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Update node-selection documentation with information about taints,

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tolerations, and alpha support for per-pod-configurable behavior
when there are node problems.
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David Oppenheimer
2017-03-11 14:04:33 -08:00
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@@ -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.