Der Mouse points out:
> > even though I've never seen a *writable* iso-fs yet - that's why I
> > said he doesn't want that - because it would be quiet useless.

> CDROM images stored in files are writable and in iso9660 format.

As long as you're willing to trust your creation tools, you would
generally hope that the CDROM image file would be valid at the time
that you try writing the CD (and if not, why would you trust the
filesystem savager tool? ;-).  Now, this might not be the case if you
crash in the middle of the writing process; however, if you're
building the CDROM image with a tool (rather than mounting it as a
writable filesystem) then you'd probably have to start over, anyway.

That makes the question really "should the iso9660 filesystem code
learn how to write iso9660 images?"  I don't know how hard this would
be, but it is likely to be of limited interest -- currently, anyway,
CD-ROM burners are still more expensive than readers.  It would seem,
then, that it is reasonable to leave the kernel-based FS code readonly
and use a user-level tool for those few who want to write CD-ROMs;
this may inconvenience them, but I think the net inconvenience is less
than the sum of stealing another 10KB of codespace from everyone
else...  (Not only that, but part of the art of laying out CD-ROMs,
for product use, anyway, is in picking *where* files go to get
acceptable performance, right?  Let's see generic filesystem code get
THAT right!)