Containers have ephemeral filesystems — when a container restarts, everything written inside is lost. Volumes solve this by providing storage that outlives individual containers. This lesson covers the primitive volume types that are built directly into the Pod spec — no external provisioning needed.

1. Volume Concepts

A volume has two parts in a Pod spec:

spec:
  volumes:                        # 1. DEFINE the volume (what type, what source)
    - name: my-volume
      emptyDir: {}
  containers:
    - name: app
      volumeMounts:               # 2. MOUNT it into a container (where)
        - name: my-volume
          mountPath: /data

Lifetime Rules

2. emptyDir — Scratch Space

Created when a Pod is assigned to a node. Starts empty. Deleted when the Pod is removed. Shared between all containers in the Pod.

spec:
  volumes:
    - name: scratch
      emptyDir: {}            # Default: node disk
    - name: cache
      emptyDir:
        medium: Memory        # tmpfs (RAM-backed) — faster, counts against memory limits
        sizeLimit: 500Mi      # Evict Pod if exceeded
  containers:
    - name: app
      volumeMounts:
        - name: scratch
          mountPath: /tmp/work
    - name: sidecar
      volumeMounts:
        - name: scratch
          mountPath: /shared    # Same volume, different mountPath — shared data!

Use Cases

3. hostPath — Node Filesystem Access

Mounts a file or directory from the host node's filesystem into the Pod. The data persists on that specific node even after the Pod is deleted.

spec:
  volumes:
    - name: host-logs
      hostPath:
        path: /var/log           # Path on the NODE
        type: Directory          # Must exist as a directory
  containers:
    - name: log-collector
      volumeMounts:
        - name: host-logs
          mountPath: /host-logs
          readOnly: true

hostPath Types

When hostPath Is Acceptable

• DaemonSets for log collection — mount /var/log read-only
• Node monitoring agents — mount /proc, /sys read-only
• Container runtime socket — mount /var/run/containerd/containerd.sock
• Static Pod manifests — /etc/kubernetes/manifests

4. Projected Volumes — Multiple Sources, One Mount

A projected volume combines multiple volume sources into a single directory. Instead of mounting 4 separate volumes, you mount one that contains files from all of them.

spec:
  volumes:
    - name: pod-info
      projected:
        sources:
          - configMap:
              name: app-config
              items:
                - key: app.properties
                  path: config/app.properties
          - secret:
              name: db-creds
              items:
                - key: password
                  path: secrets/db-password
          - downwardAPI:
              items:
                - path: metadata/labels
                  fieldRef:
                    fieldPath: metadata.labels
          - serviceAccountToken:
              path: token
              expirationSeconds: 3600
              audience: api-server
  containers:
    - name: app
      volumeMounts:
        - name: pod-info
          mountPath: /etc/pod-info
          readOnly: true

# Result in container:
/etc/pod-info/
├── config/app.properties      (from ConfigMap)
├── secrets/db-password        (from Secret)
├── metadata/labels            (from Downward API)
└── token                      (ServiceAccount token)

Sources You Can Project

5. Generic Ephemeral Volumes (K8s 1.23+)

Like emptyDir but with PVC-backed storage: you get a dynamically provisioned volume that's automatically deleted when the Pod is removed. Useful when you need more storage features (specific StorageClass, size guarantees) but don't want to manage the PVC lifecycle separately.

spec:
  volumes:
    - name: scratch-pvc
      ephemeral:
        volumeClaimTemplate:
          metadata:
            labels:
              type: scratch
          spec:
            accessModes: ["ReadWriteOnce"]
            storageClassName: fast-ssd
            resources:
              requests:
                storage: 10Gi
  containers:
    - name: etl
      volumeMounts:
        - name: scratch-pvc
          mountPath: /work

Ephemeral vs emptyDir vs PVC

6. Quick Reference: All Primitive Volume Types

Summary