Anarcat: Automating major Debian upgrades

It's major upgrade time again! The Debian project just published the Debian 11 "bullseye" release, and it's pretty awesome! This makes me realized that I have never written here about my peculiar upgrade process, and figured it was worth bringing that up to a wider audience.

My upgrade process also has a notable changes section which includes major version changes (e.g. Inkscape 1.0!), new packages (e.g. podman!) and important behavior changes (e.g. driverless scanning and printing!).

I'm particularly interested to hear about any significant change I might have missed. If you know of a cool new package that shipped with bullseye and that I forgot, do let me know!

But that's for the cool new stuff. We need to talk about the problems with Debian major upgrades.

Background

I have been maintaining detailed upgrade guides, on my wiki, starting with the jessie release, but I have actually written such guides for Koumbit.org as far back as Debian squeeze in 2011 (another worker wrote the older Debian lenny upgrade guide in 2009). Koumbit, since then, has kept maintaining those guides all the way to the latest bullseye upgrade, through 7 major releases!

Over the years, those guides evolved from a quick "cheat-sheet" format copied from the release notes into a more or less "scripted" form that I currently use.

Each guide has a procedure made of a few steps that can be basically copy-pasted to batch-upgrade a host (or multiple hosts in parallel) as quickly as possible. There is also the predict-os script which allows you to keep track of progress of the upgrades in a Puppet cluster.

Limitations of the official procedure

In comparison with my procedure, the official upgrade guide is mostly designed to upgrade a single machine, typically a workstation, with a rather slow and exhaustive process. The PDF version of the upgrade guide is 14 pages long! This, obviously, does not work when you have tens or hundreds of machines to upgrade.

Debian upgrades are notorious for being extremely reliable, but we have a lot of packages, and there are always corner cases where the upgrade will just fail because of a bug specific to your environment. Those will only fixed after some back and forth in the community (and that's assuming users report those bugs, which is not always the case). There's no obvious way to deploy "hot fixes" in this context, at least not without fixing the package and publishing it on an unofficial Debian archive while the official ones catch up. This is slow and difficult.

Or some packages require manual labor. Examples of this are the PostgreSQL or Ganeti packages which require you to upgrade your clusters by hand, while the old and new packages live side by side. Debian packages bring you far in the upgrade process, but sometimes not all the way.

Which means every Debian install needs to be manually upgraded and inspected when a new release comes out. That's slow and error prone and we can do better.

How to automate major upgrades

I have a proposal to automate this. It's been mostly dormant in the Debian wiki, for 5 years now. Fundamentally, this is a hard problem: Debian gets installed in so many different environments, from workstations to physical servers to virtual machines, embedded systems and so on, that it's extremely hard to come up with a "one size fits all" system.

The (manual) procedure I'm using is mostly targeting servers, but I'm also using it on workstations. And I'll note that it's specific to my home setup: I have a different procedure at work, although it has a lot of common code.

To automate this, I would factor out that common code with hooks where you could easily inject special code like "you need to upgrade ferm first", "you need an extra reboot here", or "this is how you finish the PostgreSQL upgrade".

With Debian getting closer to a 2 year release cycle, with the previous release being supported basically only one year after the new stable comes out, I feel more and more strongly that this needs better automation.

So I'm thinking that I should write a prototype for this. Ubuntu has do-release-upgrade that is too Ubuntu-specific to be reused. An attempt at collaborating on this has been mostly met with silence from Ubuntu's side as well.

I'm thinking that using something like Fabric, Mitogen, or Transilience: anything that will allow me to write simple, portable Python code that can run transparently on a local machine (for single systems upgrades, possibly with a GUI frontend) to remote servers (for large clusters of servers, maybe with canaries and grouping using Cumin). I'll note that Koumbit started experimenting with Puppet Bolt in the bullseye upgrade process, but that feels too site-specific to be useful more broadly.

Trade-offs

I am not sure where this stands in the XKCD time trade-off evaluation, because the table doesn't actually cover the time frequency of Debian release (which is basically "biennial") and the amount of time the upgrade would take across a cluster (which varies a lot, but that I estimate to be between one to 6 hours per machine).

Assuming I have 80 machines to upgrade, that is 80 to 480 hours (between ~3 to 20 days) of work! It's unclear how much work such an automated system would shave off, however. Assuming things are an order of magnitude faster (say I upgrade 10 machines at a time), I would shave off between 3 and 18 days of work, which implies I might allow myself to spend a minimum of 5 days working on such a project.

The other option: never upgrade

Before people mention those: I am aware of containers, Kubernetes, and other deployment mechanisms. Indeed, those may be a long-term solution, we currently can't afford to migrate everything over to containers right now: that is a huge migration and a total paradigm shift. At that point, whatever is left might not even be Debian in the first place. And besides, if you run Kubernetes, you still need to run some OS underneath and upgrade that, so that problem never completely disappears.

Still, maybe that's the final answer: never upgrade.

For some stateless machines like DNS replicas or load balancers, that might make a lot of sense as there's no or little data to carry to the new host. But this implies a seamless and fast provisioning process, and we don't have that either: at my work, installing a machine takes about as long as upgrading it, and that's after a significant amount of work automating that process, partly writing my own Debian installer with Fabric (!).

What is your process?

I'm curious to hear what people think of those ideas. It strikes me as really odd that no one has really tackled that problem yet, considering how many clusters of Debian machines are out there. Surely people are upgrading those, and not following that slow step by step guide, right?

I suspect everyone is doing the same thing: we all have our little copy-paste script we batch onto multiple machines, sometimes in parallel. That is what the Debian.org sysadmins are doing as well.

There must be a better way. What is yours?

My upgrades so far

So far, I have upgraded 2 out of my 3 home machines running buster -- others have been installed directly in bullseye -- with only my main, old, messy server left. Upgrades have been pretty painless so far, much better than the previous buster upgrade. Obviously, for me personal use, automating this is pointless.

Work-side, however, is another story: we have over 80 boxes to upgrade there and that will take a while. The last stretch to buster cycle took about two years to complete, so we might be done by the time the next release (12, "bookworm") is released, but that's actually a full year after "buster" becomes EOL, so it's actually too late...

At least I fixed the installers so that new the machines we create all ship with bullseye, so we stopped accumulating new buster hosts...

Thanks to lelutin and pabs for reviewing a draft of this post.

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