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Kubernetes - Volumes

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By thirtyone
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Kubernetes - Volumes

1. Why Storage Is Hard in Kubernetes

Containers are ephemeral by design:

  • Containers restart
  • Pods get rescheduled
  • Nodes fail

By default, container filesystems do not persist.

Kubernetes solves this problem using Volumes, which decouple data from containers.

2. Kubernetes Volumes: The Foundation

A Volume is a directory mounted into a container, backed by some storage.

Key Properties

  • Defined at the Pod level
  • Data survives container restarts
  • Shared between containers in the same Pod
  • Exists as long as the Pod exists

Volumes are the base abstraction for all Kubernetes storage.

3. Ephemeral Volumes: Pod-Scoped Temporary Storage

Ephemeral volumes exist only for the lifetime of a Pod.

Common Types

  • emptyDir – scratch space, caching, shared workspace
  • configMap – configuration files
  • secret – credentials
  • downwardAPI – Pod metadata

When to Use

  1. Caches
  2. Temporary files
  3. Shared workspace
  4. Config and secret injection

4. The Real Problem: Data That Must Survive Pods

Stateful workloads (databases, queues, file systems) must survive:

  • Pod restarts
  • Rescheduling
  • Node failures

This requires Persistent Storage.

5. PersistentVolume (PV): The Actual Storage

A PersistentVolume (PV) is a cluster-level storage resource.

Characteristics

  • Independent of Pods
  • Represents real storage
  • Created manually or dynamically
  • Has capacity, access modes(ReadWriteOnce, ReadOnlyMany, ReadWriteMany), reclaim policy

PV is infrastructure-oriented ie- it represents actual physical or cloud storage.

6. PersistentVolumeClaim (PVC): Storage Request

A PVC is a request for storage by an application.

PVC Specifies

  • Storage size
  • Access mode

Kubernetes binds the PVC to a matching PV.

PVC is application-oriented.

7. PV vs PVC; tldr

PV is supply, PVC is demand — Kubernetes does the matching.

8. StorageClass: Blueprint for Storage

A StorageClass defines how storage is provisioned.

What It Controls

  • Storage backend (CSI driver)
  • Performance tier
  • Reclaim policy
  • Volume expansion
  • Binding behavior
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apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: fast-ssd
provisioner: ebs.csi.aws.com
parameters:
  type: gp3
reclaimPolicy: Retain
allowVolumeExpansion: true
volumeBindingMode: WaitForFirstConsumer

provisioner – Specifies which CSI driver will create the storage volume (here, AWS EBS CSI). parameters – Defines storage-specific settings such as disk type, performance, or filesystem. reclaimPolicy – Determines what happens to the underlying storage when the PVC is deleted (Retain or Delete). allowVolumeExpansion – Allows the volume size to be increased after it has been created. volumeBindingMode – Controls when the volume is provisioned; WaitForFirstConsumer waits until a Pod is scheduled to avoid zone mismatch issues.

Enables dynamic provisioning — no manual PV creation.

9. CSI Drivers: How Kubernetes Talks to Storage

CSI (Container Storage Interface) is the standard interface for storage plugins.

Why CSI Exists

Before CSI:

  • Storage code lived inside Kubernetes core
  • Hard to maintain and extend

With CSI:

  • Storage plugins are external
  • Vendors ship and maintain drivers
  • Kubernetes core remains stable

CSI Responsibilities

  • Create/delete volumes
  • Attach/detach volumes
  • Mount/unmount volumes
  • Resize volumes
  • Create snapshots

10. Projected Volumes: Clean Config & Secret Injection

A Projected Volume combines multiple volume sources into one directory.

Supported Sources

  • Secret
  • ConfigMap

Why It’s Useful

  • Single mount point
  • Cleaner Pod specs
  • Easier application logic

Commonly used for config + credentials + metadata.

Final TL;DR

  • Volumes solve container filesystem ephemerality
  • Ephemeral volumes are Pod-lifetime only
  • PV/PVC decouple storage from compute
  • StorageClasses enable dynamic provisioning
  • CSI drivers are the modern storage standard
  • Projected volumes simplify config & secret management
K8s
kubernetes volumes
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