Comment 33 for bug 1874939

I have the same issue.
That's why I've been testing a few things over the last few days:

Upgrade process:
Luminous -> Mimic -> Nautilus -> Octopus
(All Versions run under Bionic)

It doesn't matter whether I activate msgr2 or not. I always get the problem after upgrading to Octopus:
2021-01-11T09: 46: 33.674 + 0000 7fb8cf2d1700 1 osd.0 194 tick checking mon for new map
2021-01-11T09: 47: 04.490 + 0000 7fb8cf2d1700 1 osd.0 194 tick checking mon for new map
2021-01-11T09: 47: 34.514 + 0000 7fb8cf2d1700 1 osd.0 194 tick checking mon for new map
2021-01-11T09: 48: 05.451 + 0000 7fb8cf2d1700 1 osd.0 194 tick checking mon for new map

With a fresh installed version from Ceph Mimic and update to Nautilus -> Octopus I don't get this problem.
The problem apparently only comes from the update process from Luminous to Mimic, which then affects Octopus at the latest.

Workaround: Execute command on one of the Ceph monitors:
ceph osd require-osd-release mimic
After that, the octupus osd's can connect again.

Perhaps it is a good idea to run the "ceph osd require-osd-release [version]" command after every update.

e.g .:
After update Luminous -> Mimic
-> command: ceph osd require-osd-release mimic

After update Mimic -> Nautilus
-> command: ceph osd require-osd-release nautilus

After update Nautilus -> Octopus
-> command: ceph osd require-osd-release octopus

Apparently this is not done by the charms yet. Maybe the charms should do that or it should be mentioned in the charm documentation. What do you think about that?