How do I get the root device through Ansible?

Depending on what system I run my Ansible against, the root device might be different. On some systems, it's /dev/sda. On others (Raspberry PIs), it's /dev/mmcblk0. I want Ansible to take a look at what device / is mounted on, and figure it out.

If I look at the facts gathered by Ansible's setup module, I see that ansible_mounts is a list, and one of the list items contains mount: /.

Unfortunately:

1. I'm not sure how to tell Ansible that I'm looking for the list item that contains mount: /
2. On a Raspberry Pi, that list item shows device: /dev/root
   • Even running df on the system shows /dev/root mounted to /
   • /dev/root doesn't even exist. Apparently it's something left behind by initrd during the boot process

Is there a reliable way to determine exactly what device / is using?

(updated in Ansible 2.16)

• Use the filter json_query

root_dev: "{{ ansible_mounts | json_query('[?mount == `/`].device') }}"

gives for example, on Ubuntu

root_dev:
  - /dev/sda3

This is correct

shell> df -h
Filesystem      Size  Used Avail Use% Mounted on
  ...
/dev/sda3       687G   26G  626G   4% /
  ...

• Optionally, the expression below gives the same result

root_dev: "{{ ansible_mounts | selectattr('mount', '==', '/')|
                               map(attribute='device') }}"

The same expressions give correct results also on FreeBSD

root_dev:
  - /dev/ada0s1a

• CentOS VM

root_dev:
  - /dev/mapper/cl-root

• and Raspberry Pi

shell> ssh admin@rp2 uname -a
FreeBSD rp2.example.org 13.2-RELEASE FreeBSD 13.2-RELEASE releng/13.2-n254617-525ecfdad597 RPI-B arm

shell> ssh admin@rp2 df
Filesystem             1K-blocks    Used   Avail Capacity  Mounted on
/dev/ufs/rootfs          7448796 2673624 4179272    39%    /
devfs                          1       1       0   100%    /dev
/dev/msdosfs/MSDOSBOOT     51140   14132   37008    28%    /boot/msdos
tmpfs                     360036       8  360028     0%    /tmp

  root_dev:
  - /dev/ufs/rootfs

- hosts: all
  
  vars:

    root_dev1: "{{ ansible_mounts | json_query('[?mount == `/`].device') }}"
    root_dev2: "{{ ansible_mounts | selectattr('mount', '==', '/') |
                                    map(attribute='device') }}"

  tasks:

    - setup:
        gather_subset: mounts

    - debug:
        var: root_dev1
    - debug:
        var: root_dev2