Update node-selection documentation with information about taints,

tolerations, and alpha support for per-pod-configurable behavior when there are node problems.
2017-03-11 14:04:33 -08:00
parent 6f9b0d9191
commit 4013c5fb60
1 changed files with 214 additions and 0 deletions
@@ -2,6 +2,7 @@
 assignees:
 - davidopp
 - kevin-wangzefeng
 - bsalamat
 title: Assigning Pods to Nodes
 ---
@@ -198,3 +199,216 @@ must be satisfied for the pod to schedule onto a node.
 For more information on inter-pod affinity/anti-affinity, see the design doc
 [here](https://github.com/kubernetes/kubernetes/blob/{{page.githubbranch}}/docs/design/podaffinity.md).
 ## Taints and tolerations (beta feature)
 Node affinity, described earlier, is a property of *pods* that *attracts* them to a set
 of nodes (either as a preference or a hard requirement). Taints are the opposite --
 they allow a *node* to *repel* a set of pods.
 Taints and tolerations work together to ensure that pods are not scheduled
 onto inappropriate nodes. One or more taints are applied to a node; this
 marks that the node should not accept any pods that do not tolerate the taints.
 Tolerations are applied to pods, and allow (but do not require) the pods to schedule
 onto nodes with matching taints.
 You add a taint to a node using [kubectl taint](https://kubernetes.io/docs/user-guide/kubectl/kubectl_taint/).
 For example,
 ```shell
 kubectl taint nodes node1 key=value:NoSchedule
 ```
 places a taint on node `node1`. The taint has key `key`, value `value`, and taint effect `NoSchedule`.
 This means that no pod will be able to schedule onto `node1` unless it has a matching toleration.
 You specify a toleration for a pod in the PodSpec. Both of the following tolerations "match" the
 taint created by the `kubectl taint` line above, and thus a pod with either toleration would be able
 to schedule onto `node1`:
 ```yaml
 tolerations: 
 - key: "key"
  operator: "Equal"
  value: "value"
  effect: "NoSchedule"
 ```
 ```yaml
 tolerations: 
 - key: "key"
  operator: "Exists"
  effect: "NoSchedule"
 ```
 A toleration "matches" a taint if the `key`s are the same and the `effect`s are the same, and:
 * the `operator` is `Exists` (in which case no `value` should be specified), or
 * the `operator` is `Equal` and the `value`s are equal
 (`Operator` defaults to `Equal` if not specified.)
 As a special case, an empty `key` with operator `Exists` matches all keys and all values.
 Also as a special case, empty `effect` matches all effects.
 The above example used `effect` of `NoSchedule`. Alternatively, you can use `effect` of `PreferNoSchedule`.
 This is a "preference" or "soft" version of `NoSchedule` -- the system will *try* to avoid placing a
 pod that does not tolerate the taint on the node, but it is not required. The third kind of `effect` is
 `NoExecute`, described later.
 You can put multiple taints on the same node and multiple tolerations on the same pod.
 The way Kubernetes processes multiple taints and tolerations is like a filter: start
 with all of a node's taints, then ignore the ones for which the pod has a matching toleration; the
 remaining un-ignored taints have the indicated effects on the pod. In particular,
 * if there is at least one un-ignored taint with effect `NoSchedule` then Kubernetes will not schedule
 the pod onto that node
 * if there is no un-ignored taint with effect `NoSchedule` but there is at least one un-ignored taint with
 effect `PreferNoSchedule` then Kubernetes will *try* to not schedule the pod onto the node
 * if there is at least one un-ignored taint with effect `NoExecute` then the pod will be evicted from
 the node (if it is already running on the node), and will not be
 scheduled onto the node (if it is not yet running on the node).
 For example, imagine you taint a node like this
 ```shell
 kubectl taint nodes node1 key1=value1:NoSchedule
 kubectl taint nodes node1 key1=value1:NoExecute
 kubectl taint nodes node1 key2=value2:NoSchedule
 ```
 And a pod has two tolerations:
 ```yaml
 tolerations: 
 - key: "key1"
  operator: "Equal"
  value: "value1"
  effect: "NoSchedule"
 - key: "key1"
  operator: "Equal"
  value: "value1"
  effect: "NoExecute"
 ```
 In this case, the pod will not be able to schedule onto the node, because there is no
 toleration matching the third taint. But it will be able to continue running if it is
 already running on the node when the taint is added, because the third taint is the only
 one of the three that is not tolerated by the pod.
 Normally, if a taint with effect `NoExecute` is added to a node, then any pods that do
 not tolerate the taint will be evicted immediately, and any pods that do tolerate the
 taint will never be evicted. However, a toleration with `NoExecute` effect can specify
 an optional `tolerationSeconds` field that dictates how long the pod will stay bound
 to the node after the taint is added. For example,
 ```yaml
 tolerations: 
 - key: "key1"
  operator: "Equal"
  value: "value1"
  effect: "NoExecute"
  tolerationSeconds: 3600
 ```
 means that if this pod is running and a matching taint is added to the node, then
 the pod will stay bound to the node for 3600 seconds, and then be evicted. If the
 taint is removed before that time, the pod will not be evicted.
 ### Example use cases
 Taints and tolerations are a flexible way to steer pods away from nodes or evict
 pods that shouldn't be running. A few of the use cases are
 * **dedicated nodes**: If you want to dedicate a set of nodes for exclusive use by
 a particular set of users, you can add a taint to those nodes (say, 
 `kubectl taint nodes nodename dedicated=groupName:NoSchedule`) and then add a corresponding
 toleration to their pods (this would be done most easily by writing a custom
 [admission controller](https://kubernetes.io/docs/admin/admission-controllers/)).
 The pods with the tolerations will then be allowed to use the tainted (dedicated) nodes as
 well as any other nodes in the cluster. If you want to dedicate the nodes to them *and*
 ensure they *only* use the dedicated nodes, then you should additionally add a label similar
 to the taint to the same set of nodes (e.g. `dedicated=groupName`), and the admission
 controller should additionally add a node affinity to require that the pods can only schedule
 onto nodes labeled with `dedicated=groupName`.
 * **nodes with special hardware**: In a cluster where a small subset of nodes have specialized
 hardware (for example GPUs), it is desirable to keep pods that don't need the specialized
 hardware off of those nodes, thus leaving room for later-arriving pods that do need the
 specialized hardware. This can be done by tainting the nodes that have the specialized
 hardware (e.g. `kubectl taint nodes nodename special=true:NoSchedule` or
 `kubectl taint nodes nodename special=true:PreferNoSchedule`) and adding a corresponding
 toleration to pods that use the special hardware. As in the dedicated nodes use case,
 it is probably easiest to apply the tolerations using a custom
 [admission controller](https://kubernetes.io/docs/admin/admission-controllers/)).
 For example, the admission controller could use
 some characteristic(s) of the pod to determine that the pod should be allowed to use
 the special nodes and hence the admission controller should add the toleration.
 To ensure that the pods that need
 the special hardware *only* schedule onto the nodes that have the special hardware, you will need some
 additional mechanism, e.g. you could represent the special resource using
 [opaque integer resources](https://kubernetes.io/docs/concepts/configuration/manage-compute-resources-container/#opaque-integer-resources-alpha-feature)
 and request it as a resource in the PodSpec, or you could label the nodes that have
 the special hardware and use node affinity on the pods that need the hardware.
 * **per-pod-configurable eviction behavior when there are node problems (alpha feature)**,
 which is described in the next section.
 ### Per-pod-configurable eviction behavior when there are node problems (alpha feature)
 Earlier we mentioned the `NoExecute` taint effect, which affects pods that are already
 running on the node as follows
 * pods that do not tolerate the taint are evicted immediately
 * pods that tolerate the taint without specifying `tolerationSeconds` in
   their toleration specification remain bound forever
 * pods that tolerate the taint with a specified `tolerationSeconds` remain
   bound for the specified amount of time
 The above behavior is a beta feature. In addition, Kubernetes 1.6 has alpha
 support for representing node problems (currently only "node unreachable" and
 "node not ready", corresponding to the NodeCondition "Ready" being "Unknown" or
 "False" respectively) as taints. When the `TaintBasedEvictions` alpha feature
 is enabled (you can do this by including `TaintBasedEvictions=true` in `--feature-gates`, such as
 `--feature-gates=FooBar=true,TaintBasedEvictions=true`), the taints are automatically
 added by the NodeController and the normal logic for evicting pods from nodes
 based on the Ready NodeCondition is disabled.
 (Note: To maintain the existing [rate limiting](https://kubernetes.io/docs/admin/node/#node-controller))
 behavior of pod evictions due to node problems, the system actually adds the taints
 in a rate-limited way. This prevents massive pod evictions in scenarios such
 as the master becoming partitioned from the nodes.)
 This alpha feature, in combination with `tolerationSeconds`, allows a pod
 to specify how long it should stay bound to a node that has one or both of these problems.
 For example, an application with a lot of local state might want to stay
 bound to node for a long time in the event of network partition, in the hope
 that the partition will recover and thus the pod eviction can be avoided.
 The toleration the pod would use in that case would look like
 ```yaml
 tolerations: 
 - key: "node.alpha.kubernetes.io/unreachable"
  operator: "Exists"
  effect: "NoExecute"
  tolerationSeconds: 6000
 ```
 (For the node not ready case, change the key to `node.alpha.kubernetes.io/notReady`.)
 Note that Kubernetes automatically adds a toleration for
 `node.alpha.kubernetes.io/notReady` with `tolerationSeconds=300`
 unless the pod configuration provided
 by the user already has a toleration for `node.alpha.kubernetes.io/notReady`.
 Likewise it adds a toleration for
 `node.alpha.kubernetes.io/unreachable` with `tolerationSeconds=300`
 unless the pod configuration provided
 by the user already has a toleration for `node.alpha.kubernetes.io/unreachable`.
 These automatically-added tolerations ensure that
 the default pod behavior of remaining bound for 5 minutes after one of these
 problems is detected is maintained.
 The two default tolerations are added by the [DefaultTolerationSeconds
 admission controller](https://github.com/kubernetes/kubernetes/tree/master/plugin/pkg/admission/defaulttolerationseconds).
 [DaemonSet](https://kubernetes.io/docs/admin/daemons/) pods are created with
 `NoExecute` tolerations for `node.alpha.kubernetes.io/unreachable` and `node.alpha.kubernetes.io/notReady`
 with no `tolerationSeconds`. This ensures that DaemonSet pods are never evicted due
 to these problems, which matches the behavior when this feature is disabled.