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Re: Splitting the distribution for CDs

On Thu, 19 Mar 1998, Ian Jackson wrote:

> Please explain what you mean by reverse dependencies.
> The package installation process (as implemented by dpkg) has been
> carefully designed so that, barring pre-dependecies (which should be
> very rare and only have a very small number of packages as targets)
> packages may be unpacked in _any_ order.

Yes, this is technically true (don't forget that conflicts cause ordering
issues), but it gives a very poor result with respect to already installed
packages. The trivial example is,

Chimera-1, Depends on elf-xlib
xlib6-1, Provides elf-xlib
xlib6-2, Does not provide elf-exlib

If you just install without considering the above you get something like
Unpack xlib6-2
Unpack Chimera
configure xlib6-2
configure chimera

Woops. By unpacking the new xlib you have wrecked the existing version of
chimera and this -might- cause problems, it certianly isn't desirable. 
The solution of course is to reverse the unpacking order so it doesn't
happen. But this makes a low priority package unpacked before a higher
priority package.. 

There are other cases, such as removal ordering, where packages are
unpacked before a package selected for removal is actually removed, this
allows the removal to procceed cleanly, especially in the case when
packages are conflicting but not providing the one being removed. Again
this may require packages from the second CD - ie removing the dependants
of a low priority package.

The basic problem here is creating a subset of packages that exists on a
single CD and can be installed cleanly, without creating breaks.

> > I'm not sure what to do..
> It is absolutely essential that we have the ability to install from
> multiple CDs which have been created in a simple manner (by applying
> glob patterns to filenames, for example).  Otherwise we won't be able
> to install from CD at all !

Well, three choices,
   - Reduce the agressiveness of the ordering, order specifically to
     reduce disc swapping
   - Copy some files to the hard drive to reduce disk swapping
   - Disc swap all you want


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