openstack-ansible/doc/source/install-guide/configure-nova.rst

Home OpenStack-Ansible Installation Guide

Configuring the Compute (Nova) Service (optional)

The compute service (nova) 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 nova role.

Availability zones

Deployers with multiple availability zones (AZ's) can set the nova_default_schedule_zone Ansible variable to specify an AZ to use for instance build requests where an AZ is not provided. This could be useful in environments with different types of hypervisors where builds are sent to certain hardware types based on their resource requirements.

For example, if a deployer has some servers with spinning hard disks and others with SSDs, they can set the default AZ to one that uses only spinning disks (to save costs). To build an instance using SSDs, users must select an AZ that includes SSDs and provide that AZ in their instance build request.

Block device tuning for Ceph (RBD)

When Ceph is enabled and nova_libvirt_images_rbd_pool is defined, two libvirt configurations will be changed by default:

• hw_disk_discard: unmap
• disk_cachemodes: network=writeback

Setting hw_disk_discard to unmap in libvirt will enable discard (sometimes called TRIM) support for the underlying block device. This allows for unused blocks to be reclaimed 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.

Deployers 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'

Deployers 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 Ceph documentation for OpenStack has additional information about these settings.

Config Drive

By default, OpenStack-Ansible will not configure Nova to force config drives to be provisioned with every instance that Nova builds. The metadata service provides configuration information that can be used by cloud-init inside the instance. Config drives are only necessary when an instance doesn't have cloud-init installed or doesn't 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 a deployer needs forced config drives and the ability to migrate instances, the config drive format should be set 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

Deployers can customize these settings using the Ansible variables shown below:

# 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. Deployers can enable multipath support in nova through a configuration override:

nova_nova_conf_overrides:
  libvirt:
      iscsi_use_multipath: true

Shared storage and synchronized UID/GID

Deployers can 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 will create the nova user and group without specifying the UID or GID. To specify custom values for the UID/GID, set the following Ansible variables:

nova_system_user_uid = <specify a UID>
nova_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.

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