commit 869185ea7be5d6b5b21c964a620839d5475196fd
Author: Mark Goddard <mark@stackhpc.com>
Date:   Thu Apr 23 12:03:45 2020 +0100

    Switch default docker storage driver to overlay2
    
    To avoid switching existing deployments from devicemapper to overlay2,
    we check the existing storage driver configuration directly with the
    Docker daemon, or if unreachable by reading the /etc/docker/daemon.json
    configuration file.
    
    Co-Authored-By: Pierre Riteau <pierre@stackhpc.com>
    Story: 2005667
    Task: 30972
    
    Change-Id: Iaf2ee8c9f302f4684ae039bb00b2e2e5969cf1fc

diff --git a/ansible/docker-devicemapper.yml b/ansible/docker-devicemapper.yml
index e61c976..ce0e012 100644
--- a/ansible/docker-devicemapper.yml
+++ b/ansible/docker-devicemapper.yml
@@ -1,11 +1,44 @@
 ---
-- name: Ensure docker devicemapper storage is configured
+- name: Verify and configure Docker storage driver
   hosts: docker
   tags:
     - docker
     - docker-devicemapper
   tasks:
-    - name: Ensure docker devicemapper storage is configured
+    - name: Check for existing Docker configuration using devicemapper
+      block:
+      - name: Query Docker daemon for storage driver
+        docker_host_info:
+        failed_when: false
+        register: docker
+
+      - block:
+        - name: Check for a Docker configuration file
+          become: True
+          stat:
+            path: /etc/docker/daemon.json
+          register: docker_config_file
+
+        - name: Check whether devicemapper is used in configuration file
+          lineinfile:
+            path: /etc/docker/daemon.json
+            regexp: 'storage-driver.*devicemapper'
+            state: absent
+          become: True
+          changed_when: False
+          # `check_mode: True` ensures that we don't modify daemon.json
+          check_mode: True
+          register: devicemapper_docker
+          when: docker_config_file.stat.exists
+        when: not docker.can_talk_to_docker
+
+      - name: Fail if devicemapper is in use while another storage driver was requested
+        fail:
+          msg: "Docker storage driver {{ docker_storage_driver }} was requested, but devicemapper is in use"
+        when: (docker.can_talk_to_docker and docker.host_info.Driver == 'devicemapper') or (devicemapper_docker.found | default(0) == 1)
+      when: docker_storage_driver != 'devicemapper'
+
+    - name: Ensure Docker devicemapper storage is configured
       include_role:
         name: docker-devicemapper
       when: docker_storage_driver == 'devicemapper'
diff --git a/ansible/group_vars/all/docker b/ansible/group_vars/all/docker
index d9ab508..861ddcc 100644
--- a/ansible/group_vars/all/docker
+++ b/ansible/group_vars/all/docker
@@ -2,8 +2,8 @@
 ###############################################################################
 # Docker configuration.
 
-# Name of the docker storage driver. Default is 'devicemapper'.
-docker_storage_driver: devicemapper
+# Name of the docker storage driver. Default is 'overlay2'.
+docker_storage_driver: overlay2
 
 # Name of the docker storage LVM volume group.
 docker_storage_volume_group: data
diff --git a/ansible/roles/docker-devicemapper/defaults/main.yml b/ansible/roles/docker-devicemapper/defaults/main.yml
index b9f813b..7c210ae 100644
--- a/ansible/roles/docker-devicemapper/defaults/main.yml
+++ b/ansible/roles/docker-devicemapper/defaults/main.yml
@@ -1,6 +1,6 @@
 ---
 # Name of the docker storage driver.
-docker_storage_driver: devicemapper
+docker_storage_driver: overlay2
 
 # Name of the docker storage LVM volume group.
 docker_storage_volume_group:
diff --git a/ansible/roles/kolla-ansible/defaults/main.yml b/ansible/roles/kolla-ansible/defaults/main.yml
index 7cf6f57..9dcc683 100644
--- a/ansible/roles/kolla-ansible/defaults/main.yml
+++ b/ansible/roles/kolla-ansible/defaults/main.yml
@@ -262,7 +262,7 @@ kolla_enable_host_ntp:
 # Docker configuration.
 
 # Name of the docker storage driver.
-docker_storage_driver: devicemapper
+docker_storage_driver: overlay2
 
 # Name of the docker storage LVM volume group.
 docker_storage_volume_group:
diff --git a/doc/source/configuration/reference/hosts.rst b/doc/source/configuration/reference/hosts.rst
index 5722df2..1bcf98c 100644
--- a/doc/source/configuration/reference/hosts.rst
+++ b/doc/source/configuration/reference/hosts.rst
@@ -690,8 +690,8 @@ Docker engine configuration is applied by both Kayobe and Kolla Ansible (during
 bootstrap-servers).
 
 The ``docker_storage_driver`` variable sets the Docker storage driver, and by
-default the ``devicemapper`` driver is used. If using this driver, see
-:ref:`configuration-hosts-lvm` for information about configuring LVM for
+default the ``overlay2`` driver is used. If using the ``devicemapper`` driver,
+see :ref:`configuration-hosts-lvm` for information about configuring LVM for
 Docker.
 
 Various options are defined in ``${KAYOBE_CONFIG_PATH}/docker.yml``
diff --git a/etc/kayobe/docker.yml b/etc/kayobe/docker.yml
index 5d30a5d..4ec00a1 100644
--- a/etc/kayobe/docker.yml
+++ b/etc/kayobe/docker.yml
@@ -2,7 +2,7 @@
 ###############################################################################
 # Docker configuration.
 
-# Name of the docker storage driver. Default is 'devicemapper'.
+# Name of the docker storage driver. Default is 'overlay2'.
 #docker_storage_driver:
 
 # Name of the docker storage LVM volume group.
diff --git a/releasenotes/notes/default-overlay2-970bdb5b7b62b60a.yaml b/releasenotes/notes/default-overlay2-970bdb5b7b62b60a.yaml
new file mode 100644
index 0000000..1c50e7f
--- /dev/null
+++ b/releasenotes/notes/default-overlay2-970bdb5b7b62b60a.yaml
@@ -0,0 +1,7 @@
+---
+upgrade:
+  - |
+    The default Docker storage driver has been changed from ``devicemapper`` to
+    ``overlay2``, which is the storage driver preferred by Docker. Environments
+    using ``devicemapper`` should set ``docker_storage_driver`` to
+    ``devicemapper`` in ``${KAYOBE_CONFIG_PATH}/docker.yml``.