Persistent Volume (PV) & Persistent Volume Claim (PVC) in Kubernetes.

1️. What is a Persistent Volume (PV)?

A Persistent Volume (PV) is a provisioned storage resource that administrators configure in the cluster. It can be backed by local storage, cloud storage (AWS EBS, Azure Disk, GCE Persistent Disk), or network storage (NFS, Ceph, etc.).

There are two main ways to create a PV volume.

# kubectl get pv -o wide

Or

# kubectl describe pv <pv_name>

Explanation of Parameters 

Detailed Explanation of Key Fields:-

1. ACCESS MODES

• RWO (ReadWriteOnce) → Can be mounted as read-write by a single node.
• ROX (ReadOnlyMany) → Can be mounted as read-only by multiple nodes.
• RWX (ReadWriteMany) → Can be mounted as read-write by multiple nodes.

2. RECLAIM POLICY

Determines what happens to the PV after the PVC using it is deleted.

• Retain → The PV is not deleted and must be manually cleaned up.
• Recycle → (Deprecated) The PV is scrubbed and made available again.
• Delete → The PV and its associated storage are automatically deleted.

3. STATUS

Shows the current state of the PV:

• Available → The PV is ready to be claimed by a PVC.
• Bound → The PV is claimed and in use by a PVC.
• Released → The PVC has been deleted, but the PV is not yet reusable.
• Failed → The PV encountered an error.

4. storageclass

• Defines the storage provisioning method.
• If manual, it means the PV was manually created.
• If using dynamic provisioning (e.g., AWS EBS, GCE Persistent Disk), it would show a class like gp2 (AWS EBS default class).
• StorageClass can be defined with any name.

5.VOLUMEATTRIBUTESCLASS

• Used in Container Storage Interface (CSI) drivers.
• <unset> means it’s not configured.
• If a CSI driver is used, this may contain extra volume attributes.

6.VOLUME MODE: Filesystem

In Kubernetes, VOLUME MODE determines how a Persistent Volume (PV) is formatted and used by a pod. The two possible values are:

1.     Filesystem (default)

•  The volume is formatted with a filesystem (e.g., ext4, xfs) and mounted into a pod as a directory.
• Used for normal file-based access.
• Example: A pod writing logs or storing application data in a directory.

2.     Block

• The volume is presented as a raw block device to the container.
• No filesystem is applied, and the application must handle reading/writing directly to the device.
• Used for databases or high-performance applications that need direct disk access.

There are two main ways to create a PV volume.

Types of Persistent Volumes (PV):

  Static Provisioning:

• Admin manually creates PVs before they are requested by PVCs.
• Example: Using an NFS server, manually setting up PVs.
• The administrator will first create the PV manually, then tell the developer, and then the developer will add the PVC size to the manifest file.     

Dynamic Provisioning:

• Kubernetes automatically provisions PVs when a PVC is created.
• Uses StorageClass to define the type of storage (AWS, Azure, GCE, etc.).
• Example: Cloud-based storage dynamically allocated to PVCs.
• It is the method in which the cluster administrator doesn’t provision the PVs manually beforehand. Rather in this process, whenever a POD requires some storage space, the Kubernetes cluster then and there provision the required PV for that POD and allocate that PV to it.

Note :-

1. In static, once the PVC is removed from the PV, the PV does not mount any PVC. If you edit the static PV, then PVC will be mounted again, but it is best to use dynamic PV.

2. bind PVC to PV.
The PVC will be mounted only when all the resources like STORAGECLASS , RECLAIM POLICY and  ACCESS MODES of the PV match with the PVC. If there is no match then the PVC will go into pending status.

############# PVC ############

2.What is a Persistent Volume Claim (PVC)?

A Persistent Volume Claim (PVC) is a request made by a pod to use a PV. The pod does not directly mount a PV; instead, it claims storage through a PVC.

# kubectl  get pvc -o wide

Explanation of Parameters

Detailed Explanation of Key Fields

1️.NAME

• This is the name of the PVC.
• In this case, mysql-data-mysql-0 is likely a PVC created by a StatefulSet for a MySQL Pod.

2️.STATUS

Indicates the current state of the PVC:

• Pending → The PVC is waiting for a matching PV to be created or bound.
• Bound → The PVC is successfully linked to a PV.
• Lost → The PV was deleted but the PVC still exists.

Why is the PVC in Pending state?

• No available PV matches the PVC's storageClass, size, or access mode , selector and label
• Dynamic provisioning is not enabled or failed.

3️.VOLUME

• Displays the name of the PV that has been bound to the PVC.

4️.CAPACITY

• Shows the allocated storage from the PV once it's bound.

5️.ACCESS MODES

Defines how the volume can be accessed:

• RWO (ReadWriteOnce) → Mounted as read/write by a single node.
• ROX (ReadOnlyMany) → Mounted as read-only by multiple nodes.
• RWX (ReadWriteMany) → Mounted as read/write by multiple nodes.

Example:

• If using local storage or hostPath, you typically see RWO.
• If using NFS or CephFS, you may see RWX or ROX.

6️.STORAGECLASS

• Defines the storage provisioning method.
• Here, mysql-storage is specified, meaning Kubernetes is looking for a PV or dynamic storage with this class.
• If dynamic provisioning is enabled, the cluster should automatically create a PV for this PVC.

Troubleshooting Pending PVC with StorageClass:

• If no PV is available with the required size and storage class, the PVC remains Pending.
• If using a dynamic storage class (e.g., AWS EBS, GCE Persistent Disk, NFS), make sure the provisioner is correctly set up.

7️.VOLUMEATTRIBUTESCLASS

• Used in Container Storage Interface (CSI) drivers.
• <unset> means it’s not in use.
• If a CSI driver is managing the volume, this field may contain extra attributes.

8️.AGE

• Shows how long ago the PVC was created.

Volume Plugin	ReadWriteOnce	ReadOnlyMany	ReadWriteMany	ReadWriteOncePod
AWSElasticBlockStore	Yes	No	No	No
AzureFile	Yes	Yes	Yes	No
AzureDisk	Yes	No	No	No
CephFS	Yes	Yes	Yes	No
Cinder	Yes	No	No	No
CSI	depends on the driver	depends on the driver	depends on the driver	depends on the driver
FC	Yes	Yes	No	No
FlexVolume	Yes	Yes	depends on the driver	No
Flocker	Yes	No	No	No
GCEPersistentDisk	Yes	Yes	No	No
Glusterfs	Yes	Yes	Yes	No
HostPath	Yes	No	No	No
iSCSI	Yes	Yes	No	No
Quobyte	Yes	Yes	Yes	No
NFS	Yes	Yes	Yes	No
RBD	Yes	Yes	No	No
VsphereVolume	Yes	No	- (works when Pods are collocated)	No
PortworxVolume	Yes	No	Yes	No
StorageOS	No	No	No	No

How PVC Binds to a PV.

Once a PVC is created, Kubernetes automatically finds a matching PV (based on storageClassName, accessModes, and capacity).

• If a matching PV is found, it is bound to the PVC.
• If no matching PV exists, the PVC remains in a "Pending" state until a PV is available.