At work, we manage a fleet of bare metal servers and virtual machines, running Debian GNU/Linux. Upgrades to a new major are often very challenging; not because of the actual upgrade path, but because of the unwanted side-effects that noone had predicted.
One of these side-effects was: “Why this shiny Buster VM takes 5 minutes to boot?“. Together with my colleague Alexandros Afentoulis, we figured out the issue, fixed it and wrote a post about it. The real root cause is a number of changes that happened to Debian GNU/Linux regarding entropy gathering (!).
You can find our work on Alexandros’ blog here.