linux-ext4 - [LSF/MM TOPIC] Drop ext2/ext3 codebase? When?

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Message-ID: <20110203144011.GA28409@quack.suse.cz>
Date:	Thu, 3 Feb 2011 15:40:11 +0100
From:	Jan Kara <jack@...e.cz>
To:	lsf-pc@...ts.linuxfoundation.org
Cc:	linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [LSF/MM TOPIC] Drop ext2/ext3 codebase? When?

  Hi,

  I'm not completely sure this is interesting for enough people but maybe
it is...

As you well know, there are three independent code bases in kernel
implementing ext-based filesystems - ext2, ext3, and ext4. Of course it
costs some effort to maintain them all in a reasonably good condition so
once in a while someone comes and proposes we should drop one of ext2, ext3
or both. So I'd like to gather input what people think about this - should
we ever drop ext2 / ext3 codebases? If yes, under what condition do we deem
it is OK to drop it?

To give some facts:
Feature-wise, ext4 should now be almost a superset of both ext2 and
ext3. ext4 has nojournal mode to simulate ext2, looking at the code I only
don't see XIP support in ext4, arguably also nobh-mode but I personally
feel that these days the complication in the code isn't worth it. As far as
I know it should be backward compatible to writeably mount ext2/ext3
filesystem with ext4 (i.e., no incompatible features should be turned on
magically).

On the other hand there are differences noticeable under some conditions -
e.g. delayed allocation, data=ordered mode of ext3 gives better data
integrity than that of ext4 in practice (it's just a side effect we never
promised but app developers somehow got used to it ;), different allocation
decisions, and I believe there are more of these subtle differences.

Then of course there is the factor of the codebase itself: Ext2 - ~9k
lines, Ext3+JBD - 24k lines, Ext4+JBD2 - 43k lines. Ext2 codebase is so
simple that it sometimes serves as a "model filesystem". But arguably it
also bitrots slowly so copy-and-pasting from ext2 need not be clever idea
anymore.

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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