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Re: from / to /usr/: a summary

Philip Hands wrote:
> On Sat, 24 Dec 2011 19:03:10 +0800, Paul Wise <pabs@debian.org> wrote:
> > On Sat, 2011-12-24 at 10:25 +0000, Philip Hands wrote:
> > > I'd still like to know what the compelling reason for the change is
> > > though.
> > 
> > Apparently the reason is simply that our upstreams (who it sounds like
> > are predominantly driven by Redhat folks) are dropping support for /
> > and /usr on different partitions and that re-adding that support or
> > maintaining the existing support is too much work for the Debian
> > maintainers involved. At least that is what started the thread.

Not quite.  Redhat and others want to make this change (moving binaries
and libraries from / into /usr) not just for ease of maintenance (though
that makes sense too) but for several rather interesting reasons.  It
would consolidate almost everything managed by the package manager under
/usr.  Configuration would live in /etc (with templates possibly
provided by packages, though more and more packages follow the "override
files in /usr with files in /etc" approach and ship no /etc
configuration by default).  /var includes bits that change, which should
not normally include package-managed bits.

This would make /usr easy to snapshot, easy to exclude from backups,
easy to share between systems, easy to mark read-only (mount --bind -o
ro /usr /usr) and various other fun possibilities.

> It would be nice to know a) which packages are actually likely to be
> involved, and what sort of breakage we might expect to see if one were
> foolish enough to carry on with a separate /usr, and what sorts of
> separate /usr might provoke that breakage.
> That might allow us to come up with solutions that are not just:
>   Everyone must have initramfs, like it or not.
> If we could break the problem down a bit, it might allow us to say
> something more nuanced, like:
>   If you're not using NFS4 for /usr, don't worry about it, but if you
>   are, you'll need to make sure that your initramfs supports mounting
>   /usr.
> at which point most of the nay-sayers would presumably shrug and find
> something more interesting to whine about. ;-)

At this point, the solution looks similar to that: "If you don't have
/usr on a separate partition, you don't care.  Otherwise, your initramfs
must mount /usr."

Debian systems without an initramfs already represent an uncommon case;
you have to go out of your way to avoid having one, and you'd need a
custom kernel.  Systems with /usr on a separate partition also represent
an uncommon case.  This change would just prohibit the intersection of
the two: you can't both have /usr on a separate partition *and* not have
an initramfs.

So, if you currently have /usr on a separate partition but you use an
initramfs, you don't care.  If you don't use an initramfs, but you have
/usr on your root partition, you also don't care.  If you have /usr on a
separate partition *and* you don't currently have an initramfs, then you
either need to start using an initramfs (which Debian makes very trivial
to do) or you need to migrate the contents of /usr onto your root
partition (which you can easily do on a live system as long as you
either have enough space already or have LVM which you can live-resize).

Does that help?

- Josh Triplett

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