Chapter 3. Using ephemeral storage

Use ephemeral storage to provide temporary local storage for stateless applications that only need data for the duration of the pod lifecycle, such as caches, scratch files, and logs that do not need to persist after the pod terminates.

Both developers and administrators can use this feature.

Pods and containers can require ephemeral or transient local storage for their operation. The lifetime of this ephemeral storage does not extend beyond the life of the individual pod, and this ephemeral storage cannot be shared across pods.

Issues related to the lack of local storage accounting and isolation include the following:

Unlike persistent volumes, ephemeral storage is unstructured and the space is shared between all pods running on the node, other uses by the system, and Red Hat build of MicroShift. The ephemeral storage framework allows pods to specify their transient local storage needs. It also allows Red Hat build of MicroShift to protect the node against excessive use of local storage.

While the ephemeral storage framework allows administrators and developers to better manage local storage, I/O throughput and latency are not directly affected.

Provision ephemeral local storage by creating the primary partition using either root or runtime methods. Choose the method that aligns with your node configuration to ensure temporary storage is available for your workloads.

Cluster administrators can manage ephemeral storage within a project by setting quotas that define limit ranges and request counts for all pods in a non-terminal state. Developers can also set requests and limits on this resource at the pod and container level.

You can manage local ephemeral storage by specifying requests and limits. Each container in a pod can specify the following:

apiVersion: v1
kind: Pod
metadata:
  name: frontend
spec:
  containers:
  - name: app
    image: images.my-company.example/app:v4
    resources:
      requests:
        ephemeral-storage: "2Gi"
      limits:
        ephemeral-storage: "4Gi"
    volumeMounts:
    - name: ephemeral
      mountPath: "/tmp"
  - name: log-aggregator
    image: images.my-company.example/log-aggregator:v6
    resources:
      requests:
        ephemeral-storage: "2Gi"
      limits:
        ephemeral-storage: "4Gi"
    volumeMounts:
    - name: ephemeral
      mountPath: "/tmp"
  volumes:
    - name: ephemeral
      emptyDir: {}

Monitor ephemeral storage usage with the /bin/df utility to track disk space consumption on /var/lib/kubelet and /var/lib/containers. Regular monitoring helps you identify storage-hungry workloads and adjust resource limits before kubelet evicts pods due to storage exhaustion.

When you use the df command, the available space for only /var/lib/kubelet is shown if /var/lib/containers is placed on a separate disk by the cluster administrator.

You can use /bin/df as a tool to monitor ephemeral storage usage on the volume where ephemeral container data is located, which is /var/lib/kubelet and /var/lib/containers. The available space for only /var/lib/kubelet is shown when you use the df command if /var/lib/containers is placed on a separate disk by the cluster administrator.