---
approvers:
- davidopp
- kevin-wangzefeng
- bsalamat
title: Taints and Tolerations
---

Node affinity, described [here](/docs/concepts/configuration/assign-pod-node/#node-affinity-beta-feature),
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.

* TOC
{:toc}

## Concepts

You add a taint to a node using [kubectl taint](/docs/user-guide/kubectl/{{page.version}}/#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 keys are the same and the effects 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.

**NOTE:** There are two special cases:

* An empty `key` with operator `Exists` matches all keys, values and effects which means this
will tolerate everything.

```yaml
tolerations:
- operator: "Exists"
```

* An empty `effect` matches all effects with key `key`.

```yaml
tolerations:
- key: "key"
  operator: "Exists"
```

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](/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](/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](/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.

* **Taint based Evictions (alpha feature)**: A per-pod-configurable eviction behavior
when there are node problems, which is described in the next section.

## Taint based Evictions

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. In other words, the node controller
automatically taints a node when certain condition is true. The built-in taints
currently include:

 * `node.kubernetes.io/not-ready`: Node is not ready. This corresponds to
   the NodeCondition `Ready` being "`False`".
 * `node.alpha.kubernetes.io/unreachable`: Node is unreachable from the node
   controller. This corresponds to the NodeCondition `Ready` being "`Unknown`".
 * `node.kubernetes.io/out-of-disk`: Node becomes out of disk.
 * `node.kubernetes.io/memory-pressure`: Node has memory pressure.
 * `node.kubernetes.io/disk-pressure`: Node has disk pressure.
 * `node.kubernetes.io/network-unavailable`: Node's network is unavailable.
 * `node.cloudprovider.kubernetes.io/uninitialized`: When kubelet is started
   with "external" cloud provider, it sets this taint on a node to mark it
   as unusable. When a controller from the cloud-controller-manager initializes
   this node, kubelet removes this taint.

When the `TaintBasedEvictions` alpha feature is enabled (you can do this by
including `TaintBasedEvictions=true` in `--feature-gates` for Kubernetes controller manager,
such as `--feature-gates=FooBar=true,TaintBasedEvictions=true`), the taints are automatically
added by the NodeController (or kubelet) and the normal logic for evicting pods from nodes
based on the Ready NodeCondition is disabled.
(Note: To maintain the existing [rate limiting](/docs/concepts/architecture/nodes/)
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
```

Note that Kubernetes automatically adds a toleration for
`node.kubernetes.io/not-ready` with `tolerationSeconds=300`
unless the pod configuration provided
by the user already has a toleration for `node.kubernetes.io/not-ready`.
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://git.k8s.io/kubernetes/plugin/pkg/admission/defaulttolerationseconds).

[DaemonSet](/docs/concepts/workloads/controllers/daemonset/) pods are created with
`NoExecute` tolerations for the following taints with no `tolerationSeconds`:

  * `node.alpha.kubernetes.io/unreachable`
  * `node.kubernetes.io/not-ready`

This ensures that DaemonSet pods are never evicted due to these problems,
which matches the behavior when this feature is disabled.

## Taint Nodes by Condition

Version 1.8 introduces an alpha feature that causes the node controller to create taints corresponding to
Node conditions. When this feature is enabled (you can do this by including `TaintNodesByCondition=true` in the `--feature-gates` command line flag to the scheduler, such as
`--feature-gates=FooBar=true,TaintNodesByCondition=true`), the scheduler does not check Node conditions; instead the scheduler checks taints. This assures that Node conditions don't affect what's scheduled onto the Node. The user can choose to ignore some of the Node's problems (represented as Node conditions) by adding appropriate Pod tolerations.

To make sure that turning on this feature doesn't break DaemonSets, starting in version 1.8, the  DaemonSet controller automatically adds the following `NoSchedule` tolerations to all daemons:

  * `node.kubernetes.io/memory-pressure`
  * `node.kubernetes.io/disk-pressure`
  * `node.kubernetes.io/out-of-disk` (*only for critical pods*)

The above settings ensure backward compatibility, but we understand they may not fit all user's needs, which is why
cluster admin may choose to add arbitrary tolerations to DaemonSets.
