CKAD — Ephemeral Volumes

Why Ephemeral Volumes?

Pods can use ephemeral volumes to share data between containers or store temporary files. These volumes last only as long as the Pod — when the Pod is deleted, the data is gone.

Ephemeral volumes are part of the Application Design and Build domain (20% of the exam). You need to know when and how to use them.

emptyDir

An emptyDir volume is created when a Pod starts and deleted when the Pod is removed. It is the most commonly used ephemeral volume type.

Use Cases

Use Case	Description
Shared data between containers	Writer container produces files, reader container consumes them
Scratch space	Temporary storage for sorting, caching, or processing
Checkpointing	Save intermediate results for long computations

Basic emptyDir Example

apiVersion: v1
kind: Pod
metadata:
  name: shared-data
spec:
  containers:
  - name: writer
    image: busybox:1.36
    command: ["/bin/sh", "-c", "echo 'hello from writer' > /data/msg.txt && sleep 3600"]
    volumeMounts:
    - name: shared
      mountPath: /data
  - name: reader
    image: busybox:1.36
    command: ["/bin/sh", "-c", "sleep 5 && cat /data/msg.txt && sleep 3600"]
    volumeMounts:
    - name: shared
      mountPath: /data
  volumes:
  - name: shared
    emptyDir: {}

kubectl apply -f shared-data.yaml
kubectl logs shared-data -c reader
# Expected: hello from writer

emptyDir with Memory-Backed Storage

For high-performance temporary storage, you can use memory (tmpfs) instead of disk:

volumes:
- name: cache
  emptyDir:
    medium: Memory
    sizeLimit: 256Mi

Warning: Memory-backed emptyDir counts against the container’s memory limit. If the volume grows beyond the size limit, the Pod gets evicted.

emptyDir with Size Limit

volumes:
- name: scratch
  emptyDir:
    sizeLimit: 1Gi

hostPath

A hostPath volume mounts a file or directory from the host node into the Pod. It provides access to the node’s filesystem.

hostPath Types

Type	Behavior
"" (empty)	No checks. Creates path if it does not exist.
DirectoryOrCreate	Creates directory if missing.
Directory	Directory must already exist.
FileOrCreate	Creates file if missing.
File	File must already exist.

hostPath Example

apiVersion: v1
kind: Pod
metadata:
  name: log-reader
spec:
  containers:
  - name: reader
    image: busybox:1.36
    command: ["/bin/sh", "-c", "tail -f /host-logs/syslog"]
    volumeMounts:
    - name: host-logs
      mountPath: /host-logs
      readOnly: true
  volumes:
  - name: host-logs
    hostPath:
      path: /var/log
      type: Directory

When to Use hostPath

Scenario	Appropriate?
Development/testing	Yes — quick access to host files
Reading node logs	Yes — mount /var/log read-only
Production workloads	No — data is node-specific, not portable
Shared storage between Pods	No — use PersistentVolumes instead

Exam tip: hostPath is rarely the right choice in production. Data is tied to a specific node and is not portable. Use emptyDir for temporary data sharing and PersistentVolumes for data that must survive Pod restarts.

Ephemeral Volume Comparison

Feature	emptyDir	hostPath
Created	When Pod starts	References existing host path
Deleted	When Pod is deleted	Data persists on host
Scope	Pod-level	Node-level
Portable	Yes (works on any node)	No (tied to specific node)
Use case	Temp data, inter-container sharing	Access host files, development
Memory option	Yes (medium: Memory)	No
Size limit	Yes (sizeLimit)	No built-in limit

Resource Requests and Limits (Preview)

Every Pod should declare how much CPU and memory it needs. We cover this in detail in Chapter 11: Resource Management, but here is the basic idea:

apiVersion: v1
kind: Pod
metadata:
  name: app
spec:
  containers:
  - name: app
    image: nginx:1.27
    resources:
      requests:
        memory: "64Mi"
        cpu: "250m"
      limits:
        memory: "128Mi"
        cpu: "500m"

• Requests — minimum guaranteed resources. Used for scheduling.
• Limits — maximum allowed resources. Container gets killed (OOMKilled) if it exceeds the memory limit.

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Next: Learn how Kubernetes handles container exits with Restart Policies →