Storage in Kubernetes (Volumes, PV, PVC, StorageClass)

A container's filesystem dies with it, and a pod can be rescheduled to another node at any time. Apps that need durable data require storage that is decoupled from the pod (and node) lifecycle.

Solution: Decouple storage with PersistentVolumeClaims that bind PersistentVolumes (static or dynamically provisioned via StorageClasses), so data outlives pods.

To use a warehouse you do not build one yourself; you fill out a claim form for the size and terms you need, and the operator hands you a rented unit that matches. Kubernetes splits storage the same way: a pod files a PersistentVolumeClaim describing the capacity and access it wants, and that claim binds to a matching PersistentVolume, the real piece of storage, which the pod then mounts without ever provisioning it directly.

The same cluster entities appear in every day's notes; the <== marks what this day touches.

A container's filesystem dies with it, and a pod can be rescheduled anytime. Volumes decouple data from the container lifecycle.

   emptyDir  -> scratch space, lives with the POD (gone when pod dies)
   hostPath  -> a directory on the NODE (tied to that one node)
   PV / PVC  -> real, durable storage that outlives pods (and nodes)

Good for cache / scratch / sharing files between containers in the same pod.

spec:
  containers:
    - name: redis
      image: redis
      volumeMounts:
        - name: redis-storage
          mountPath: /data/redis
  volumes:
    - name: redis-storage
      emptyDir: {}            # deleted when the pod is removed

Separation of concerns:

The PVC is matched to a PV with enough capacity and compatible accessModes + storageClassName, then they bind 1:1.

A volume's access mode declares how many nodes can mount it and whether they get read-write or read-only access.

   RWO  ReadWriteOnce      -> mounted read-write by ONE node
   ROX  ReadOnlyMany       -> read-only by MANY nodes
   RWX  ReadWriteMany      -> read-write by MANY nodes (needs NFS-like backend)

The reclaim policy decides the fate of the PV and its data once the claim is removed.

   Retain  -> keep the PV + data (manual cleanup) — safest
   Delete  -> delete the PV and the underlying storage
   Recycle -> deprecated

The PV is the supply side: it declares capacity, access mode, and the backing storage (here a node hostPath for simplicity).

apiVersion: v1
kind: PersistentVolume
metadata:
  name: task-pv-volume
  labels:
    type: local
spec:
  storageClassName: standard
  capacity:
    storage: 1Gi
  accessModes:
    - ReadWriteOnce
  hostPath:
    path: "/home/ubuntu/day29-storage-k8s"

The PVC is the demand side: the app asks for a size, access mode, and class, and Kubernetes binds it to a matching PV.

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: task-pv-claim
spec:
  storageClassName: standard
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 500Mi          # binds to a PV with >= this capacity

The pod references the PVC by name in volumes, then mounts it into a container path — it never touches the PV directly.

apiVersion: v1
kind: Pod
metadata:
  name: task-pv-pod
spec:
  volumes:
    - name: task-pv-storage
      persistentVolumeClaim:
        claimName: task-pv-claim
  containers:
    - name: task-pv-container
      image: nginx
      ports:
        - containerPort: 80
      volumeMounts:
        - mountPath: "/usr/share/nginx/html"
          name: task-pv-storage

kubectl get pv,pvc
# STATUS Bound means the PVC found and claimed a PV

PVs can be hand-created by an admin ahead of time, or auto-created on demand by a StorageClass when a PVC appears.

   STATIC  -> admin pre-creates PVs by hand (like the demo above)
   DYNAMIC -> a StorageClass auto-creates a PV when a PVC is made

StorageClass = a template/provisioner for on-demand volumes:

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: fast
provisioner: kubernetes.io/aws-ebs   # or a CSI driver
reclaimPolicy: Delete
volumeBindingMode: WaitForFirstConsumer

A PVC that names a StorageClass gets its PV created automatically — no admin step. The cluster's default StorageClass is used when none is specified.

This is the picture from the video. A Storage Admin wires a StorageClass to a real backend once; after that, app teams just submit PVCs and the cluster creates a PV for each — you don't have to specify (or pre-create) the PersistentVolume at all.

Developer/User writes only the PVC (size + accessMode + storageClassName). No kind: PersistentVolume YAML is authored by hand.

Each pod owns its claim — the PVC YAML names a class, not a volume:

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: redis-pvc            # one of these per app (nginx-pvc, nodejs-pvc, ...)
spec:
  storageClassName: standard # the class the admin created; "" disables dynamic
  accessModes:
    - ReadWriteOnce          # RWO: this claim is mounted by one node at a time
  resources:
    requests:
      storage: 10Gi          # the provisioner carves a PV of (at least) this size

# the pod just references its claim by name — never the PV
spec:
  volumes:
    - name: data
      persistentVolumeClaim:
        claimName: redis-pvc
  containers:
    - name: redis
      image: redis
      volumeMounts:
        - mountPath: /data
          name: data

Division of labor: the admin owns the StorageClass (and which backend + reclaim policy it uses); developers own PVCs. Because the class provisions on demand, three pods means three PVCs and three auto-created PVs — no manual PV step for any of them.

kubectl get storageclass            # the template/provisioner the admin set up
kubectl get pvc                      # one Bound claim per app (redis/nginx/nodejs)
kubectl get pv                       # PVs the provisioner created automatically

A PVC binds a PV; the pod can die and be recreated, the data stays.

A PVC binds durable storage so data outlives the pod that writes it.

• emptyDir = pod-scoped scratch; hostPath = one node; PV/PVC = durable.
• PV = supply, PVC = demand; they bind 1:1 on capacity + accessMode + class.
• Access modes RWO/ROX/RWX; reclaim Retain/Delete.
• Static = hand-made PVs; dynamic = StorageClass provisions on demand.
• With a StorageClass you write only the PVC — the PV is auto-created, so many pods (nginx/redis/nodejs) each just bring their own claim.

• [ ] Used an emptyDir and saw it vanish with the pod
• [ ] Created a PV + PVC and confirmed STATUS Bound
• [ ] Mounted the PVC in a pod and wrote/read data
• [ ] Explained static vs dynamic provisioning and StorageClasses
• [ ] Created several PVCs against one StorageClass and saw PVs auto-provisioned