Controlling CRUSH placement

Learn how to control CRUSH placement.

The default CRUSH rule provides OSDs that are on different hosts. For instance:

chunk nr    01234567

step 1      _cDD_cDD
step 2      cDDD____
step 3      ____cDDD

needs exactly 8 OSDs, one for each chunk. If the hosts are in two adjacent racks, the first four chunks can be placed in the first rack and the last four in the second rack. Recovering from the loss of a single OSD does not require using bandwidth between the two racks.

For instance:

crush-steps='[ [ "choose", "rack", 2 ], [ "chooseleaf", "host", 4 ] ]'

creates a rule that selects two crush buckets of type rack and for each of them choose four OSDs, each of them located in a different bucket of type host.

The rule can also be created manually for finer control.