metadata out to disk, the associated data blocks
 are written first.  In general, this mode performs slightly slower than
 writeback but significantly faster than journal mode.