there is recommendation:
6) After all new nodes are running, run nodetool cleanup
on each of the previously existing nodes to remove the keys that no longer
belong to those nodes. Wait for cleanup to complete on one node before
running nodetool cleanup on the next node.
I had added a new node to the cluster, and running nodetool cleanup
according to this recommendation - but it takes near 10 days to complete on
a single node. Is it safe to start it on all nodes?