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Merge pull request #39392 from arrikto/feature-ds-schedule
Update DaemonSet guide
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‎content/en/docs/concepts/workloads/controllers/daemonset.md

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@@ -105,30 +105,24 @@ If you do not specify either, then the DaemonSet controller will create Pods on
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## How Daemon Pods are scheduled
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### Scheduled by default scheduler
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{{< feature-state for_k8s_version="1.17" state="stable" >}}
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A DaemonSet ensures that all eligible nodes run a copy of a Pod. Normally, the
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node that a Pod runs on is selected by the Kubernetes scheduler. However,
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DaemonSet pods are created and scheduled by the DaemonSet controller instead.
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That introduces the following issues:
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* Inconsistent Pod behavior: Normal Pods waiting to be scheduled are created
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and in `Pending` state, but DaemonSet pods are not created in `Pending`
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state. This is confusing to the user.
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* [Pod preemption](/docs/concepts/scheduling-eviction/pod-priority-preemption/)
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is handled by default scheduler. When preemption is enabled, the DaemonSet controller
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will make scheduling decisions without considering pod priority and preemption.
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`ScheduleDaemonSetPods` allows you to schedule DaemonSets using the default
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scheduler instead of the DaemonSet controller, by adding the `NodeAffinity` term
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to the DaemonSet pods, instead of the `.spec.nodeName` term. The default
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scheduler is then used to bind the pod to the target host. If node affinity of
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the DaemonSet pod already exists, it is replaced (the original node affinity was
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taken into account before selecting the target host). The DaemonSet controller only
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performs these operations when creating or modifying DaemonSet pods, and no
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changes are made to the `spec.template` of the DaemonSet.
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A DaemonSet ensures that all eligible nodes run a copy of a Pod. The DaemonSet
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controller creates a Pod for each eligible node and adds the
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`spec.affinity.nodeAffinity` field of the Pod to match the target host. After
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the Pod is created, the default scheduler typically takes over and then binds
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the Pod to the target host by setting the `.spec.nodeName` field. If the new
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Pod cannot fit on the node, the default scheduler may preempt (evict) some of
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the existing Pods based on the
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[priority](/docs/concepts/scheduling-eviction/pod-priority-preemption/#pod-priority)
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of the new Pod.
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The user can specify a different scheduler for the Pods of the DamonSet, by
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setting the `.spec.template.spec.schedulerName` field of the DaemonSet.
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The original node affinity specified at the
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`.spec.template.spec.affinity.nodeAffinity` field (if specified) is taken into
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consideration by the DaemonSet controller when evaluating the eligible nodes,
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but is replaced on the created Pod with the node affinity that matches the name
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of the eligible node.
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```yaml
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nodeAffinity:
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- target-host-name
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```
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In addition, `node.kubernetes.io/unschedulable:NoSchedule` toleration is added
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automatically to DaemonSet Pods. The default scheduler ignores
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`unschedulable` Nodes when scheduling DaemonSet Pods.
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### Taints and Tolerations
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Although Daemon Pods respect
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[taints and tolerations](/docs/concepts/scheduling-eviction/taint-and-toleration/),
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the following tolerations are added to DaemonSet Pods automatically according to
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the related features.
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| Toleration Key | Effect | Version | Description |
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| ---------------------------------------- | ---------- | ------- | ----------- |
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| `node.kubernetes.io/not-ready` | NoExecute | 1.13+ | DaemonSet pods will not be evicted when there are node problems such as a network partition. |
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| `node.kubernetes.io/unreachable` | NoExecute | 1.13+ | DaemonSet pods will not be evicted when there are node problems such as a network partition. |
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| `node.kubernetes.io/disk-pressure` | NoSchedule | 1.8+ | DaemonSet pods tolerate disk-pressure attributes by default scheduler. |
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| `node.kubernetes.io/memory-pressure` | NoSchedule | 1.8+ | DaemonSet pods tolerate memory-pressure attributes by default scheduler. |
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| `node.kubernetes.io/unschedulable` | NoSchedule | 1.12+ | DaemonSet pods tolerate unschedulable attributes by default scheduler. |
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| `node.kubernetes.io/network-unavailable` | NoSchedule | 1.12+ | DaemonSet pods, who uses host network, tolerate network-unavailable attributes by default scheduler. |
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### Taints and tolerations
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The DaemonSet controller automatically adds a set of {{< glossary_tooltip
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text="tolerations" term_id="toleration" >}} to DaemonSet Pods:
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{{< table caption="Tolerations for DaemonSet pods" >}}
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| Toleration key | Effect | Details |
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| --------------------------------------------------------------------------------------------------------------------- | ------------ | --------------------------------------------------------------------------------------------------------------------------------------------- |
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| [`node.kubernetes.io/not-ready`](/docs/reference/labels-annotations-taints/#node-kubernetes-io-not-ready) | `NoExecute` | DaemonSet Pods can be scheduled onto nodes that are not healthy or ready to accept Pods. Any DaemonSet Pods running on such nodes will not be evicted. |
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| [`node.kubernetes.io/unreachable`](/docs/reference/labels-annotations-taints/#node-kubernetes-io-unreachable) | `NoExecute` | DaemonSet Pods can be scheduled onto nodes that are unreachable from the node controller. Any DaemonSet Pods running on such nodes will not be evicted. |
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| [`node.kubernetes.io/disk-pressure`](/docs/reference/labels-annotations-taints/#node-kubernetes-io-disk-pressure) | `NoSchedule` | DaemonSet Pods can be scheduled onto nodes with disk pressure issues. |
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| [`node.kubernetes.io/memory-pressure`](/docs/reference/labels-annotations-taints/#node-kubernetes-io-memory-pressure) | `NoSchedule` | DaemonSet Pods can be scheduled onto nodes with memory pressure issues. |
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| [`node.kubernetes.io/pid-pressure`](/docs/reference/labels-annotations-taints/#node-kubernetes-io-pid-pressure) | `NoSchedule` | DaemonSet Pods can be scheduled onto nodes with process pressure issues. |
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| [`node.kubernetes.io/unschedulable`](/docs/reference/labels-annotations-taints/#node-kubernetes-io-unschedulable) | `NoSchedule` | DaemonSet Pods can be scheduled onto nodes that are unschedulable. |
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| [`node.kubernetes.io/network-unavailable`](/docs/reference/labels-annotations-taints/#node-kubernetes-io-network-unavailable) | `NoSchedule` | **Only added for DaemonSet Pods that request host networking**, i.e., Pods having `spec.hostNetwork: true`. Such DaemonSet Pods can be scheduled onto nodes with unavailable network.|
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{{< /table >}}
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You can add your own tolerations to the Pods of a Daemonset as well, by
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defining these in the Pod template of the DaemonSet.
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Because the DaemonSet controller sets the
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`node.kubernetes.io/unschedulable:NoSchedule` toleration automatically,
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Kubernetes can run DaemonSet Pods on nodes that are marked as _unschedulable_.
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If you use a DaemonSet to provide an important node-level function, such as
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[cluster networking](/docs/concepts/cluster-administration/networking/), it is
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helpful that Kubernetes places DaemonSet Pods on nodes before they are ready.
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For example, without that special toleration, you could end up in a deadlock
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situation where the node is not marked as ready because the network plugin is
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not running there, and at the same time the network plugin is not running on
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that node because the node is not yet ready.
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## Communicating with Daemon Pods
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