Hi Radoslaw/Laura, I discovered an issue while fixing the broken upgrade suite for the final reef release, ticket here: https://tracker.ceph.com/issues/75034 The short of it is that Reef chose a feature bit for SERVER_REEF that was still in use by Pacific: https://github.com/ceph/ceph/commit/35d483af5425eb08593bc1c3065cfaec99938244 that bit was deprecated in https://github.com/ceph/ceph/commit/6c097015bbc1bcfa8abe518680a3d3a17ff39884 and retired in https://github.com/ceph/ceph/commit/f1ecf99a86edfe899392b6b734351f1015a93be6 Thus Pacific OSDs are advertising SERVER_REEF from the Reef Monitor's perspective. This was only discovered because this backport was merged for v18.2.5: https://github.com/ceph/ceph/commit/25e8b22c6f29cd3947b501f6aaf7614ba204a2c8 https://tracker.ceph.com/issues/69150 That PR would cause these warnings to be raised during stress splits of reef upgrades (4/8 OSDs upgraded to reef): https://pulpito.ceph.com/pdonnell-2026-03-04_15:16:52-upgrade:pacific-x-reef... Unfortunately the bug was shadowed by other failures during QA for v18.2.5: https://tracker.ceph.com/issues/70563 in https://pulpito.ceph.com/yuriw-2025-03-20_16:00:43-upgrade:pacific-x-reef-re... I have worked around the problem by muting the OSD_UPGRADE_FINISHED health warning: https://github.com/ceph/ceph/pull/67657/changes/5a52bfaf4400a7d7632602f455c6... However, I do wonder if there may be other unintended failures resulting from the Pacific and Reef OSDs believing they are operating with the same version. This problem would have existed since the first reef release (v18.2.0) but to my knowledge there are no documented problems that may have arisen during a rolling upgrade. I request the core team have a second look so we can document the problem properly for users. -- Patrick Donnelly, Ph.D. He / Him / His Red Hat Partner Engineer IBM, Inc. GPG: 19F28A586F808C2402351B93C3301A3E258DD79D