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Quick note on kubectl taint
The main page about Taints and Tolerations in kubernetes does not go into some corner cases of how you can set or remove taints, so here we are.
The aim of taints and tolerations in Kubernetes is described well in the documentation:
Taints […] 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.
- you set taints on nodes to mark them as having some “bad disease” that everyone should keep away from;
- you set tolerations onto resources that are allowed to deal with those “diseases” and should therefore be admitted in the node even though it has the taint.
The documentation (including the manual page) describe how to set and remove taints, but I’ve found that they don’t tell 100% of the story.
The following is valid as of early January, 2020.
Structure of a Taint
The generic syntax of a taint is the following:
The key is like the name of the disease on the node. It can be further specialized with a value, to allow for finer selection granularity, although setting it is optional (i.e. you can leave it empty).
The effect is what consequence the taint has. You can set the following effects:
NoSchedule: prevent scheduling of new Pods;
PreferNoSchedule: avoid as much as possible scheduling of new Pods (this is a soft alternative of the previous effect);
NoExecute: prevent execution of Pods.
So the main goal of setting a taint is to affect either scheduling or execution, nothing more.
For example, setting the following taint on a node:
means that, unless Pods are able to deal with
has-feature=sriov (via a
suitable toleration), they will not be scheduled on the node. Something
similar would happen with this taint:
only that in this case there’s an empty value associated to the
has-sriov. How you want to use all of this (i.e. with a value or not) is
totally up to you.
You can set a taint with the kubectl taint command. The generic syntax is as follows:
kubectl taint node <node> <key>=[<value>]:<effect> [...]
You can use
nodes instead of
node if you wish.
We will assume to work on node whose name is in environment variable
NODEfrom now on. If you set it to the string
--all, the operation will apply to all nodes.
# set taint NoSchedule associated to key 'has-feature' and value 'sriov' kubectl taint node "$NODE" has-feature=sriov:NoSchedule
Setting a taint with an empty value always requires the equal sign:
# THIS GIVES AN ERROR kubectl taint node "$NODE" has-feature:NoSchedule # this is correct and associates an empty value to key 'has-feature' kubectl taint node "$NODE" has-feature=:NoSchedule
Removing a taint can be as easy as setting it, just append a minus sign
-) at the end of the effect to indicate that you want to get rid of it.
The following gets rid of the first taint set in the previous section:
kubectl taint node "$NODE" has-feature=sriov:NoSchedule-
It works also for empty taints of course:
kubectl taint node "$NODE" has-feature=:NoSchedule-
While you must be very precise when setting a taint (i.e. you always have
to put a value, even if it’s empty, and always specify an effect), you
can be more liberal when removing them. Suppose you set the following two
taints associated to key
kubectl taint node "$NODE" somekey=val1:NoSchedule somekey=val2:NoExecute
You can remove all of them in a single sweep by just “removing” the key:
kubectl taint node "$NODE" somekey-
This will work whatever value and/or effect were set for that key.
A real world example
This post started from reading the ovn-kubernetes documentation, that contains the following:
On Kubernetes master, label it to run daemonsets. ``` kubectl taint nodes --all node-role.kubernetes.io/master- ```
Now I understand that:
- by default, master nodes are set with taint
node-role.kubernetes.io/master=:NoSchedule, i.e. with key
node-role.kubernetes.io/master, empty value and effect of
- the command above removes all taints with the specific key, including the default one above;
- removing that taint means that everything can be scheduled on all nodes
(unless there are additional taints resulting in
NoSchedule, of course).
This is all I had to write about for this post. One final observation though: you should not normally need to remove a taint, you probably want to set the right tolerations on the resources. Read the Taints and Tolerations page to learn more!