Understanding Kubernetes: Part 38 Container Storage Interface (CSI)

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Understanding Kubernetes: Part 38 Container Storage Interface (CSI)

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Container Storage Interface (CSI) in Kubernetes

The Container Storage Interface (CSI) is a standardized API that allows Kubernetes to interact with various storage systems in a uniform way. Before CSI, Kubernetes relied on in-tree storage plugins, which required updating Kubernetes itself to add support for new storage providers. CSI decouples storage management from Kubernetes, enabling the use of external storage solutions without modifying the core Kubernetes code.

Why is CSI Important?

Flexibility — Allows Kubernetes to work with different storage providers (AWS EBS, Azure Disk, Ceph, etc.).
Extensibility — New storage solutions can be integrated without modifying Kubernetes itself.
Consistency — Provides a standard way to provision, mount, and manage storage across different environments.
Better Maintenance — In-tree storage plugins are being deprecated in favor of CSI-based solutions.

How CSI Works in Kubernetes

CSI introduces a plugin-based architecture that enables storage providers to develop their own CSI drivers. These drivers communicate with external storage systems, allowing Kubernetes to perform storage-related operations like:

Provisioning — Creating new storage volumes dynamically.
Attachment — Attaching storage volumes to specific Kubernetes nodes.
Mounting — Making storage volumes available to containers.
Snapshot & Cloning — Creating backups and replicas of storage volumes.
Resizing & Deletion — Expanding or removing storage volumes when no longer needed.

CSI Components

A CSI-based storage solution typically consists of:

CSI Driver — The implementation provided by storage vendors (e.g., AWS EBS CSI driver, Azure Disk CSI driver).
CSI Controller Plugin — Manages volume provisioning, attachment, and snapshots.
CSI Node Plugin — Runs on each Kubernetes node and handles volume mounting and unmounting.
External Provisioner — Creates persistent volumes dynamically based on storage class settings.

Capabilities of CSI

Dynamic Volume Provisioning — Storage is allocated on demand, eliminating the need for pre-provisioned volumes.
Volume Expansion — Supports resizing persistent volumes without downtime.
Volume Snapshots & Cloning — Enables backups and restores of storage.
ReadWriteMany (RWX) Support — Allows multiple pods to share the same volume.
Customizability — Different storage backends can implement advanced features like encryption, caching, and replication.

Example: Checking CSI Health

To check if CSI is functioning correctly, you can use the following Kubernetes commands:

List Installed CSI Drivers

kubectl get csidrivers

This lists all CSI drivers installed on the cluster.

Check Storage Classes Managed by CSI

kubectl get storageclass

This displays available storage classes that use CSI drivers.

List Persistent Volumes (PVs) Created via CSI

kubectl get pv

This shows all persistent volumes provisioned by CSI.

Real-World Usage of CSI in Kubernetes

In my previous role as a DevOps Engineer, I managed Kubernetes clusters using AWS EBS CSI Driver for dynamically provisioning block storage. We ensured CSI was working properly by:

Monitoring volume health using Kubernetes events and logs.
Configuring snapshots for disaster recovery using CSI volume snapshot capabilities.
Testing volume expansion to ensure persistent volumes could scale as needed.
Migrating from in-tree EBS to CSI-based EBS drivers to align with Kubernetes best practices.

By leveraging CSI, we were able to achieve a highly scalable and flexible storage infrastructure that integrated seamlessly with Kubernetes.

Example: YAML Configuration for CSI Storage in Kubernetes

Define a Storage Class Using CSI (AWS EBS Example)

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: ebs-csi
provisioner: ebs.csi.aws.com
parameters:
  type: gp3
  encrypted: "true"
  fsType: ext4
reclaimPolicy: Delete
volumeBindingMode: WaitForFirstConsumer

This storage class dynamically provisions AWS EBS volumes using the EBS CSI driver.

Create a Persistent Volume Claim (PVC) Using CSI

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: ebs-pvc
spec:
  accessModes:
    - ReadWriteOnce
  storageClassName: ebs-csi
  resources:
    requests:
      storage: 10Gi

This PVC requests a 10GiB volume from the ebs-csi storage class.

Use the PVC in a Pod

apiVersion: v1
kind: Pod
metadata:
  name: csi-app
spec:
  containers:
    - name: app-container
      image: nginx
      volumeMounts:
        - mountPath: "/data"
          name: storage
  volumes:
    - name: storage
      persistentVolumeClaim:
        claimName: ebs-pvc

The pod mounts the 10GiB CSI-backed volume at /data.

Key Takeaways

CSI standardizes storage management in Kubernetes, making it more flexible and extensible.
It allows Kubernetes to support multiple storage backends without modifying the kubelet.
Popular CSI implementations include AWS EBS, Azure Disk, Ceph, NFS, and Portworx.
You can verify CSI health using kubectl get csidrivers and kubectl get storageclass.
Migrating from in-tree storage to CSI is recommended for better scalability and maintainability.

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