Set default qemu settings for RBD

When Nova and/or Cinder are using Ceph as backend, qemu will need
to open a connection and two threads for each and every Ceph OSD.
Since all connections occur in the same qemu process, this may result
in hitting default max open files limit.
Thus in case of more then 10 volumes are attached
to the same instance and are used actively can end up in blocked
IO operations inside VMs.

We increase these limits by default when RBD backend is used.

Change-Id: Ib3081280cdbae1eb2235083c95c27e2efd0b413e

defaults/main.yml

@@ -491,7 +491,7 @@ nova_qemu_group: kvm
 
 # Define the following variable to drop a replacement
 # file for /etc/libvirt/qemu.conf
-#qemu_conf_dict: {}
+qemu_conf_dict: {}
 
 ## Tunable overrides
 nova_nova_conf_overrides: {}

tasks/drivers/kvm/nova_compute_kvm.yml

@@ -100,7 +100,7 @@
     group: "root"
     mode: "0600"
   when:
-    - qemu_conf_dict is defined
+    - _nova_qemu_conf | length > 0
       or nova_network_type == 'calico'
   notify: Restart libvirt-bin
   tags:

templates/qemu.conf.j2

@@ -15,8 +15,7 @@ cgroup_device_acl = [
 ]
 {% endif %}
 
-{% if qemu_conf_dict is defined %}
-{% for key, value in qemu_conf_dict.items() %}
+
+{% for key, value in _nova_qemu_conf.items() %}
 {{ key }} = {{ value }}
 {% endfor %}
-{% endif %}

vars/main.yml

@@ -104,3 +104,13 @@ _nova_scheduler_filters: |-
   {%   set filters = default_filters %}
   {% endif %}
   {{ filters + nova_scheduler_extra_filters }}
+
+_qemu_default_conf_dict: |-
+  {% set qemu_dict = {} %}
+  {% if nova_rbd_inuse or nova_cinder_rbd_inuse %}
+  {%   set _ = qemu_dict.update({'max_files': '32768'}) %}
+  {%   set _ = qemu_dict.update({'max_processes': '131072'}) %}
+  {% endif %}
+  {{ qemu_dict }}
+
+_nova_qemu_conf: "{{ _qemu_default_conf_dict | combine(qemu_conf_dict) }}"