Manage a multi-datacenter crush map with the command line

A new datacenter is added to the crush map of a Ceph cluster:

# ceph osd crush add-bucket fsf datacenter
added bucket fsf type datacenter to crush map
# ceph osd crush move fsf root=default
moved item id -13 name 'fsf' to location {root=default} in crush map
# ceph osd tree
# id    weight  type name       up/down reweight
-13     0               datacenter fsf
-5      7.28            datacenter ovh
-2      1.82                    host bm0014
0       1.82                            osd.0   up      1
...

The datacenter bucket type already exists by default in the default crush map that is provided when the cluster is created. The fsf bucket is moved ( with crush move ) to the root of the crush map.

A new rule is created to take objects in the fsf datacenter and ensure no host holds more than one copy:

# ceph osd crush rule create-simple fsf-rule fsf host
# ceph osd crush rule dump
...
    { "rule_id": 6,
      "rule_name": "fsf",
      "ruleset": 6,
      "type": 1,
      "min_size": 1,
      "max_size": 10,
      "steps": [
            { "op": "take",
              "item": -13},
            { "op": "chooseleaf_firstn",
              "num": 0,
              "type": "host"},
            { "op": "emit"}]}]

A new pool is created and associated with the newly created rule ( id 6 ):

# ceph osd pool create fsf 128
pool 'fsf' created
# ceph osd pool set fsf crush_ruleset 6
set pool 7 crush_ruleset to 6

The OSDs are automatically added to the fsf bucket by adding the following to /etc/ceph/ceph.conf:

osd_crush_update_on_start = 1
osd_crush_location = datacenter=fsf

It is interpreted by the ceph-osd upstart script that is triggered when a new OSD is created or when the machine boots.

# ceph-deploy osd create bm0101.the.re:/dev/sdb:/dev/sdc
...
# ceph osd tree
...
-13     3.64            datacenter fsf
-14     3.64                    host bm0101
8       3.64                            osd.8   up      1
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2 Responses to Manage a multi-datacenter crush map with the command line

  1. The “crush map” link is broken: double hostname info.

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