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Date: Fri, 24 Jun 2005 19:06:44 -0400
From: "Theodore Ts'o" <tytso@mit.edu>
To: Hans Reiser <reiser@namesys.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>, David Masover <ninja@slaphack.com>,
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Subject: Re: reiser4 plugins
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On Fri, Jun 24, 2005 at 12:21:18PM -0700, Hans Reiser wrote:
> There is an area where we suffered from writing fsck last.  When there
> are two leaf nodes with the same key range AND the bitmap cannot be
> trusted to tell us which is the valid one, we don't know which is the
> most recent, and pick arbitrarily.  Also, if you store a backup of V3,
> and you don't compress it, and you wipe out the bitmap blocks and need
> to use fsck, we don't know what blocks are backup image and what blocks
> are the fs.  We advise users to never store a V3 backup on V3 without
> compressing it.

Unfortunately, there are plenty of reasons why you might want to store
a filesystem image on disk besides for backup purposes:

	* Regression tests --- I have some 70+ small filesystem images
		used for e2fsck's regression test suite.  (I am always
		amazed how many filesystem fsck programs don't have
		regression test suites.)
	* Initial ram-disk images
	* Image files for qemu or user-mode-linux

... and probably many more.  None of these are safe to store on a
reiserfs3 filesystem if you're worried about fsck being robust after a
disk failure.

Funny thing.  When I tell system administrators who have been around
the block more than a few times about this particular "feature" of
reiserfs3, they usually very quickly decide that it's time to switch
to another filesystem.....

						- Ted