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Ephemeral storage

When local ephemeral (temporal) storage is needed, an emptyDir should be issued. It is local to the node, on Rahti this is RAID-1 SSD storage. It can be shared across several containers in the same Pod, and it the fastest filesystem available in Rahti but it will be lost when the Pod is killed or restarted. It is declared directly in the Pod definition:

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: centos:7
    volumeMounts:
    - mountPath: /outputdata
      name: volume-a
  - name: container-b
    image: centos:7
    volumeMounts:
    - mountPath: /interm
      name: volume-a

emptyDir

Using memory as medium

It is possible to make an emptyDir even faster by using memory as storage medium, i.e.: use tmpfs. The two drawbacks of this approach compared with a standard emptyDir is (1) the memory is shared with all the processes of the Pod, so its maximum size will be the same as the memory limit of the Pod, and (2) if the emptyDir (together with the processes of the Pod) uses all the available memory, the Pod will be killed. You can create one by adding medium: Memory under emptyDir. It is recommended to configure the sizeLimit to something 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