Ephemeral storage

When local ephemeral (temporal) storage is needed, an emptyDir should volume should be created. The volume is local to the node in which the pod is running, on LUMI-K this is a local SSD disk. The volume can be shared across several containers in the same Pod, and it is the fastest filesystem storage available in LUMI-K. However, the emptyDir volume will be deleted if the Pod is deleted or migrated to another node. It is declared directly in the Pod definition as following example:

podWithEmptydDir.yaml:

apiVersion: v1
kind: Pod
metadata:
  name: my-app
  labels:
    app: my-application
spec:
  volumes:
  - name: volume-a
    emptyDir: {}
  containers:
  - name: container-a
    image: almalinux:10
    volumeMounts:
    - mountPath: /outputdata
      name: volume-a
  - name: container-b
    image: almalinux:10
    volumeMounts:
    - mountPath: /interm
      name: volume-a

Using memory as medium

It is possible to make an emptyDir even faster by using the memory as storage medium instead (i.e., tmpfs) of the local disks. The size of data stored in a memory backed emptyDir is counted toward the Pod’s memory usage; this means the maximum size of data that can be stored is equal to the Pod memory limit (i.e., the size of memory limits of containers inside the Pod). If the Pod exceeds its memory limit, Kubernetes may terminate one or more containers with an OutOfMemory (OOMKilled) status. This can happen even if the application itself is not using too much memory — the contents of the tmpfs volume contribute to the limit. You can create memory backed emptyDir by adding medium: Memory under emptyDir field. It is recommended to configure the sizeLimit to a value lower than the Pod memory limit.

• podWithEmptyDirMemory.yaml

apiVersion: v1
kind: Pod
metadata:
  name: test-pd
spec:
  containers:
  - image: busybox:stable
    name: test-container
    command: ['sh', '-c', 'while true; do sleep 50; done']
    volumeMounts:
    - mountPath: /cache
      name: cache-volume
    resources:
      limits:
        memory: 2Gi
  volumes:
  - name: cache-volume
    emptyDir:
      sizeLimit: 500Mi
      medium: Memory