Visually Identify Raspberry Pi In A Cluster

Visually Identify Raspberry Pi

Raspberry Pi are great! Low power, do not use much space, and so on.

However, I run a cluster of about nine nodes, which makes finding a specific node almost as challenging as finding a random server in a datacenter.

Good news is, you can make one of the onboard LEDs flash to give you an idea of which node it is.

Make the node blink

The following instructions assume you have both remote access (SSH) to your Raspberry Pi, as well as sudo or root permissions.

1. Verify the current kernel module for LED0:

    $ cat /sys/class/leds/led0/trigger
    none rc-feedback kbd-scrolllock kbd-numlock kbd-capslock
    kbd-kanalock kbd-shiftlock  kbd-altgrlock kbd-ctrllock
    kbd-altlock kbd-shiftllock kbd-shiftrlock kbd-ctrlllock
    kbd-ctrlrlock timer oneshot heartbeat backlight gpio cpu
    cpu0 cpu1 cpu2 cpu3 default-on input panic actpwr [mmc0]
    rfkill-any rfkill-none
   

2. Enable the ledtrig_heartbeat kernel module:

    $ sudo su -
    # modprobe ledtrig_heartbeat
   

3. Set LED0 to blinking:

    # echo "heartbeat" > /sys/class/leds/led0/trigger
    # cat /sys/class/leds/led0/trigger
    none rc-feedback kbd-scrolllock kbd-numlock kbd-capslock
    kbd-kanalock kbd-shiftlock  kbd-altgrlock kbd-ctrllock
    kbd-altlock kbd-shiftllock kbd-shiftrlock kbd-ctrlllock
    kbd-ctrlrlock timer oneshot [heartbeat] backlight gpio cpu
    cpu0 cpu1 cpu2 cpu3 default-on input panic actpwr mmc0
    rfkill-any rfkill-none
   

4. Reset back to SD card access:

    # echo "mmc0" > /sys/class/leds/led0/trigger
    # cat /sys/class/leds/led0/trigger
    none rc-feedback kbd-scrolllock kbd-numlock kbd-capslock
    kbd-kanalock kbd-shiftlock  kbd-altgrlock kbd-ctrllock
    kbd-altlock kbd-shiftllock kbd-shiftrlock kbd-ctrlllock
    kbd-ctrlrlock timer oneshot heartbeat backlight gpio cpu
    cpu0 cpu1 cpu2 cpu3 default-on input panic actpwr [mmc0]
    rfkill-any rfkill-none
   

Explanation

The status LED (the flashy green one, or LED0) on a Raspberry Pi will flash on SD card access. To identify a specific node, we take advantage of the fact that LED0 is programmable, and set it to heartbeat, which makes it flash on and off in a steady pattern.

You can also configure the red power LED to heartbeat as well by changing led0 to led1 in the preceding commands.

Bonus - Enable and disable blinking with Ansible

The commands discussed before work for a single node, but, honestly, they can be hard to remember when you are in a pinch. Have no fear, the following is an Ansible playbook that will set them to blinking for you:

---
# Sets led0 on an rPI to heartbeat
# To allow for identification of a node in a cluster of rPI.

- name: Host Provisioning
  hosts: swarm
  become: true
  gather_facts: false
  become_method: sudo
  vars_files:
    - vars/provision.yaml
  tasks:
    - name: Modprobe led heartbeat
      modprobe:
        name: ledtrig_heartbeat
        state: present
    - name: Set LED0 to blink
      shell: "echo heartbeat > /sys/class/leds/led0/trigger"
      args:
        executable: /bin/bash
      when:
        - identify_me is defined
        - identify_me
    - name: Set LED0 to normal
      shell: "echo mmc0 > /sys/class/leds/led0/trigger"
      args:
        executable: /bin/bash
      when:
        - identify_me is not defined

Then run the playbook as follows:

# Enable heartbeat LED across the cluster
$ ansible-playbook -i swarm.yml -e "identify_me=true" identify-rpi.yml

# Disable it across the cluster
$ ansible-playbook -i swarm.yml identify-rpi.yml

# Enable heartbeat LED for a specific node
$ ansible-playbook -i swarm.yml -e "identify_me=true" --limit swarm-02 identify-rpi.yml

Resources

• https://www.raspberrypi.org/forums/viewtopic.php?t=12530