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readme: RemovePodsViolatingNodeAffinity: reword description of the strategy

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Compared to https://kubernetes.io/docs/concepts/configuration/assign-pod-node/#node-affinity
the strategy kinda implements requiredDuringSchedulingRequiredDuringExecution node affinity type
for kubelets. Only addition to kubelet is the strategy checks whether is at least another node
capable of respecting the node affinity rules.

When requiredDuringSchedulingRequiredDuringExecution node affinity type is implemented in kubelet,
it's likely the strategy either gets removed or re-implemented. Stressing the relation with
requiredDuringSchedulingRequiredDuringExecution will helps consumers of descheduler
to keep in mind the kubelet will eventually take over the strategy when implemented.
This commit is contained in:
Jan Chaloupka
2020-04-15 11:51:17 +02:00
parent e3a562aea0
commit e0c101c5ae
1 changed files with 18 additions and 6 deletions
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24
README.md
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@@ -138,11 +138,23 @@ strategies:
### RemovePodsViolatingNodeAffinity
This strategy makes sure that pods violating node affinity are removed from nodes. For example, there is
podA that was scheduled on nodeA which satisfied the node affinity rule `requiredDuringSchedulingIgnoredDuringExecution`
at the time of scheduling, but over time nodeA no longer satisfies the rule, then if another node nodeB
is available that satisfies the node affinity rule, then podA will be evicted from nodeA. The policy file
should look like:
This strategy makes sure all pods violating
[node affinity](https://kubernetes.io/docs/concepts/configuration/assign-pod-node/#node-affinity)
are eventually removed from nodes. Node affinity rules allow a pod to specify
`requiredDuringSchedulingIgnoredDuringExecution` type, which tells the scheduler
to respect node affinity when scheduling the pod but kubelet to ignore
in case node changes over time and no longer respects the affinity.
When enabled, the strategy serves as a temporary implementation
of `requiredDuringSchedulingRequiredDuringExecution` and evicts pod for kubelet
that no longer respects node affinity.
For example, there is podA scheduled on nodeA which satisfies the node
affinity rule `requiredDuringSchedulingIgnoredDuringExecution` at the time
of scheduling. Over time nodeA stops to satisfy the rule. When the strategy gets
executed and there is another node available that satisfies the node affinity rule,
podA gets evicted from nodeA.
The policy file should look like:
```
apiVersion: "descheduler/v1alpha1"
@@ -180,7 +192,7 @@ never evicted because these pods won't be recreated.
* Pods associated with DaemonSets are never evicted.
* Pods with local storage are never evicted.
* Best efforts pods are evicted before burstable and guaranteed pods.
* All types of pods with the annotation descheduler.alpha.kubernetes.io/evict are evicted. This
* All types of pods with the annotation descheduler.alpha.kubernetes.io/evict are evicted. This
annotation is used to override checks which prevent eviction and users can select which pod is evicted.
Users should know how and if the pod will be recreated.