This will probably not hit many people but it bit me and should be in the doc, at least until Octopus packages are upgraded to 15.2.14.
The Bug that hit me:
https://tracker.ceph.com/issues/51673
Fixed in 15.2.14:
It was not easy to downgrade to Octopus but it can be done and everything is working again.
Best regards,
Jonas Stunkat
The Bug that hit me:
https://tracker.ceph.com/issues/51673
Fixed in 15.2.14:
CephFS: old clusters (pre-Jewel) that did not use CephFS have legacy data structures in the ceph-mon stores. These structures are not understood by Pacific monitors. With Octopus v15.2.14, the monitors have been taught to flush and trim these old structures out in preparation for an upgrade to Pacific or Quincy. For more information, see Issue 51673 <https://tracker.ceph.com/issues/51673>.
It was not easy to downgrade to Octopus but it can be done and everything is working again.
Best regards,
Jonas Stunkat