ganeti clusters don't like automatic upgrades
this weekend the ganeti cluster had a partial outage: nodes were reachable, but networking was broken on all instances.
this is, presumably, because of the Debian buster point release that occured on saturday (!). last time this happened, weasel identified openvswitch as the culprit, and hiro deployed a fix that would make it survive such situations. but either something else came up or the fix didn't work, because the problem happened again this weekend.
i fixed it by rebooting all nodes forcibly (without migrating first).