In Ceph, developers calculate the placement of data by making an enhancement to the CRUSH algorithm. Developers have introduced a series of CRUSH tunable options to support the change in behavior. These options control the improved variation or legacy of the algorithm that is used. Both Ceph servers and clients must support the new version of CRUSH for using new tunables.

Hence, Ceph developers have named CRUSH tunable profiles in the name of the Ceph version in which they were introduced. For example, the Firefly release supports the firefly tunables that will not work with the older clients. The ceph-osd and ceph-mon will prevent older clients from connecting to the cluster, as soon as a given set of tunables are changed from the legacy default behavior. These old clients do not support the new CRUSH features.

For more information, please visit http://docs.ceph.com/docs/jewel/rados/operations/crush-map/#tunables.