Generally, we update to the new version of Debian, it installs the new version of Ganeti, we do a little gnt-cluster upgrade, and it rolls.
Alas, not for Debian Bullseye (Debian 11)! Indeed, the Debian update releases the packages relating to Ganeti 2.16, leaving only the Ganeti 3 files in place. However, you need the old binaries to update to the new ones!
So, before switching to Bullseye, install ganeti, ganeti-3.0, ganeti-haskell-3.0and ganeti-htools-3.0from the buster backports and upgrade the ganeti cluster to 3.0 ( gnt-cluster upgrade --to=3.0).
apt install ganeti ganeti-3.0 ganeti-haskell-3.0 ganeti-htools-3.0 -t buster-backports
You can then update the machines to Bullseye with peace of mind.
NB: it is said in the release notes of Ganeti that Ganeti must be updated to version 2.16.2 in order to be able to update to version 3.0 . Debian doesn’t provide a 2.16.2 version (just 2.16.0–5), but they’ve made it work (it’s in the list of changes when updating to Bullseye), don’t worry.