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Debian buster shipped with a "merged /usr", which means that /bin, /lib and /sbin are now symlinks to their counterparts in /usr. There are concerns that this behavior is buggy and triggers problems in all sorts of places. In particular, the dpkg maintainers are quite unhappy about the change and do not support it as a configuration:

https://wiki.debian.org/Teams/Dpkg/MergedUsr

... which is disturbing, considering the dpkg is such a core component of a Debian system.

That wiki page provides a hackish script to "migrate away" from usrmerge but no one, as far as I know, has done that in production. It definitely looks nasty.

We should consider :

• whether this is a real problem (probably?)
• which machines have usrmerge (20 machines or 27%, detailed below)
• whether new machines should have it (probably not? not having usrmerge is not a problem, and having it has risks, so let's not risk it?)
• whether we need to fix old machines

There are two ways of fixing the installers:

• pass --no-merged-usr to deboostrap
• use mmdebstrap

The latter has the advantage of being faster, at the cost of being possibly less reliable and compatible.

Next steps:

1. fix cloud installer - fixed in the wiki and tsa-misc
2. fix robot installer - fixed in the wiki and tsa-misc
3. fix ganeti installer - reported as bug 959745, mentioned in the wiki, reported in the puppet module