Node affinity, described here, 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. For example,
kubectl taint nodes node1 key=value:NoSchedule
places a taint on node
node1. The taint has key
value, and taint effect
This means that no pod will be able to schedule onto
node1 unless it has a matching toleration.
To remove the taint added by the command above, you can run:
kubectl taint nodes node1 key:NoSchedule-
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
tolerations: - key: "key" operator: "Equal" value: "value" effect: "NoSchedule"
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:
Exists(in which case no
valueshould be specified), or
values are equal
Operator defaults to
Equal if not specified.
NOTE: There are two special cases:
Existsmatches all keys, values and effects which means this will tolerate everything.
tolerations: - operator: "Exists"
effectmatches all effects with key
tolerations: - key: "key" operator: "Exists"
The above example used
NoSchedule. Alternatively, you can use
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
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,
NoSchedulethen Kubernetes will not schedule the pod onto that node
NoSchedulebut there is at least one un-ignored taint with effect
PreferNoSchedulethen Kubernetes will try to not schedule the pod onto the node
NoExecutethen 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
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:
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
tolerationSeconds field that dictates how long the pod will stay bound
to the node after the taint is added. For example,
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.
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
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
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
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
For example, it is recommended to use Extended
to represent the special hardware, taint your special hardware nodes with the
extended resource name and run the
admission controller. Now, because the nodes are tainted, no pods without the
toleration will schedule on them. But when you submit a pod that requests the
extended resource, the
ExtendedResourceToleration admission controller will
automatically add the correct toleration to the pod and that pod will schedule
on the special hardware nodes. This will make sure that these special hardware
nodes are dedicated for pods requesting such hardware and you don’t have to
manually add tolerations to your pods.
Taint based Evictions (alpha feature): A per-pod-configurable eviction behavior when there are node problems, which is described in the next section.
Earlier we mentioned the
NoExecute taint effect, which affects pods that are already
running on the node as follows
tolerationSecondsin their toleration specification remain bound forever
tolerationSecondsremain bound for the specified amount of time
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
node.alpha.kubernetes.io/unreachable: Node is unreachable from the node controller. This corresponds to the NodeCondition
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.
TaintBasedEvictions alpha feature is enabled (you can do this by
--feature-gates for Kubernetes controller manager,
--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
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
tolerations: - key: "node.alpha.kubernetes.io/unreachable" operator: "Exists" effect: "NoExecute" tolerationSeconds: 6000
Note that Kubernetes automatically adds a toleration for
unless the pod configuration provided
by the user already has a toleration for
Likewise it adds a toleration for
unless the pod configuration provided
by the user already has a toleration for
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.
DaemonSet pods are created with
NoExecute tolerations for the following taints with no
This ensures that DaemonSet pods are never evicted due to these problems, which matches the behavior when this feature is disabled.
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/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.