MSDN Documentation

Kubernetes Storage Deep Dive: Understanding Persistent Storage in Containers

Kubernetes has revolutionized application deployment and management, but a critical aspect that often requires careful consideration is persistent storage. Unlike traditional applications that might have direct access to a file system, containerized applications in Kubernetes need a robust and flexible way to store data that survives pod restarts and scaling events. This article explores the fundamental concepts, APIs, and best practices for managing persistent storage in your Kubernetes environments.

The Challenge of Ephemeral Storage

By default, the storage attached to a container is ephemeral. When a pod terminates, all data within its containers is lost. This is acceptable for stateless applications, but for databases, caches, user uploads, or any application that needs to retain state, ephemeral storage is not an option. Kubernetes addresses this challenge through its storage primitives.

Key Kubernetes Storage Concepts

• Volumes: The most basic unit of storage in Kubernetes. Volumes are mounted into pods and provide a directory that can be shared among containers in the same pod. They have a lifecycle independent of the container, meaning data persists even if the container restarts.
• PersistentVolumes (PVs): These are cluster-level resources representing a piece of storage in the cluster. They are provisioned by an administrator or dynamically. PVs are abstractions of actual storage implementations (e.g., NFS, cloud provider disks).
• PersistentVolumeClaims (PVCs): These are requests for storage by users. A PVC consumes PV resources. Pods request storage by referencing a PVC, abstracting the underlying storage details from the application developer.
• StorageClasses: These provide a way for administrators to describe the "classes" of storage they offer. Different StorageClasses might map to different quality-of-service levels, backup policies, or access modes. They enable dynamic provisioning of PVs.

Understanding the Storage Workflow

The typical workflow for using persistent storage in Kubernetes involves these steps:

1. Administrator Setup: A cluster administrator configures available storage resources, potentially defining StorageClasses that map to various storage backends like AWS EBS, Google Persistent Disks, Azure Disk, Ceph, or NFS.
2. User Request: A developer or application operator creates a PersistentVolumeClaim (PVC) specifying the desired storage capacity, access modes (e.g., ReadWriteOnce, ReadOnlyMany, ReadWriteMany), and optionally a StorageClass.
3. Provisioning: If a StorageClass is specified, Kubernetes dynamically provisions a PersistentVolume (PV) matching the PVC's requirements. If no StorageClass is found or dynamic provisioning is disabled, an administrator must pre-create a PV that the PVC can bind to.
4. Binding: The PVC is then bound to a suitable PV.
5. Pod Consumption: A pod is configured to use the PVC by referencing it in its volume definitions. Kubernetes mounts the volume into the pod's containers.

Types of Volumes

Kubernetes supports various volume types, each suited for different use cases:

• `emptyDir`: A temporary directory created when a pod is assigned to a node. It exists as long as the pod is running on that node. Useful for scratch space or passing data between containers in a pod.
• `hostPath`: Mounts a file or directory from the host node's filesystem into a pod. Use with caution, as it ties the pod to a specific node and can have security implications.
• `persistentVolumeClaim`: The most common way to provide persistent storage. References a PVC that is already bound to a PV.
• Cloud Provider Volumes: Integrations with cloud provider storage services (e.g., `awsElasticBlockStore`, `gcePersistentDisk`, `azureDisk`).
• Network Storage: Support for network file systems like NFS, iSCSI, Glusterfs, Ceph, etc.

Access Modes Explained

Access modes define how a volume can be mounted to nodes:

• `ReadWriteOnce` (RWO): The volume can be mounted as read-write by a single node. This is the most common mode and is suitable for most single-instance applications or applications with a single writer.
• `ReadOnlyMany` (ROX): The volume can be mounted read-only by many nodes. Useful for distributing static configuration files or read-heavy datasets.
• `ReadWriteMany` (RWX): The volume can be mounted as read-write by many nodes. This requires a distributed storage solution (e.g., NFS, CephFS, GlusterFS) and is essential for multi-writer applications.
• `ReadWriteOncePod` (RWOP): The volume can be mounted as read-write by a single pod. This is a newer access mode providing a stronger guarantee than RWO, ensuring that only one pod across the entire cluster can access the volume at a time.

Dynamic vs. Static Provisioning

Static Provisioning involves a cluster administrator manually creating `PersistentVolume` objects that represent existing storage. A `PersistentVolumeClaim` then binds to one of these pre-existing PVs.

Dynamic Provisioning, facilitated by `StorageClass` objects, automates the creation of `PersistentVolume` objects on demand. When a `PersistentVolumeClaim` requests storage using a `StorageClass`, Kubernetes invokes the provisioner specified in that `StorageClass` to create a new PV. This is the preferred method in most modern Kubernetes deployments for its flexibility and ease of management.

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: standard
provisioner: kubernetes.io/aws-ebs # Example for AWS
parameters:
  type: gp2 # Example EBS volume type
  fsType: ext4
---
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: my-database-pvc
spec:
  storageClassName: standard
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 10Gi
            

Best Practices

• Prefer Dynamic Provisioning: Use `StorageClass` for automatic PV creation to simplify administration.
• Choose Appropriate Access Modes: Select access modes that match your application's requirements (RWO, ROX, RWX).
• Understand PV/PVC Lifecycle: Be aware of the `reclaimPolicy` of your PVs (`Retain`, `Delete`, `Recycle`) to prevent accidental data loss.
• Monitor Storage Usage: Implement monitoring for disk usage and IOPS to ensure performance and capacity.
• Back Up Your Data: Persistent storage in Kubernetes is not inherently backed up. Implement a robust backup strategy for your persistent volumes.
• Consider StatefulSets: For stateful applications like databases, `StatefulSets` provide stable network identifiers, stable persistent storage, and ordered graceful deployment and scaling.

Mastering Kubernetes storage is crucial for running stateful applications reliably and efficiently. By understanding PVs, PVCs, StorageClasses, and access modes, you can build resilient and scalable containerized applications that leverage the full power of Kubernetes.