第6章 Defining performance tiers for varying workloads in a Ceph Storage cluster with director

You can use Red Hat OpenStack Platform (RHOSP) director to deploy different Red Hat Ceph Storage performance tiers. You can combine Ceph CRUSH rules and the CephPools director parameter to use the device classes feature and build different tiers to accommodate workloads that have different performance requirements. For example, you can define a HDD class for normal workloads and an SSD class that distributes data only over SSDs for high performance loads. In this scenario, when you create a new Block Storage volume, you can choose the performance tier, either HDDs or SSDs.

WARNING
Defining performance tiers in an existing environment might result in massive data movement in the Ceph cluster. ceph-ansible, which director triggers during the stack update, does not have logic to check if a pool is already defined in the cluster and if it contains data. This means that defining performance tiers in an existing environment can be dangerous because the change of the default CRUSH rule that is associated with a pool results in data movement. If you require assistance or recommendations for adding or removing nodes, contact Red Hat support.
注記

Ceph autodetects the disk type and assigns it to the corresponding device class, either HDD, SSD, or NVMe based on the hardware properties exposed by the Linux kernel. However, you can also customize the category according to your needs.

Prerequisites

  • For new deployments, Red Hat Ceph Storage (RHCS) version 4.1 or later.
  • For existing deployments, Red Hat Ceph Storage (RHCS) version 4.2 or later.

To deploy different Red Hat Ceph Storage performance tiers, create a new environment file that contains the CRUSH map details and then include it in the deployment command.

In the following procedures, each Ceph Storage node contains three OSDs, sdb and sdc are spinning disks and sdc is a SSD. Ceph automatically detects the correct disk type. You then configure two CRUSH rules, HDD and SSD, to map to the two respective device classes. The HDD rule is the default and applies to all pools unless you configure pools with a different rule.

Finally, you create an extra pool called fastpool and map it to the SSD rule. This pool is ultimately exposed through a Block Storage (cinder) back end. Any workload that consumes this Block Storage back end is backed by SSD only for fast performances. You can leverage this for either data or boot from volume.

6.1. Configuring the performance tiers

WARNING
Defining performance tiers in an existing environment might result in massive data movement in the Ceph cluster. ceph-ansible, which director triggers during the stack update, does not have logic to check if a pool is already defined in the cluster and if it contains data. This means that defining performance tiers in an existing environment can be dangerous because the change of the default CRUSH rule that is associated with a pool results in data movement. If you require assistance or recommendations for adding or removing nodes, contact Red Hat support.

Director does not expose specific parameters to cover this feature, however, you can generate the ceph-ansible expected variables by completing the following steps.

Procedure

  1. Log in to the undercloud node as the stack user.
  2. Create an environment file, such as /home/stack/templates/ceph-config.yaml, to contain the Ceph config parameters and the device classes variables. Alternatively, you can add the following configurations to an existing environment file.

  3. In the environment file, use the CephAnsibleDisksConfig parameter to list the block devices that you want to use as Ceph OSDs: 

    CephAnsibleDisksConfig:
    devices:
      - /dev/sdb
      - /dev/sdc
      - /dev/sdd
    osd_scenario: lvm
    osd_objectstore: bluestore

  4. Optional: Ceph automatically detects the type of disk and assigns it to the corresponding device class. However, you can also use the crush_device_class property to force a specific device to belong to a specific class or create your own custom classes. The following example contains the same list of OSDs with specified classes: 

    CephAnsibleDisksConfig:
    lvm_volumes:
      - data: '/dev/sdb'
        crush_device_class: 'hdd'
      - data: '/dev/sdc'
        crush_device_class: 'hdd'
      - data: '/dev/sdd'
        crush_device_class: 'ssd'
    osd_scenario: lvm
    osd_objectstore: bluestore

  5. Add the CephAnsibleExtraVars parameters. The crush_rules parameter must contain a rule for each class that you define or that Ceph detects automatically. When you create a new pool, if no rule is specified, the rule that you want Ceph to use as the default is selected. 

    CephAnsibleExtraConfig:
    crush_rule_config: true
    create_crush_tree: true
    crush_rules:
      - name: HDD
        root: default
        type: host
        class: hdd
        default: true
      - name: SSD
        root: default
        type: host
        class: ssd
        default: false

  6. Add the CephPools parameter:

    • Use the rule_name parameter to specify the tier for each pool that does not use the default rule. In the following example, the fastpool pool uses the SSD device class that is configured as a fast tier, to manage Block Storage volumes.

    • Replace <appropriate_PG_num> with the appropriate number of placement groups (PGs). Alternatively, use the placement group auto-scaler to calculate the number of PGs for the Ceph pools.

      For more information, see Assigning custom attributes to different Ceph pools.

    • Use the CinderRbdExtraPools parameter to configure fastpool as a Block Storage back end. 

      CephPools:
  - name: fastpool
    pg_num: <appropraiate_PG_num>
    rule_name: SSD
    application: rbd
CinderRbdExtraPools: fastpool

  7. Use the following example to ensure that your environment file contains the correct values: 

    parameter_defaults:
    CephAnsibleDisksConfig:
        devices:
            - '/dev/sdb'
            - '/dev/sdc'
            - '/dev/sdd'
        osd_scenario: lvm
        osd_objectstore: bluestore
    CephAnsibleExtraConfig:
        crush_rule_config: true
        create_crush_tree: true
        crush_rules:
            - name: HDD
              root: default
              type: host
              class: hdd
              default: true
            - name: SSD
              root: default
              type: host
              class: ssd
              default: false
    CinderRbdExtraPools: fastpool
    CephPools:
        - name: fastpool
          pg_num: <appropriate_PG_num>
          rule_name: SSD
          application: rbd

  8. Include the new environment file in the openstack overcloud deploy command. 

    $ openstack overcloud deploy \
--templates \
…
-e <other_overcloud_environment_files> \
-e /home/stack/templates/ceph-config.yaml  \
…

    Replace <other_overcloud_environment_files> with the list of environment files that are part of your deployment.

重要

If you apply the environment file to an existing Ceph cluster, the pre-existing Ceph pools are not updated with the new rules. For this reason, you must enter the following command after the deployment completes to set the rules to the specified pools. 

$ ceph osd pool set <pool> crush_rule <rule>
  • Replace <pool> with the name of the pool that you want to apply the new rule to.
  • Replace <rule> with one of the rule names that you specified with the crush_rules parameter.
  • Replace <appropriate_PG_num> with the appropriate number of placement groups or a target_size_ratio and set pg_autoscale_mode to true.

For every rule that you change with this command, update the existing entry or add a new entry in the CephPools parameter in your existing templates: 

CephPools:
    - name: <pool>
      pg_num: <appropriate_PG_num>
      rule_name: <rule>
      application: rbd