The default value for mon_crush_min_required_version has been

changed from firefly to hammer , which means the cluster will

issue a health warning if your CRUSH tunables are older than hammer.

There is generally a small (but non-zero) amount of data that will

move around by making the switch to hammer tunables; for more information,

see Tunables.

If possible, we recommend that you set the oldest allowed client to hammer

or later. You can tell what the current oldest allowed client is with:

ceph osd dump | min_compat_client

If the current value is older than hammer, you can tell whether it

is safe to make this change by verifying that there are no clients

older than hammer current connected to the cluster with:

ceph features