Home OpenStack-Ansible Installation Guide
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.
When Ceph is enabled and nova_libvirt_images_rbd_pool is defined, two libvirt configurations will be changed by default:
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.