Advanced scheduling involves configuring a pod so that the pod is required to run on particular nodes or has a preference to run on particular nodes.
Generally, advanced scheduling is not necessary, as the OKD automatically places pods in a reasonable manner. For example, the default scheduler attempts to distribute pods across the nodes evenly and considers the available resources in a node. However, you might want more control over where a pod is placed.
If a pod needs to be on a machine with a faster disk speed (or prevented from being placed on that machine) or pods from two different services need to be located so they can communicate, you can use advanced scheduling to make that happen.
To ensure that appropriate new pods are scheduled on a dedicated group of nodes and prevent other new pods from being scheduled on those nodes, you can combine these methods as needed.
There are several ways to invoke advanced scheduling in your cluster:
- Pod Affinity and Anti-affinity
Pod affinity allows a pod to specify an affinity (or anti-affinity) towards a group of pods (for an application’s latency requirements, due to security, and so forth) it can be placed with. The node does not have control over the placement.
Pod affinity uses labels on nodes and label selectors on pods to create rules for pod placement. Rules can be mandatory (required) or best-effort (preferred).
- Node Affinity
Node affinity allows a pod to specify an affinity (or anti-affinity) towards a group of nodes (due to their special hardware, location, requirements for high availability, and so forth) it can be placed on. The node does not have control over the placement.
Node affinity uses labels on nodes and label selectors on pods to create rules for pod placement. Rules can be mandatory (required) or best-effort (preferred).
See Using Node Affinity.
- Node Selectors
Node selectors are the simplest form of advanced scheduling. Like node affinity, node selectors also use labels on nodes and label selectors on pods to allow a pod to control the nodes on which it can be placed. However, node selectors do not have required and preferred rules that node affinities have.
See Using Node Selectors.
- Taints and Tolerations
Taints/Tolerations allow the node to control which pods should (or should not) be scheduled on them. Taints are labels on a node and tolerations are labels on a pod. The labels on the pod must match (or tolerate) the label (taint) on the node in order to be scheduled.
Taints/tolerations have one advantage over affinities. For example, if you add to a cluster a new group of nodes with different labels, you would need to update affinities on each of the pods you want to access the node and on any other pods you do not want to use the new nodes. With taints/tolerations, you would only need to update those pods that are required to land on those new nodes, because other pods would be repelled.