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Bug#978636: move to merged-usr-only?

Am Di., 29. Dez. 2020 um 17:39 Uhr schrieb Marco d'Itri <md@linux.it>:
> On Dec 29, Ansgar <ansgar@debian.org> wrote:
> > as suggested in [1], I would like to see Debian to move to support
> > only the merged-usr filesystem layout.  This would simplfy things for
> > the future and also address the problem with installing files under
> > aliased trees that dpkg has to do for both variants to be supported.
> As the architect of the Debian merged-usr transition, I agree: removing
> support for unmerged systems would simplify many things.
> Thank you Ansgar for bringing this to the CTTE.


> > I'm not asking the committee to decide on a concrete technical
> > implementation for this.  Obviously we would need to also implement a
> > migration path for legacy installations for a move to merged-usr-only
> > to be implemented.  This also isn't relevant for Debian 11 (bullseye),
> > but I would like to have enough time in the Debian 12 (bookworm)
> > cycle.
> Agreed.
> FWIW: we have had for a long time a reliable migration method, the
> usrmerge package.
> Except that on a live system it requires a reboot mid-conversion (due to
> bind mounts created by systemd): since this is inconvenient and mildly
> scary I did not propose wider adoption for buster.
> The best workaround would probably be to run it from the initramfs, but
> I have not been able to actually make this[1] work: I expect that
> somebody who knows initramfs-tools better than I do can fix it quickly.

For package upgrades, we can already perform so-called "offline
upgrades", where the system reboots into a smaller systemd target,
applies all updates and then reboots again into the updated system.
This is implemented in PackageKit as an option and used by GNOME.
Fwupd uses similar concepts for certain firmware updates as well.
Maybe hooking into these facilities is also an option for the usrmerge
upgrade? (unless of course systemd is still interfering even in a
minimal target, that would be a dealbreaker).
More info about offline-upgrades can be found at [1], could be
interesting to look into.

This discussion is probably OT through for the issue at hand (*if* we
make that switch, not the *how* we make it in particular).


[1]: https://www.freedesktop.org/software/systemd/man/systemd.offline-updates.html

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