ceph: CRUSH mapping algorithm

ceph: CRUSH mapping algorithm

CRUSH is a fancy hash function designed to map inputs onto a dynamic
hierarchy of devices while minimizing the extent to which inputs are
remapped when the devices are added or removed.  It includes some
features that are specifically useful for storage, most notably the
ability to map each input onto a set of N devices that are separated
across administrator-defined failure domains.  CRUSH is used to
distribute data across the cluster of Ceph storage nodes.

More information about CRUSH can be found in this paper:

    http://www.ssrc.ucsc.edu/Papers/weil-sc06.pdf