Disk by id

We’ve been using an (openstack based) cloud provider, that can’t guarantee a stable device name for an attached volume.

This was causing problems when used in /etc/fstab; on reboot, if the device name was incorrect, the instance would hang.

It’s pretty straight forward to use the UUID instead, with ansible:

- name: Mount vol
  become: yes
    path: "{{ mount_point }}"
    src: "UUID={{ ansible_devices[device_name].partitions[device_name + '1'].uuid }}"
    fstype: ext4
    state: mounted

but we still needed the device_name in group vars. Our provider explained that a stable id was provided, in /dev/disk/by-id, which could be used directly for most tasks:

- name: Create a new primary partition
    device: "/dev/disk/by-id/{{ device_id }}"
    number: 1
    state: present
  become: yes

- name: Create ext4 filesystem on vol
  become: yes
    fstype: ext4
    dev: "/dev/disk/by-id/{{ device_id }}-part1"

But how do you get from the id, to the device name?

$ ls /dev/disk/by-id/
virtio-c11c38e5-7021-48d2-a  virtio-c11c38e5-7021-48d2-a-part1
"ansible_devices": {
        "vda": {
        "vdb": {
        "vdc": {
            "links": {
                "ids": [

This seemed like a job for json_query but, after a fruitless hour or two, I gave up and used this (slightly hacky) solution suggested on SO:

- name: Get device name
    device_name: "{{ item.key }}"
  with_dict: "{{ ansible_devices }}"
  when: "(item.value.links.ids[0] | default()) == device_id"
  no_log: yes

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s