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02-OS-Upgrades.md

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OS Upgrades

In this section, we will take a look at os upgrades.

  • We wull discuss about scenarios where you might have to take down node as part of your cluster for maintenance purposes like upgrading a base software or applying patches like security patches etc.

If the node was down for more than 5 minutes, then the pods are terminated from that node

  • If the pods where part of a replicaset then they are recreated on other nodes.

  • The time it waits for a pod to come back online is known as the pod eviction timeout and is set on the controller manager with a default value of 5 minutes.

    os

  • When the node comes online after the pod eviction timeout it comes up blank without any pods scheduled on it.

  • Thus, if you have maintenance tasks to be performed on a node if you know that the workloads running on the node have other replicas and if it's okay that they go down for a short period of time and if you're sure the node will come back online within 5 minutes you make a quick upgrade and reboot. But you are not sure if a node is going to be back online in 5 mintues.

  • You can purposefully drain the node of all the workloads so that the workloads are moved to other nodes.

  • Well technically they are not moved. When you drain the node the pods are gracefully terminated from the node that they're on and recreated on another.

    $ kubectl drain node-1

  • The node is also cordoned or marked as unschedulable.

  • When the node is back online after a maintenance, it is still unschedulable. You then need to uncordorn it.

    $ kubectl uncordorn node-1

  • There is also another command called cordorn. Cordorn simply marks a node unschedulable. Unlike drain it does not terminate or move the pods on an existing node.

    drain