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Add note about existing PriorityClasses (#9279)

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* Add note about existing PriorityClasses

* Addressed the reviewer's comments
This commit is contained in:
Bobby (Babak) Salamat
2018-07-04 01:15:25 -07:00
committed by k8s-ci-robot
parent 97eada0ebe
commit a706ae9bcd
1 changed files with 18 additions and 3 deletions
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content/en/docs/concepts/configuration/pod-priority-preemption.md
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@@ -111,12 +111,27 @@ object for each such mapping that they want.
PriorityClass also has two optional fields: `globalDefault` and `description`. PriorityClass also has two optional fields: `globalDefault` and `description`.
The `globalDefault` field indicates that the value of this PriorityClass should The `globalDefault` field indicates that the value of this PriorityClass should
be used for Pods without a `priorityClassName`. Only one PriorityClass with be used for Pods without a `priorityClassName`. Only one PriorityClass with
`globalDefault` set to true can exist in the system. If there is no PriorityClass `globalDefault` set to true should exist in the system. If there is more than one
PriorityClass with `globalDefault` set to true, the value of the smallest
of those PriorityClasses is used as the default priority. If there is no PriorityClass
with `globalDefault` set, the priority of Pods with no `priorityClassName` is zero. with `globalDefault` set, the priority of Pods with no `priorityClassName` is zero.
The `description` field is an arbitrary string. It is meant to tell users of The `description` field is an arbitrary string. It is meant to tell users of
the cluster when they should use this PriorityClass. the cluster when they should use this PriorityClass.
Kubernetes comes with two predefined PriorityClasses: `system-node-critical`, and
`system-cluster-critical`. The former has the highest priority and is used for critical
system Pods that must run on all or most of the nodes and should never
be preempted. The latter is used for system critical Pods that must always
run in a cluster, but they can be terminated on one node and started on a
different node.
{{< note >}}
**Note**: You should never use these PriorityClasses that are dedicated to
system components for non-system Pods. Otherwise, Pods with such high priorities
might preempt system critical components, causing disruption of cluster services.
{{< /note >}}
### Notes about PodPriority and existing clusters ### Notes about PodPriority and existing clusters
- If you upgrade your existing cluster and enable this feature, the priority - If you upgrade your existing cluster and enable this feature, the priority
of your existing Pods is effectively zero. of your existing Pods is effectively zero.
@@ -138,7 +153,7 @@ metadata:
name: high-priority name: high-priority
value: 1000000 value: 1000000
globalDefault: false globalDefault: false
description: "This priority class should be used for XYZ service pods only." description: "This priority class should be used for XYZ service Pods only."
``` ```
## Pod priority ## Pod priority
@@ -242,7 +257,7 @@ preceding question must be yes. If the answer is no, the Node is not considered
for preemption. for preemption.
{{< /note >}} {{< /note >}}
If a pending Pod has inter-pod affinity to one or more of the lower-priority Pods If a pending Pod has inter-Pod affinity to one or more of the lower-priority Pods
on the Node, the inter-Pod affinity rule cannot be satisfied in the absence of those on the Node, the inter-Pod affinity rule cannot be satisfied in the absence of those
lower-priority Pods. In this case, the scheduler does not preempt any Pods on the lower-priority Pods. In this case, the scheduler does not preempt any Pods on the
Node. Instead, it looks for another Node. The scheduler might find a suitable Node Node. Instead, it looks for another Node. The scheduler might find a suitable Node