CSI

Introduction

Container Storage Interface (CSI) is an industry standard that enables storage vendors to develop plugins for container orchestrators like Kubernetes without touching the core orchestrator code. It defines a common API between container orchestrators and storage providers.

Key Concepts

• Volume — A unit of storage made available to a container workload.
• CSI Driver — A storage plugin that implements the CSI specification (Node, Controller, and Identity services).
• PersistentVolume (PV) — A piece of storage provisioned by the CSI driver.
• PersistentVolumeClaim (PVC) — A request for storage by a user/pod.
• StorageClass — Defines a class of storage with a specific CSI driver and parameters.
• VolumeSnapshot — A point-in-time copy of a volume.

Architecture

┌────────────────────────────────────────────────────┐
│                 Kubernetes Cluster                  │
│                                                    │
│  ┌─────────────┐         ┌───────────────────┐    │
│  │  kube-       │         │  CSI Driver       │    │
│  │  controller- │────────▶│  Controller Plugin │    │
│  │  manager     │         │  (CreateVolume,    │    │
│  └─────────────┘         │   DeleteVolume,    │    │
│                           │   Snapshot...)     │    │
│  ┌─────────────┐         ├───────────────────┤    │
│  │  kubelet     │────────▶│  Node Plugin       │    │
│  │  (per node)  │         │  (NodeStageVolume, │    │
│  └─────────────┘         │   NodePublish...)  │    │
│                           └────────┬──────────┘    │
│                                    │               │
│                           ┌────────▼──────────┐    │
│                           │  Storage Backend  │    │
│                           │  (AWS EBS, GCE PD,│    │
│                           │   Ceph, NFS...)   │    │
│                           └───────────────────┘    │
└────────────────────────────────────────────────────┘

CSI Sidecar Containers

Kubernetes provides standard sidecar containers that communicate with CSI drivers:

Sidecar	Description
external-provisioner	Watches PVCs and triggers CreateVolume / DeleteVolume
external-attacher	Watches VolumeAttachment and triggers ControllerPublish / Unpublish
external-snapshotter	Watches VolumeSnapshot and triggers CreateSnapshot
external-resizer	Watches PVCs for size changes and triggers ControllerExpandVolume
node-driver-registrar	Registers the CSI driver with kubelet
livenessprobe	Monitors CSI driver health

StorageClass Example

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: fast-ssd
provisioner: ebs.csi.aws.com
parameters:
  type: gp3
  iops: "5000"
  throughput: "250"
reclaimPolicy: Delete
volumeBindingMode: WaitForFirstConsumer
allowVolumeExpansion: true

PVC Example

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: my-data
spec:
  accessModes:
    - ReadWriteOnce
  storageClassName: fast-ssd
  resources:
    requests:
      storage: 50Gi

Common CSI Drivers

Driver	Storage Backend
aws-ebs-csi-driver	Amazon EBS
gcp-pd-csi-driver	Google Persistent Disk
azuredisk-csi-driver	Azure Managed Disks
csi-driver-nfs	NFS
rook-ceph-csi	Ceph (via Rook)
minio-csi	MinIO Object Storage
longhorn-csi	Longhorn Distributed Storage

Reference:

1. CSI Specification
2. Kubernetes CSI Developer Docs
3. CSI Drivers List