Chapter 4. Configuring Red Hat Ceph Storage stretch cluster

Procedure

1. Check the current election strategy being used by the monitors with the ceph mon dump command. By default in a ceph cluster, the connectivity is set to classic.

   ceph mon dump | grep election_strategy

   Example output:

   dumped monmap epoch 9
   election_strategy: 1

2. Change the monitor election to connectivity.

   ceph mon set election_strategy connectivity

3. Run the previous ceph mon dump command again to verify the election_strategy value.

   $ ceph mon dump | grep election_strategy

   Example output:

   dumped monmap epoch 10
   election_strategy: 3

   To know more about the different election strategies, see Configuring monitor election strategy.

4. Set the location for all our Ceph monitors:

   ceph mon set_location ceph1 datacenter=DC1
   ceph mon set_location ceph2 datacenter=DC1
   ceph mon set_location ceph4 datacenter=DC2
   ceph mon set_location ceph5 datacenter=DC2
   ceph mon set_location ceph7 datacenter=DC3

5. Verify that each monitor has its appropriate location.

   $ ceph mon dump

   Example output:

   epoch 17
   fsid dd77f050-9afe-11ec-a56c-029f8148ea14
   last_changed 2022-03-04T07:17:26.913330+0000
   created 2022-03-03T14:33:22.957190+0000
   min_mon_release 16 (pacific)
   election_strategy: 3
   0: [v2:10.0.143.78:3300/0,v1:10.0.143.78:6789/0] mon.ceph1; crush_location {datacenter=DC1}
   1: [v2:10.0.155.185:3300/0,v1:10.0.155.185:6789/0] mon.ceph4; crush_location {datacenter=DC2}
   2: [v2:10.0.139.88:3300/0,v1:10.0.139.88:6789/0] mon.ceph5; crush_location {datacenter=DC2}
   3: [v2:10.0.150.221:3300/0,v1:10.0.150.221:6789/0] mon.ceph7; crush_location {datacenter=DC3}
   4: [v2:10.0.155.35:3300/0,v1:10.0.155.35:6789/0] mon.ceph2; crush_location {datacenter=DC1}

6. Create a CRUSH rule that makes use of this OSD crush topology by installing the ceph-base RPM package in order to use the crushtool command:

   $ dnf -y install ceph-base

   To know more about CRUSH ruleset, see Ceph CRUSH ruleset.

7. Get the compiled CRUSH map from the cluster:

   $ ceph osd getcrushmap > /etc/ceph/crushmap.bin

8. Decompile the CRUSH map and convert it to a text file in order to be able to edit it:

   $ crushtool -d /etc/ceph/crushmap.bin -o /etc/ceph/crushmap.txt

9. Add the following rule to the CRUSH map by editing the text file /etc/ceph/crushmap.txt at the end of the file.

   $ vim /etc/ceph/crushmap.txt

   rule stretch_rule {
           id 1
           type replicated
           min_size 1
           max_size 10
           step take DC1
           step chooseleaf firstn 2 type host
           step emit
           step take DC2
           step chooseleaf firstn 2 type host
           step emit
   }
   
   # end crush map

   Note

   The rule id has to be unique. In the example, we only have one more crush rule with id 0 hence we are using id 1. If your deployment has more rules created, then use the next free id.

   The CRUSH rule declared contains the following information:

   • Rule name:

     • Description: A unique whole name for identifying the rule.
     • Value: stretch_rule

   • id:

     • Description: A unique whole number for identifying the rule.
     • Value: 1

   • type:

     • Description: Describes a rule for either a storage drive replicated or erasure-coded.
     • Value: replicated

   • min_size:

     • Description: If a pool makes fewer replicas than this number, CRUSH will not select this rule.
     • Value: 1

   • max_size:

     • Description: If a pool makes more replicas than this number, CRUSH will not select this rule.
     • Value: 10

   • step take DC1

     • Description: Takes a bucket name (DC1), and begins iterating down the tree.

   • step chooseleaf firstn 2 type host

     • Description: Selects the number of buckets of the given type, in this case is two different hosts located in DC1.

   • step emit

     • Description: Outputs the current value and empties the stack. Typically used at the end of a rule, but may also be used to pick from different trees in the same rule.

   • step take DC2

     • Description: Takes a bucket name (DC2), and begins iterating down the tree.

   • step chooseleaf firstn 2 type host

     • Description: Selects the number of buckets of the given type, in this case, is two different hosts located in DC2.

   • step emit

     • Description: Outputs the current value and empties the stack. Typically used at the end of a rule, but may also be used to pick from different trees in the same rule.

10. Compile the new CRUSH map from the file /etc/ceph/crushmap.txt and convert it to a binary file called /etc/ceph/crushmap2.bin:

    $ crushtool -c /etc/ceph/crushmap.txt -o /etc/ceph/crushmap2.bin

11. Inject the new crushmap we created back into the cluster:

    $ ceph osd setcrushmap -i /etc/ceph/crushmap2.bin

    Example output:

    17

    Note

    The number 17 is a counter and it will increase (18,19, and so on) depending on the changes you make to the crush map.

12. Verify that the stretched rule created is now available for use.

    ceph osd crush rule ls

    Example output:

    replicated_rule
    stretch_rule

13. Enable the stretch cluster mode.

    $ ceph mon enable_stretch_mode ceph7 stretch_rule datacenter

    In this example, ceph7 is the arbiter node, stretch_rule is the crush rule we created in the previous step and datacenter is the dividing bucket.

14. Verify all our pools are using the stretch_rule CRUSH rule we have created in our Ceph cluster:

    $ for pool in $(rados lspools);do echo -n "Pool: ${pool}; ";ceph osd pool get ${pool} crush_rule;done

    Example output:

    Pool: device_health_metrics; crush_rule: stretch_rule
    Pool: cephfs.cephfs.meta; crush_rule: stretch_rule
    Pool: cephfs.cephfs.data; crush_rule: stretch_rule
    Pool: .rgw.root; crush_rule: stretch_rule
    Pool: default.rgw.log; crush_rule: stretch_rule
    Pool: default.rgw.control; crush_rule: stretch_rule
    Pool: default.rgw.meta; crush_rule: stretch_rule
    Pool: rbdpool; crush_rule: stretch_rule

    This indicates that a working Red Hat Ceph Storage stretched cluster with arbiter mode is now available.