Create symlink for libvirt save directory

Nova can fill up the partition that /var/lib/libvirt/qemu/save is on
when nova calls `image-create` on an instance.  This occurs because
nova calls libvirt with managedsave which creates a temporary file in
the /var/lib/libvirt/qemu/save location.  Libvirt does not allow the
location of /var/lib/libvirt/qemu/save to be configurable so we have
to manage it ourselves via bindmounts or symlinks.

In this commit we remove the /var/lib/libvirt/qemu/save directory and
create a symlink in it's place, pointing to
`{{ nova_system_home_folder }}/save` instead (which we also create).
This location was chosen as deployers often have more space here for
nova instances.

Change-Id: I5c96f70aa355cc02379fe7e0681416eca53447eb
Partial-Bug: #1585325

defaults/main.yml

@@ -52,6 +52,7 @@ nova_system_group_name: nova
 nova_system_shell: /bin/bash
 nova_system_comment: nova system user
 nova_system_home_folder: "/var/lib/{{ nova_system_user_name }}"
+nova_libvirt_save_path: "{{ nova_system_home_folder }}/save"
 
 ## Manually specified nova UID/GID
 # Deployers can specify a UID for the nova user as well as the GID for the
@@ -397,6 +398,8 @@ nova_pip_packages:
   - nova
   - python-cephlibs
 
+nova_qemu_user: libvirt-qemu
+nova_qemu_group: kvm
 
 ## Tunable overrides
 nova_nova_conf_overrides: {}

releasenotes/notes/symlink_libvirt_save_dir_to_nova_dir-3b1b278cb7e5831f.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - The ``/var/lib/libvirt/qemu/save`` directory is now a
+    symlink to ``{{ nova_system_home_folder }}/save`` to
+    resolve an issue where the default location used by the
+    libvirt managed save command can result with the root
+    partitions on compute nodes becoming full when
+    ``nova image-create`` is run on large instances.

tasks/nova_compute_kvm_install.yml

@@ -45,6 +45,30 @@
     - nova-apt-packages
     - nova-compute-kvm-apt-packages
 
+- name: Check if qemu save directory is a directory
+  stat:
+    path: "/var/lib/libvirt/qemu/save"
+  register: qemu_savedir_stat
+
+- name: Check if qemu save directory is empty
+  shell:
+    cmd: 'ls -1A /var/lib/libvirt/qemu/save'
+  register: qemu_savedir_empty
+
+# We force the link creation if our tests passed, this removes the directory as
+# well
+- name: Symlink qemu save dir to nova_libvirt_save_path
+  file:
+    path: "/var/lib/libvirt/qemu/save"
+    state: link
+    owner: "{{ nova_qemu_user }}"
+    group: "{{ nova_qemu_group }}"
+    src: "{{ nova_libvirt_save_path }}"
+    force: True
+  when:
+    - qemu_savedir_stat.stat.isdir
+    - qemu_savedir_empty.stdout_lines | length == 0
+
 - name: Install pip packages (venv)
   pip:
     name: "{{ item }}"

tasks/nova_pre_install.yml

@@ -68,6 +68,7 @@
     - { path: "{{ nova_system_home_folder }}/.ssh", mode: "0700" }
     - { path: "{{ nova_system_home_folder }}/cache/api" }
     - { path: "{{ nova_system_home_folder }}/instances" }
+    - { path: "{{ nova_libvirt_save_path }}", mode: "0750" }
     - { path: "/var/lock/nova" }
     - { path: "/var/run/nova" }
   tags: