Volumes & Persistent Storage

Volumes

Docs: K8s Docs - Volumes

• Provide storage to containers in a pod
• Lifetime is tied to the pod (except for persistent volumes)

emptyDir

• Created when a pod is assigned to a node, deleted when the pod is removed
• Shared between all containers in the same pod
• Use cases: scratch space, caching, sharing files between containers

spec:
  containers:
    - name: app
      image: myapp:1.0
      volumeMounts:
        - name: cache
          mountPath: /tmp/cache
  volumes:
    - name: cache
      emptyDir: {}

hostPath

• Mounts a file or directory from the host node's filesystem into a pod
• Data persists beyond the pod's lifetime (but is node-specific)
• Use cases: accessing host logs, Docker socket, node-level config

spec:
  containers:
    - name: app
      image: myapp:1.0
      volumeMounts:
        - name: host-logs
          mountPath: /var/log/host
          readOnly: true
  volumes:
    - name: host-logs
      hostPath:
        path: /var/log
        type: Directory          # Directory, DirectoryOrCreate, File, FileOrCreate

Warning: hostPath is a security risk and should be avoided in production — pods can access the host filesystem.

---

PersistentVolumes (PV)

Docs: K8s Docs - Persistent Volumes

• Cluster-scoped storage resource provisioned by an admin or dynamically via a StorageClass
• Lifecycle is independent of any pod

Access Modes

Mode	Abbreviation	Description
ReadWriteOnce	RWO	Mounted as read-write by a single node
ReadOnlyMany	ROX	Mounted as read-only by many nodes
ReadWriteMany	RWX	Mounted as read-write by many nodes
ReadWriteOncePod	RWOP	Mounted as read-write by a single pod

Reclaim Policies

Policy	Behavior
Retain	PV is kept after PVC is deleted (data preserved, manual cleanup)
Delete	PV and underlying storage are deleted when PVC is deleted

PV Manifest

apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv-data
spec:
  capacity:
    storage: 10Gi
  accessModes:
    - ReadWriteOnce
  persistentVolumeReclaimPolicy: Retain
  storageClassName: manual
  hostPath:                          # for local/testing — use cloud storage in production
    path: /mnt/data

PV Phases

• Available — free, not yet bound to a PVC
• Bound — bound to a PVC
• Released — PVC deleted, but resource not yet reclaimed
• Failed — automatic reclamation failed

---

PersistentVolumeClaims (PVC)

Docs: K8s Docs - PersistentVolumeClaims

• A request for storage by a user
• Kubernetes binds a PVC to a matching PV (by capacity, access mode, storageClass)

PVC Manifest

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: pvc-data
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 5Gi
  storageClassName: manual           # must match PV's storageClassName

Using a PVC in a Pod

apiVersion: v1
kind: Pod
metadata:
  name: app
spec:
  containers:
    - name: app
      image: myapp:1.0
      volumeMounts:
        - name: data
          mountPath: /data
  volumes:
    - name: data
      persistentVolumeClaim:
        claimName: pvc-data

---

StorageClasses

Docs: K8s Docs - Storage Classes

• Enable dynamic provisioning — PVs are created automatically when a PVC is created
• No need to manually create PVs

StorageClass Manifest

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: fast
  annotations:
    storageclass.kubernetes.io/is-default-class: "true"    # make this the default
provisioner: kubernetes.io/aws-ebs       # depends on cloud provider / CSI driver
parameters:
  type: gp3
reclaimPolicy: Delete                     # Delete (default) or Retain
volumeBindingMode: WaitForFirstConsumer   # Immediate or WaitForFirstConsumer
allowVolumeExpansion: true

volumeBindingMode

Mode	Behavior
Immediate	PV is provisioned as soon as the PVC is created
WaitForFirstConsumer	PV provisioning is delayed until a pod using the PVC is scheduled

• WaitForFirstConsumer is preferred — ensures the PV is created in the same zone as the pod

Using a StorageClass in a PVC

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: dynamic-pvc
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 20Gi
  storageClassName: fast             # references the StorageClass

• If no storageClassName is specified and a default StorageClass exists, it will be used

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Volume Expansion

• Allows growing the size of an existing PVC
• The StorageClass must have allowVolumeExpansion: true

Expand a PVC

kubectl edit pvc <pvc-name>
# change spec.resources.requests.storage to the new size

spec:
  resources:
    requests:
      storage: 50Gi    # increased from 20Gi

Some storage backends require the pod to be restarted for the filesystem to be resized.

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Useful Commands

# --- PersistentVolumes ---
kubectl get pv
kubectl describe pv <pv-name>

# --- PersistentVolumeClaims ---
kubectl get pvc
kubectl get pvc -A
kubectl describe pvc <pvc-name>

# --- StorageClasses ---
kubectl get storageclass
kubectl get sc                         # shorthand
kubectl describe sc <name>

# check the default StorageClass
kubectl get sc -o wide

# see which PV is bound to which PVC
kubectl get pv,pvc

Notes

1. A PVC can only bind to a PV with equal or greater capacity.
2. storageClassName: "" (empty string) in a PVC explicitly disables dynamic provisioning.
3. Deleting a PVC with reclaimPolicy: Retain keeps the PV in Released state — you must manually clean it up.