How to mount a FreeBSD-5.1 partition from FreeBSD-4.9

[Juan Rodriguez Hervella]

On Wednesday 21 January 2004 17:26, Sergey 'DoubleF' Zaharchenko wrote:
> On Wed, 21 Jan 2004 15:52:25 +0100
>
> Juan Rodriguez Hervella <jfrherve at ing.uc3m.es> probably wrote:
> > On Wednesday 21 January 2004 14:53, Sergey 'DoubleF' Zaharchenko wrote:
> > > On Wed, 21 Jan 2004 13:26:25 +0100
> > >
> > > Juan Rodriguez Hervella <jfrherve at ing.uc3m.es> probably wrote:
> > > > Hello, please send the reply to myself cause Im not
> > > > subscribe to the list
> > > >
> > > > I've got a FreeBSD-5.1 installation in /dev/ad0s3,
> > > > but I usually run FreeBSD-4.9 from /dev/ad0s2.
> > >
> > > 5.x uses UFS2 by default. 4.x does not understand UFS2. In short, you
> > > either re-newfs the 5.x partition to be UFS1, or you are short of luck
> > > this time.
> >
> > I can not believe you !,
> >
> > I guess there is (or there will be) some work-in-progress to have
> > UFS2 support on FreeBSD-4.X systems.
>
> If only by you:(. I'd suggest you make your 5.x partition UFS1 and be
> satisfied with that - that's pretty much all you can do.
>
> > Or are we following "Windows way of life" here ?
>
> If 5.x couldn't understand 4.x, that would be a bad thing. But forcing
> 5.x to be absolutely compatible with 4.x is another bad thing.
>
> If you were forced to stay that compatible all the time, you wouldn't be
> able to do major architectural changes. If everyone thought the same
> way, an Athlon or a P4 would be a 80286, only MUCH faster (which it is
> for most olden DOS or Windows/16 programs, so your definition of
> `Windows way of life' is definitely contrary to mine).
>
> You don't complain 4.x can't run 5.x binaries, do you?:)

But the filesystem is a different thing, imho.

For example, if you use FreeBSD-4.X you can mount 
ext2,ntfs,msdos,cd9660,smbfs..... if you use Linux, you
can mount ufs..... what's the reason it is not possible to
make a program which understands the UFS2 filesystem
under FreeBSD-4.X ? Is there any tecnical barrier ? Even
if the filesystem was mounted "read-only" (like ntfs), 
that would fit me....

Besides, is there any way to make my UFS2 filesystem go
back to UFS1 without losing the data ?

Thanks.
I