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Date: Mon, 11 Aug 2003 09:55:18 -0600
From: Andreas Dilger <adilger@clusterfs.com>
To: Jeff Garzik <jgarzik@pobox.com>
Cc: Alex Tomas <bzzz@tmi.comex.ru>, linux-kernel@vger.kernel.org,
       ext2-devel@lists.sourceforge.net
Subject: Re: [RFC] file extents for EXT3
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In-Reply-To: <3F3791C8.4090903@pobox.com>; from jgarzik@pobox.com on Mon, Aug 11, 2003 at 08:53:28AM -0400
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On Aug 11, 2003  08:53 -0400, Jeff Garzik wrote:
> Changing the underlying disk format without bumping the filesystem 
> revision is a hugely bad idea.  I disagreed with merging htree (even 
> though its backward compat) without bumping the filesystem version, too.
> 
> Vendors, distributors, OEMs, etc. all test against existing on-disk 
> formats, when they release their products.  When the filesystem format 
> for an existing filesystem, in production, changes underneath them, they 
> tend to get worried and annoyed.  So, to all ext developers,
> 
> Please add <it> to ext4 not ext3!

Ext2/3 uses feature flags instead of version numbers to indicate such
changes.  Version numbers are a poor way of indicating whether a change
is compatible or not compared to feature flags.  For example, if you bump
the minor number to indicate a "compatible" change it means that any
code that pretends to support version x.y features also needs to support
all features <= y and all features <= x.

If you really want to have a feature number to be happy, just think of

	s.feature_incompat.s_feature_ro_compat.s_feature_compat

as something like a version number and you will nearly be happy.

Cheers, Andreas
--
Andreas Dilger
http://sourceforge.net/projects/ext2resize/
http://www-mddsp.enel.ucalgary.ca/People/adilger/