Re: [RFC 0/13] extents and 48bit ext3

[Theodore Tso <tytso@mit.edu>]

On Fri, Jun 09, 2006 at 01:38:03PM -0700, Joel Becker wrote:
> 	Filesystem features are different.  There is a permanent state
> that the older code cannot read.  Alex claims people just shouldn't use
> "-o extents", but the fact is their distro will choose it for them.  We
> have multiboot machines in our test lab, because like many people we
> don't have unlimited funds.  What happened when we installed the 2.6
> distros?  All of a sudden the older 2.4 distros wouldn't mount the
> shared filesystems, becuase of ext3 features.  

This is going to happen regardless of whether we call the code base
"ext3" or "ext4".  Anytime you make format changes (in this case, to
support larger disk sizes) older kernels won't support it any more.
Surprise!  

In the case you were referring to, one specific distribution, Red Hat,
silently added the extended attributes feature to the filesystem
because it was needed by SELINUX.  This was actually a backwards
compatible feature, so that older 2.4 based distributions could
*mount* the filesystem.  Unfortunately e2fsck needs to be more
careful, and so the problem was that the older distribution's fsck
wasn't able to check the filesystem.  But this was actually Red Hat's
fault, in that they shouldn't have transparently added the extended
attribute feature without first asking the user's permission.   

Being able to forward upgrade to newer filesystem formats is a good
thing, and has a long history; users don't like to do a backup,
reformat, and restore pass if they can't help that.  Heck, Microsoft
Windows even has a way that they can upgrade a FAT filesystem to their
latest NTFSv5 filesystem using a userspace progam.  Providing such a
capability is not a bad thing, and in fact it is a good thing.  The
bad thing to do is to do the conversion without first asking the
user's permission (for example just as Windows XP does when you first
boot a preinstalled system on a laptop for the first time).

People seem to be arguing that just because an distribution installer
_could_ do a backwards incompatible upgrade without first asking
permission first, we must not provide the capability at all, and make
it be the case that the only way to upgrade from ext3 to ext4 is with
a backup, reformat, and restore.  Surely that doesn't make sense!

> This wasn't the kernel driver, this was merely the tools!  Surprise!
> We made no choice to use new features, and they were thrust upon us.
> This will happen to others.

I suspect that Red Hat has learned from that past experience, and
won't be making that mistake again, at least without explicitly
requesting the user's permission.  So how about we trust the
distributions to be a bit more careful this time around?

						- Ted