Ephemeral storage is a type of storage that is temporary and non-persistent, meaning it does not retain data once the process is terminated. In the context of Kubernetes, ephemeral storage is used for storing temporary files that a running container can write and read.

Why is this an issue?

Without a storage request, a container can potentially be scheduled on a node where there are not enough resources for it. This can lead to unpredictable behavior of the container and the node itself.

What is the potential impact?

Unpredictable Resource Allocation

Kubernetes doesn’t know how much of a particular resource to allocate to a container without defined requests. This can lead to unpredictable behavior, as the Kubernetes scheduler may not make optimal decisions about pod placement and resource contention management. For instance, a container might not get the resources it needs to function correctly, leading to performance issues or even failure of the container.

System Instability

In the worst-case scenario, if a container uses more resources than a node can handle (due to lack of defined requests), it can cause the node to run out of resources. This can lead to system instability, and in extreme cases, the node might crash, causing downtime for all containers running on that node.

How to fix it

Code examples

To avoid potential issues, specify a storage request for each container using ephemeral storage with resources.requests.ephemeral-storage, or create a LimitRange resource, that sets a default storage request for all containers in all pod specifications belonging to the same namespace.

Noncompliant code example

apiVersion: v1
kind: Pod
metadata:
  name: example
spec:
  containers:
    - name: web # Noncompliant
      image: nginx
      volumeMounts:
        - name: ephemeral
          mountPath: "/tmp"

apiVersion: v1
kind: Pod
metadata:
  name: example
spec:
  containers:
    - name: web # Noncompliant
      image: nginx
      volumeMounts:
        - name: ephemeral
          mountPath: "/tmp"

Compliant solution

apiVersion: v1
kind: Pod
metadata:
  name: example
spec:
  containers:
    - name: web
      image: nginx
      resources:
        requests:
          ephemeral-storage: "2Gi"
      volumeMounts:
        - name: ephemeral
          mountPath: "/tmp"

apiVersion: v1
kind: LimitRange
metadata:
  name: storage-limit-range
  namespace: namespace-with-limit-range
spec:
  limits:
  - defaultRequest:
      ephemeral-storage: "10Mi"
    type: Container
---
apiVersion: v1
kind: Pod
metadata:
  name: example
  namespace: namespace-with-limit-range
spec:
  containers:
    - name: web
      image: nginx
      volumeMounts:
        - name: ephemeral
          mountPath: "/tmp"

How does this work?

You can set a request through the property resources.requests.ephemeral-storage of a container. Alternatively, you can set a default request for a namespace with LimitRange through spec.limits[].defaultRequest.ephemeral-storage.

Resources

Documentation