Chapter 15. Add OSD Hosts/Chassis to the CRUSH Hierarchy
Once you have added OSDs and created a CRUSH hierarchy, add the OSD hosts/chassis to the CRUSH hierarchy so that CRUSH can distribute objects across failure domains. For example:
ceph osd crush set osd.0 1.0 root=default datacenter=dc1 room=room1 row=row1 rack=rack1 host=node2 ceph osd crush set osd.1 1.0 root=default datacenter=dc1 room=room1 row=row1 rack=rack2 host=node3 ceph osd crush set osd.2 1.0 root=default datacenter=dc1 room=room1 row=row1 rack=rack3 host=node4
The foregoing example uses three different racks for the exemplary hosts (assuming that is how they are physically configured). Since the exemplary Ceph configuration file specified "rack" as the largest failure domain by setting osd_crush_chooseleaf_type = 3, CRUSH can write each object replica to an OSD residing in a different rack. Assuming osd_pool_default_min_size = 2, this means (assuming sufficient storage capacity) that the Ceph cluster can continue operating if an entire rack were to fail (e.g., failure of a power distribution unit or rack router).
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