Many vendors are moving container registries away from docker hub. One of them is ceph. This means: Recent container images are no longer published via docker hub and will prevent you from upgrading using cephadm if your installation is still pointing to docker hub.
Issue: failed to pull target image
Trying to upgrade you might notice that the ceph version you want to upgrade to is unavailable.
This occurs because the base image is no longer present on docker hub, as stated above.
Fix: set container image base
The solution is straight forward: just set the image base using ceph config and start the upgrade.
This will effectively apply quay.io/ceph/ceph as image base.
Upgrading can then be started easily using this command: