I just tried upgrading cloudcephmon2004-dev to bookworm + ceph 16.2. It's configured so that it should pool with the other mons running 15.x.
It seems happy but it also consumes all the RAM on the system (64G) and the system locks up a few minutes after boot.
2025-07-02T01:47:57.112+0000 7f1ca049a640 0 starting mon.cloudcephmon2004-dev rank 1 at public addrs [v2:10.192.20.19:3300/0,v1:10.192.20.19:6789/0] at bind addrs [v2:10.192.20.19:3300/0,v1:10.192.20.19:6789/0] mon_data /var/lib/ceph/mon/ceph-cloudcephmon2004-dev fsid 489c4187-17bc-44dc-9aeb-1d044c9bba9e 2025-07-02T01:47:57.116+0000 7f1ca049a640 1 mon.cloudcephmon2004-dev@-1(???) e0 preinit fsid 489c4187-17bc-44dc-9aeb-1d044c9bba9e 2025-07-02T01:47:57.116+0000 7f1ca049a640 1 mon.cloudcephmon2004-dev@-1(???) e0 initial_members cloudcephmon2004-dev,cloudcephmon2005-dev,cloudcephmon2006-dev, filtering seed monmap 2025-07-02T01:47:57.120+0000 7f1ca049a640 0 mon.cloudcephmon2004-dev@-1(probing) e0 my rank is now 0 (was -1)