Add cluster sizing chapter

slides/k8s/cluster-sizing.md

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+# Cluster sizing
+
+- What happens when the cluster gets full?
+
+- How can we scale up the cluster?
+
+- Can we do it automatically?
+
+- What are other methods to address capacity planning?
+
+---
+
+## When are we out of resources?
+
+- kubelet monitors node resources:
+
+  - memory
+
+  - node disk usage (typically the root filesystem of the node)
+
+  - image disk usage (where container images and RW layers are stored)
+
+- For each resource, we can provide two thresholds:
+
+  - a hard threshold (if it's met, it provokes immediate action)
+
+  - a soft threshold (provokes action only after a grace period)
+
+- Resource thresholds and grace periods are configurable
+
+  (by passing kubelet command-line flags)
+
+---
+
+## What happens then?
+
+- If disk usage is too high:
+
+  - kubelet will try to remove terminated pods
+
+  - then, it will try to *evict* pods
+
+- If memory usage is too high:
+
+  - it will try to evict pods
+
+- The node is marked as "under pressure"
+
+- This temporarily prevents new pods from being scheduled on the node
+
+---
+
+## Which pods get evicted?
+
+- kubelet looks at the pods QoS and PriorityClass
+
+- First, pods with BestEffort QoS are considered
+
+- Then, pods with Burstable QoS exceeding their *requests*
+
+  (but only if the exceeding resource is the one that is low on the node)
+
+- Finally, pods with Guaranteed QoS, and Burstable pods within their requests
+
+- Within each group, pods are sorted by PriorityClass
+
+- If there are pods with the same PriorityClass, they are sorted by usage excess
+
+  (i.e. the pods whose usage exceeds their requests the most are evicted first)
+
+---
+
+class: extra-details
+
+## Eviction of Guaranteed pods
+
+- *Normally*, pods with Guaranteed QoS should not be evicted
+
+- A chunk of resources is reserved for node processes (like kubelet)
+
+- It is expected that these processes won't use more than this reservation
+
+- If they do use more resources anyway, all bets are off!
+
+- If this happens, kubelet must evict Guaranteed pods to preserve node stability
+
+  (or Burstable pods that are still within their requested usage)
+
+---
+
+## What happens to evicted pods?
+
+- The pod is terminated
+
+- It is marked as `Failed` at the API level
+
+- If the pod was created by a controller, the controller will recreate it
+
+- The pod will be recreated on another node, *if there are resources available!*
+
+- For more details about the eviction process, see:
+
+  - [this documentation page](https://kubernetes.io/docs/tasks/administer-cluster/out-of-resource/) about resource pressure and pod eviction,
+
+  - [this other documentation page](https://kubernetes.io/docs/concepts/configuration/pod-priority-preemption/) about pod priority and preemption.
+
+---
+
+## What if there are no resources available?
+
+- Sometimes, a pod cannot be scheduled anywhere:
+
+  - all the nodes are under pressure,
+
+  - or the pod requests more resources than are available
+
+- The pod then remains in `Pending` state until the situation improves
+
+---
+
+## Cluster scaling
+
+- One way to improve the situation is to add new nodes
+
+- This can be done automatically with the [Cluster Autoscaler](https://github.com/kubernetes/autoscaler/tree/master/cluster-autoscaler)
+
+- The autoscaler will automatically scale up:
+
+  - if there are pods that failed to be scheduled
+
+- The autoscaler will automatically scale down:
+
+  - if nodes have a low utilization for an extended period of time
+
+---
+
+## Restrictions, gotchas ...
+
+- The Cluster Autoscaler only supports a few cloud infrastructures
+
+  (see [here](https://github.com/kubernetes/autoscaler/tree/master/cluster-autoscaler/cloudprovider) for a list)
+
+- The Cluster Autoscaler cannot scale down nodes that have pods using:
+
+  - local storage
+
+  - affinity/anti-affinity rules preventing them from being rescheduled
+
+  - a restrictive PodDisruptionBudget
+
+---
+
+## Other way to do capacity planning
+
+- "Running Kubernetes without nodes"
+
+- Systems like Virtual Kubelet or Kiyot can run pods using on-demand resources
+
+  - Virtual Kubelet can leverage e.g. ACI or Fargate to run pods
+
+  - Kiyot runs pods in ad-hoc EC2 instances (1 instance per pod)
+
+- Economic advantage (no wasted capacity)
+
+- Security advantage (stronger isolation between pods)
+
+Check [this blog post](http://jpetazzo.github.io/2019/02/13/running-kubernetes-without-nodes-with-kiyot/) for more details.