Kubernetes provides powerful tools to manage storage for your applications. However, understanding concepts like Persistent Volumes (PVs), Persistent Volume Claims (PVCs), and Storage Classes can initially feel overwhelming. In this post, I’ll break these concepts down into simple, digestible pieces to help you get started.
What are Persistent Volumes (PVs)?
A Persistent Volume (PV) is storage in a Kubernetes cluster that exists independently of the lifecycle of pods. Think of it as a “storage unit” that your pods can use. Unlike ephemeral storage (which is deleted when a pod is deleted), PVs ensure your data persists.
PVs can be backed by various storage solutions, including:
- Local storage (like the disk of your node).
- Network File Systems (NFS).
- Cloud storage (AWS EBS, Azure Disk, Google Persistent Disk).
Admins create PVs, and these storage units remain in the cluster until they are manually deleted.
What are Persistent Volume Claims (PVCs)?
A Persistent Volume Claim (PVC) is how pods request storage in Kubernetes. If a PV is the “storage unit,” the PVC is the “rental agreement” for that storage. A pod defines how much storage it needs and the access type through a PVC.
PVCs specify:
- Size: How much storage is needed.
- Access Mode:
ReadWriteOnce(one node can write).ReadOnlyMany(multiple nodes can read).ReadWriteMany(multiple nodes can read and write).
When a PVC is created, Kubernetes searches for an available PV that matches the requirements and binds them together.
What is a Storage Class?
A Storage Class is like a template that defines how Kubernetes should provision storage dynamically when a PVC is created. Instead of relying on pre-existing PVs, the Storage Class can create PVs on demand.
With a Storage Class, you can:
- Specify storage type (e.g., SSD or HDD).
- Define cloud-specific parameters (e.g., AWS zone or replication settings).
- Automate the creation of PVs.
Admins or developers can set up multiple Storage Classes based on the application’s needs, such as fast-ssd, standard-hdd, or replicated.
How They Work Together
To understand how PVs, PVCs, and Storage Classes interact, let’s walk through the process step-by-step:
- A Pod Requests Storage
Your application pod needs persistent storage, so you define a PVC in your deployment manifest. - Kubernetes Looks for a Matching PV
3. If a suitable PV already exists in the cluster, Kubernetes binds it to the PVC.
4. If no matching PV exists, Kubernetes refers to the associated Storage Class (if defined) to dynamically create a new PV.
5. Storage is Mounted to the Pod
Once the PVC is bound to a PV, Kubernetes mounts the storage to your pod. Your application can now use it as a persistent storage space.
A Simple Analogy
Imagine you’re renting a storage unit:
- The Storage Unit (PV): The physical space that holds your stuff.
- The Rental Agreement (PVC): The paperwork you fill out, specifying the size and type of unit you need.
- The Storage Manager (Storage Class): The person who creates or allocates storage units on demand if none are available.
Example Use Case
Let’s consider an example where an application pod needs 5GB of storage:
- Pod Creates PVC:
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
name: my-pvc
spec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 5Gi
2. Kubernetes Searches for a PV:
3. If a 5GB PV exists, it binds the PVC to it.
4. If no PV is available, a Storage Class automatically provisions a new one.
5. Pod Accesses Storage:
The PVC is mounted to the pod as persistent storage.
Why Use Persistent Volumes and PVCs?
- Data Persistence: Your application data survives even if the pod is deleted or crashes.
- Flexibility: Applications request storage without worrying about the underlying implementation.
- Scalability: Use dynamic provisioning with Storage Classes to automate resource management.
Key Takeaways
- PVs: Pre-provisioned or dynamically created storage in your cluster.
- PVCs: Requests for storage made by pods.
- Storage Classes: Templates that define how storage should be provisioned dynamically.
By understanding these components, you can effectively manage persistent storage in your Kubernetes applications, ensuring data reliability and scalability.
Do you have questions about implementing these concepts in your projects? Let me know in the comments below! and please don’t forget to leave a lot of claps 😁😁😁