Ansible role to set 'noop' i/o scheduler (CentOS 7)

gistfile1.txt

---
- name: test if grub configured for noop i/o scheduler
  command: egrep -q 'elevator=noop' /boot/grub2/grub.cfg
  register: grub
  changed_when: no
  failed_when: grub_test.rc == 2

- name: configure grub for noop i/o scheduler
  sudo: yes
  command: grubby --update-kernel=ALL --args=elevator=noop
  when: grub_test.rc == 1

- name: get list of block devices to test
  shell: ls /sys/block/ | egrep '^([shv]|xv])d[a-z]$'
  register: block_devs
  changed_when: no

- name: test if block device using noop i/o scheduler
  shell: cat /sys/block/{{ item }}/queue/scheduler | egrep -q '\[noop\]'
  with_items: block_devs.stdout
  register: noop_test
  changed_when: noop_test.rc == 1
  failed_when: noop_test.rc == 2

- name: set block device to use noop i/o scheduler
  sudo: yes
  shell: echo noop > /sys/block/{{ item }}/queue/scheduler
  with_items: block_devs.stdout
  when: noop_test.changed

Why so complicated to set the current scheduler via /sys/block/*/queue/scheduler when this can also be done in a single Ansible task? At least with enabled facts gathering which already does most of the work.

I got these variants of that task and they even cope with newer releases requiring none instead of noop because it should work with more than just one release of one distro and also with NVMe SSDs:

Variant 1 (shortest, but hackish):

- name: Set noop or none as scheduler (shell hack), whichever works
  shell: "printf 'noop\nnone' > /sys/block/{{ item }}/queue/scheduler 2> /dev/null || true"
  when: "ansible_devices[item].scheduler_mode not in ['', 'none', 'noop']"
  with_items: "{{ ansible_devices }}"

Variant 2 (longer, but doesn't require suppressing expected errors):

- name: Set noop or none as scheduler (shell safe), whichever works
  shell: "( fgrep -q noop /sys/block/{{ item }}/queue/scheduler && printf 'noop' || printf 'none') > /sys/block/{{ item }}/queue/scheduler"
  when: "ansible_devices[item].scheduler_mode not in ['', 'none', 'noop']"
  with_items: "{{ ansible_devices }}"

The only downside (of these two variants) I'm currently aware of is that they say "skipped" instead of "ok" if it is actually already ok.

So I wrote variant 3, based on variant 1:

- name: Set noop or none as scheduler, whichever works
  shell: "printf 'noop\nnone' > /sys/block/{{ item }}/queue/scheduler 2> /dev/null || true"
  when: "ansible_devices[item].scheduler_mode != ''"
  changed_when: "ansible_devices[item].scheduler_mode not in ['none', 'noop']"
  with_items: "{{ ansible_devices }}"

This correctly says ok if it is ok, but it though executes the command. So let's do a variant 4 which also defines when the command actually failed (having write error: Invalid argument' in STDERR is actually a sign of success in this case):

- name: Set noop or none as scheduler, whichever works
  shell: "grep -qP '\\[no(op|ne)\\]' /sys/block/{{ item  }}/queue/scheduler || printf 'noop\nnone' > /sys/block/{{ item }}/queue/scheduler"
  register: executed_command
  when: "ansible_devices[item].scheduler_mode != ''"
  changed_when: executed_command.rc == 1
  failed_when:
    - executed_command.rc != 0
    - executed_command.rc != 1
    - "'write error: Invalid argument' not in executed_command.stderr"
  with_items: "{{ ansible_devices }}"

Since I wasn't happy with that error prone looking failed_when statement, my final variant is now this one:

- name: Set noop or none as scheduler, whichever works
  shell: "if ! grep -qP '\\[no(op|ne)\\]' /sys/block/{{ item }}/queue/scheduler; then ( fgrep -q noop /sys/block/{{ item }}/queue/scheduler && printf noop || printf none) > /sys/block/{{ item }}/queue/scheduler; else cat /sys/block/{{ item }}/queue/scheduler; fi"
  register: executed_command
  changed_when: "'[no' not in executed_command.stdout"
  failed_when: "executed_command.rc != 0 or executed_command.stderr != ''"
  when: "ansible_devices[item].scheduler_mode != ''"
  with_items: "{{ ansible_devices }}"

In you case, this simple variant would likely suffice:

- name: Set noop as scheduler
  shell: "printf 'noop' > /sys/block/{{ item }}/queue/scheduler"
  when: "ansible_devices[item].scheduler_mode not in ['', 'noop']"
  with_items: "{{ ansible_devices }}"