Configuring the Compute (nova) service (optional)¶

Availability zones¶

Deployers with multiple availability zones can set the nova_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 nova_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):

nova_libvirt_hw_disk_discard: 'unmap'
nova_libvirt_disk_cachemodes: 'network=writeback'

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

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

nova_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>

• The Ceph documentation for OpenStack has additional information about these settings.
• OpenStack-Ansible and Ceph Working Example

Config drive¶

By default, OpenStack-Ansible does not configure nova to force config drives to be provisioned with every instance that nova 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:

nova_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 nova_nova_conf_overrides variable:

nova_nova_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
nova_libvirtd_listen_tls: 1

# Disable libvirtd's plaintext TCP listener
nova_libvirtd_listen_tcp: 0

# Use SASL for authentication
nova_libvirtd_auth_tcp: sasl

Multipath¶

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

nova_nova_conf_overrides:
  libvirt:
      iscsi_use_multipath: true

Shared storage and synchronized UID/GID¶

Specify a custom UID for the nova user and GID for the nova 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 nova user and group without specifying the UID or GID. To specify custom values for the UID or GID, set the following Ansible variables:

nova_system_user_uid = <specify a UID>
nova_system_group_gid = <specify a GID>