Allow operators to change hw_disk_discard in Nova

Some base image distros don't ship new enough versions of libvirt/qemu
to support hw_disk_discard in Nova (libvirt (1, 0, 6) or later and qemu
(1, 6, 0) or later).

When deploying Nova with Ceph enabled this config item is automatically
turned on, we should give operators a chance to disable it.

Change-Id: I0b2615c64a9569c09567bf456815f93254d90ade
Closes-Bug: #1632348

ansible/roles/nova/defaults/main.yml

@@ -13,6 +13,10 @@ nova_pool_name: "{{ ceph_nova_pool_name }}"
 nova_pool_type: "{{ ceph_nova_pool_type }}"
 nova_cache_mode: "{{ ceph_nova_cache_mode }}"
 
+# Discard option for nova managed disks. Requires libvirt (1, 0, 6) or later and
+# qemu (1, 6, 0) or later. Set to "" to disable.
+nova_hw_disk_discard: "unmap"
+
 
 ####################
 # Database

ansible/roles/nova/templates/nova.conf.j2

@@ -175,7 +175,9 @@ images_rbd_pool = {{ ceph_nova_pool_name }}
 images_rbd_ceph_conf = /etc/ceph/ceph.conf
 rbd_user = nova
 disk_cachemodes="network=writeback"
-hw_disk_discard = unmap
+{% if nova_hw_disk_discard != '' %}
+hw_disk_discard = {{ nova_hw_disk_discard }}
+{% endif %}
 {% endif %}
 {% if nova_backend == "rbd" %}
 rbd_secret_uuid = {{ rbd_secret_uuid }}