Our future upgrade wave of Ubuntu 18.04 machines
We have long had a mix of Ubuntu versions. The short explanation is that most machines users log in to (our login servers and compute servers) get upgraded every LTS version, while other machines that are less accessible only get upgraded every other LTS (the longer version is How we handle Ubuntu LTS versions). Under normal circumstances, this would currently give us a relatively even mix of 20.04 machines and 18.04 machines. These aren't normal times.
The result of these abnormal times is that we have a lot more 18.04 machines and a lot fewer 20.04 machines than we normally would. None of our user login machines have been upgraded, our mail servers had to move to 18.04 instead of 20.04, and until late last year we lacked much experience with 20.04, so the path of least resistance was using 18.04 for new or upgraded machines because it was a known quantity. This is fine by itself, as Ubuntu 18.04 LTS is a perfectly good Ubuntu release.
Our future issue is that having a lot of 18.04 machines (some of them very critical ones) means that when Ubuntu 22.04 comes out next April, we'll have a lot of machines to upgrade in less than a year (since 18.04 will stop being supported at the end of April 2023). This is probably more unique machines than we've ever had to upgrade in one cycle, even if we assume that the machines users log in to are mostly simple to rebuild. Some of the machines, such as our fileservers, will take extensive testing all on their own.
If we get enough time in the office this summer we may try to upgrade our user login machines to 20.04, even though it's a year behind the usual schedule. We have a test user login machine built, although it hasn't seen much use, and that would let us slip their upgrade to 22.04 until late, perhaps even the summer of 2023.
Beyond that, we could upgrade some machines early, moving we normally wouldn't touch from 18.04 to 20.04 just so we have fewer to move to 22.04 later. This would also give us more of a spread between LTS versions for the long term; otherwise, if we just upgrade all of our 18.04 machines to 22.04 we'll have much the same problem in 2026, with a lot of 22.04 machines to suddenly move to 26.04.
I have no conclusions, but at least this is now an issue I'm going to be thinking about.
(I'm aware that in some places, planning for 2026 would be a laughable idea. It may be optimistic even for us, but I've had long term planning pay off before and in general we exist in an environment with long term stability, although there are somewhat more clouds on the horizon than usual.)