OCP Pod scheduling - Explanation of Toleration , Taints , nodeSelector , Affinity - Pod status reports: didn't match Pod's node affinity/selector

Issue

The purpose of this document is to clarify and summarize in a few lines the concept and purpose of :

• tolerations
• taints
• nodeSelector
• affinity

If these are incorrectly configured, we may end up having pods in Pending status, failing to be started with errors like

status:
  conditions:
  - lastProbeTime: null
    lastTransitionTime: '2022-10-05T12:01:38Z'
    message: '0/29 nodes are available: 23 node(s) didn''t match Pod''s node affinity/selector,
      3 node(s) had taint {node-role.kubernetes.io/infra: }, that the pod didn''t
      tolerate, 3 node(s) had taint {node-role.kubernetes.io/master: }, that the pod
      didn''t tolerate.'
    reason: Unschedulable
    status: 'False'
    type: PodScheduled
  phase: Pending