Chapter 7. Ceph OSDs in CRUSH

Adding an OSD to a CRUSH hierarchy is the final step before you start an OSD (rendering it up and in) and Ceph assigns placement groups to the OSD. You must prepare an OSD before you add it to the CRUSH hierarchy. Deployment tools such as ceph-deploy may perform this step for you. Refer to Adding/Removing OSDs for additional details.

The CRUSH hierarchy is notional, so the ceph osd crush add command allows you to add OSDs to the CRUSH hierarchy wherever you wish. The location you specify should reflect its actual location. If you specify at least one bucket, the command will place the OSD into the most specific bucket you specify, and it will move that bucket underneath any other buckets you specify.

To add an OSD to a CRUSH hierarchy, execute the following:

ceph osd crush add {id-or-name} {weight}  [{bucket-type}={bucket-name} ...]

ceph osd crush add {id-or-name} {weight}  [{bucket-type}={bucket-name} ...]

The following example adds osd.0 to the hierarchy:

ceph osd crush add osd.0 1.0 root=default datacenter=dc1 room=room1 row=foo rack=bar host=foo-bar-1

ceph osd crush add osd.0 1.0 root=default datacenter=dc1 room=room1 row=foo rack=bar host=foo-bar-1

If your deployment tool (e.g., ceph-deploy) added your OSD to the CRUSH map at a sub-optimal CRUSH location, or if your cluster topology changes, you may move an OSD in the CRUSH hierarchy to reflect its actual location.

To move an OSD within the CRUSH hierarchy, execute the following:

ceph osd crush set {id-or-name} {weight} root={pool-name}  [{bucket-type}={bucket-name} ...]

ceph osd crush set {id-or-name} {weight} root={pool-name}  [{bucket-type}={bucket-name} ...]

Removing an OSD from a CRUSH hierarchy is the first step when you want to remove an OSD from your cluster. When you remove the OSD from the CRUSH map, CRUSH will recompute which OSDs will get the placement groups and data will rebalance accordingly. Refer to Adding/Removing OSDs for additional details.

To remove an OSD from the CRUSH map of a running cluster, execute the following:

ceph osd crush remove {name}

ceph osd crush remove {name}