Implement base container for LVM-backed CoW containers

This patch implements a base container which may be used by a
deployer to deploy copy-on-write container backing stores backed
by LVM.

This process may be used to speed up the container creation
process without compromising security through the user of
backing stores like overlayfs.

Change-Id: I0bf227891a85bd7c8db53ca73fc5380b95e514fa

defaults/main.yml

@@ -31,6 +31,15 @@ lxc_container_cache_path: "/var/cache/lxc/download"
 # which is when overlayfs was merged into the mainline kernel
 # lxc_container_backing_store: overlayfs
 
+# The container backing method can be set to 'copy-on-write' to use LVM
+# snapshot-backed containers when the container backing store is set to
+# 'lvm'.
+# lxc_container_backing_method: copy-on-write
+
+# When using a base container to snapshot from for the overlayfs or LVM
+# copy-on-write backing stored, the base container can be set.
+lxc_container_base_name: "{{ lxc_cache_map.distro }}-{{ lxc_cache_map.release }}-{{ lxc_cache_map.arch }}"
+
 # lxc container net network
 lxc_net_bridge: lxcbr0
 lxc_net_bridge_port: none

releasenotes/notes/base-container-lvm-cow-2faa824f6cd4b083.yaml

@@ -0,0 +1,14 @@
+---
+features:
+  - The container cache preparation process now allows ``copy-on-write`` to be
+    set as the ``lxc_container_backing_method`` when the
+    ``lxc_container_backing_store`` is set to ``lvm``. When this is set a base
+    container will be created using a name of the form
+    `<linux-distribution>`-`distribution-release>`-`<host-cpu-architecture>`.
+    The container will be stopped as it is not used for anything except to be
+    a backing store for all other containers which will be based on a snapshot
+    of the base container.
+  - When using copy-on-write backing stores for containers, the base container
+    name may be set using the variable ``lxc_container_base_name`` which
+    defaults to
+    `<linux-distribution>`-`distribution-release>`-`<host-cpu-architecture>`.

tasks/lxc_cache_create.yml

@@ -36,7 +36,7 @@
 
 - name: Create base container to use for overlayfs containers
   lxc_container:
-    name: "{{ lxc_cache_map.distro }}-{{ lxc_cache_map.release }}-{{ lxc_cache_map.arch }}"
+    name: "{{ lxc_container_base_name }}"
     template: "download"
     state: stopped
     backing_store: "dir"
@@ -48,3 +48,19 @@
   when:
     - lxc_container_backing_store is defined
     - lxc_container_backing_store == 'overlayfs'
+
+- name: Create base container to use for LVM-backed copy-on-write containers
+  lxc_container:
+    name: "{{ lxc_container_base_name }}"
+    template: "download"
+    state: stopped
+    backing_store: "lvm"
+    template_options: "{{ lxc_cache_download_template_options }}"
+  register: cache_download
+  retries: 3
+  delay: 10
+  until: cache_download|success
+  when:
+    - lxc_container_backing_store is defined
+    - lxc_container_backing_store == 'lvm'
+    - lxc_container_backing_method == 'copy-on-write'