Configuring the Compute (zun) service (optional)

The Compute service (zun) handles the creation of virtual machines within an OpenStack environment. Many of the default options used by OpenStack-Ansible are found within defaults/main.yml within the zun role.

Availability zones

Deployers with multiple availability zones can set the zun_default_schedule_zone Ansible variable to specify an availability zone for new requests. This is useful in environments with different types of hypervisors, where builds are sent to certain hardware types based on their resource requirements.

For example, if you have servers running on two racks without sharing the PDU. These two racks can be grouped into two availability zones. When one rack loses power, the other one still works. By spreading your containers onto the two racks (availability zones), you will improve your service availability.

Block device tuning for Ceph (RBD)

Enabling Ceph and defining zun_libvirt_images_rbd_pool changes two libvirt configurations by default:

  • hw_disk_discard: unmap

  • disk_cachemodes: network=writeback

Setting hw_disk_discard to unmap in libvirt enables discard (sometimes called TRIM) support for the underlying block device. This allows reclaiming of unused blocks on the underlying disks.

Setting disk_cachemodes to network=writeback allows data to be written into a cache on each change, but those changes are flushed to disk at a regular interval. This can increase write performance on Ceph block devices.

You have the option to customize these settings using two Ansible variables (defaults shown here):

zun_libvirt_hw_disk_discard: 'unmap'
zun_libvirt_disk_cachemodes: 'network=writeback'

You can disable discard by setting zun_libvirt_hw_disk_discard to ignore. The zun_libvirt_disk_cachemodes can be set to an empty string to disable network=writeback.

The following minimal example configuration sets zun to use the ephemeral-vms Ceph pool. The following example uses cephx authentication, and requires an existing cinder account for the ephemeral-vms pool:

zun_libvirt_images_rbd_pool: ephemeral-vms
ceph_mons:
  - 172.29.244.151
  - 172.29.244.152
  - 172.29.244.153

If you have a different Ceph username for the pool, use it as:

cinder_ceph_client: <ceph-username>

Config drive

By default, OpenStack-Ansible does not configure zun to force config drives to be provisioned with every instance that zun builds. The metadata service provides configuration information that is used by cloud-init inside the instance. Config drives are only necessary when an instance does not have cloud-init installed or does not have support for handling metadata.

A deployer can set an Ansible variable to force config drives to be deployed with every virtual machine:

zun_force_config_drive: True

Certain formats of config drives can prevent instances from migrating properly between hypervisors. If you need forced config drives and the ability to migrate instances, set the config drive format to vfat using the zun_zun_conf_overrides variable:

zun_zun_conf_overrides:
  DEFAULT:
    config_drive_format: vfat
    force_config_drive: True

Libvirtd connectivity and authentication

By default, OpenStack-Ansible configures the libvirt daemon in the following way:

  • TLS connections are enabled

  • TCP plaintext connections are disabled

  • Authentication over TCP connections uses SASL

You can customize these settings using the following Ansible variables:

# Enable libvirtd's TLS listener
zun_libvirtd_listen_tls: 1

# Disable libvirtd's plaintext TCP listener
zun_libvirtd_listen_tcp: 0

# Use SASL for authentication
zun_libvirtd_auth_tcp: sasl

Multipath

Nova supports multipath for iSCSI-based storage. Enable multipath support in zun through a configuration override:

zun_zun_conf_overrides:
  libvirt:
      iscsi_use_multipath: true

Shared storage and synchronized UID/GID

Specify a custom UID for the zun user and GID for the zun group to ensure they are identical on each host. This is helpful when using shared storage on Compute nodes because it allows instances to migrate without filesystem ownership failures.

By default, Ansible creates the zun user and group without specifying the UID or GID. To specify custom values for the UID or GID, set the following Ansible variables:

zun_system_user_uid = <specify a UID>
zun_system_group_gid = <specify a GID>

Warning

Setting this value after deploying an environment with OpenStack-Ansible can cause failures, errors, and general instability. These values should only be set once before deploying an OpenStack environment and then never changed.