PersistentVolume (pv)

• PersistentVolume (PV) provisions volumes statically which can be claimed by PVCs
• On the other hand StorageClass (SC) provisions volumes dynamically
• A PV represents a space from the actual physical storage (e.g., local disk, NFS server, cloud storage)

• The PV is an interface with the actual storage

• Storage Requirements

• Storage does not depend on the pod lifecycle
• Storage does not depend on a node (it must be available on all nodes, pods can restart and spawn in another node)

• Storage must be independent from the cluster (it needs to survive even if the cluster crashes)

• Local Volumes violates the 2nd and 3rd requirements for data persistence: it's tied to 1 node, does not survive in cluster crashes. It's bad for DB persistence.

PV Phases

• PV phases are not affected by the pods/deployments. It is affected by PVCs

• If a pod is deleted or recreated, the PV does not transition to other phase (it is independent from the pod lifecycle)

• Available

• Not yet claimed by any PVC
• Bound
• Claimed and bound to a PVC
• Released
• Claimed by a PVC, but already released
• Data still intact, but cannot be bound to any other PVC

Properties

spec.capacity

apiVersion: v1
kind: PersistentVolume
metadata:
  name: my-pv
spec:
  capacity:
    storage: 50Mi
  accessModes:
    - ReadWriteOnce
  hostPath:
    path: /tmp/data
  volumeMode: Filesystem

spec.persistentVolumeReclaimPolicy

• Delete (default)
• Cloud storage resource (and the PV) is deleted as soon as the PVC is deleted
• Retain
• Cloud storage resource (and the PV) is kept when PVC is deleted
• The PV cannot be reused by any other PVC
• Recycle
• Cloud storage resource (and the PV) is kept when PVC is deleted
• The PV can be reused by other PVCs

apiVersion: v1
kind: PersistentVolume
metadata:
  name: my-pv
spec:
  capacity:
    storage: 50Mi
  accessModes:
    - ReadWriteOnce
  hostPath:
    path: /tmp/data
  persistentVolumeReclaimPolicy: Retain

spec.accessModes

• ReadOnlyMany: multiple nodes can read-only the volume
• ReadWriteOnce: can read and write by only one node
• ReadWriteMany: multiple nodes can read and write to the volume

apiVersion: v1
kind: PersistentVolume
metadata:
  name: my-pv
spec:
  capacity:
    storage: 50Mi
  accessModes:
    - ReadOnlyMany
    - ReadWriteOnce
    - ReadWriteMany
  hostPath:
    path: /tmp/data

spec.volumeMode

• Filesystem (default)
• The underlying volume is formatted with a filesystem (e.g., ext4, xfs, etc)
• Useful for scenarios in which you want to read/write files
• Block
• Storage is exposed as a raw block device
• Does not have any filesystem formatting (which the application can create)

apiVersion: v1
kind: PersistentVolume
metadata:
  name: my-pv
spec:
  capacity:
    storage: 50Mi
  accessModes:
    - ReadWriteOnce
  hostPath:
    path: /tmp/data
  volumeMode: Filesystem

spec.nodeAffinity

apiVersion: v1
kind: PersistentVolume
metadata:
  name: my-pv
spec:
  capacity:
    storage: 50Mi
  accessModes:
    - ReadWriteOnce
  hostPath:
    path: /tmp/data
  nodeAffinity:
    required:
      nodeSelectorTerms:
        - matchExpressions:
            - key: kubernetes.io/hostname
              operator: In
              values:
                - node01

Storage Sources

spec.local

• Local storage devices mounted on nodes.

apiVersion: v1
kind: PersistentVolume
metadata:
  name: my-pv
spec:
  capacity:
    storage: 100Gi
  accessModes:
    - ReadWriteOnce
  local:
    path: /mnt/disks/sdd1
  #storageClassName: local-storage

spec.hostPath

• Take a slice of the host HDD and make it a PV
• For single node testing only; will not work in a multi-node cluster (consider using local volume instead)

apiVersion: v1
kind: PersistentVolume
metadata:
  name: my-pv
spec:
  capacity:
    storage: 50Mi
  accessModes:
    - ReadWriteOnce
  hostPath:
    path: /tmp/data # path in the host machine

spec.nfs

• Network File System (NFS) storage

apiVersion: v1
kind: PersistentVolume
metadata:
  name: my-pv
spec:
  capacity:
    storage: 5Gi
  accessModes:
    - ReadWriteOnce
  nfs: # nfs parameters
    path: /dir/path/on/nfs/server
    server: nfs-server-ip-address
  mountOptions:
    - hard
    - nfsvers=4.0
  #storageClassName: slow

spec.awsElasticBlockStore

• AWS Elastic Block Store (EBS)

apiVersion: v1
kind: PersistentVolume
metadata:
  name: my-pv
spec:
  capacity:
    storage: 1Gi
  accessModes:
    - ReadWriteOnce
  awsElasticBlockStore:
    volumeID: <volume-id>
    fsType: ext4

spec.gcePersistentDisk

apiVersion: v1
kind: PersistentVolume
metadata:
  name: my-pv
  labels:
    failure-domain.beta.kubernetes.io/zone: us-central1-a__us-central1-b
spec:
  capacity:
    storage: 400Gi
  accessModes:
    - ReadWriteOnce
  gcePersistentDisk:
    pdName: my-data-disk
    fsType: ext4

csi

• Use a Container Storage Interface (CSI) as a backend
• Static provisioning

apiVersion: v1
kind: PersistentVolume
metadata:
  name: my-pv
spec:
  capacity:
    storage: 5Gi
  csi:
    driver: ebs.csi.aws.com
    fsType: ext4
    volumeHandle: vol-02813386e90b6186c # EBS volume must be created beforehand
  accessModes:
    - ReadWriteOnce
  nodeAffinity:
    required:
      nodeSelectorTerms:
        - matchExpressions:
            - key: topology.kubernetes.io/zone
              operator: In
              values:
                - us-east-1a