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Stateful applications (like databases) require persistent storage that survives Pod restarts and Node crashes. Kubernetes abstracts storage using Persistent Volumes (PVs) and Persistent Volume Claims (PVCs). Advanced storage management goes beyond these primitives to handle dynamic provisioning, resizing, and specialized storage classes.
A StorageClass provides a way for administrators to describe the "classes" of storage they offer. Different classes might map to quality-of-service levels (e.g., standard HDD vs. ultra-fast NVMe SSDs), or to backup policies.
When a developer creates a PVC, they specify the storageClassName. The cluster's storage provisioner automatically creates the physical disk in the cloud (like an AWS EBS volume) and binds it to the PVC.
Historically, expanding a persistent volume required significant downtime. Modern Kubernetes supports Online Volume Expansion. If your StorageClass has allowVolumeExpansion: true, you can simply edit your PVC YAML and increase the storage request.
Kubernetes will automatically tell the underlying cloud provider to resize the disk, and then resize the file system on the node while the Pod is still running!
The Container Storage Interface (CSI) is a standard for exposing arbitrary block and file storage systems to containerized workloads. Before CSI, cloud provider storage code (like AWS or GCP storage drivers) was baked directly into the core Kubernetes source code ("in-tree").
Now, storage vendors write independent CSI drivers ("out-of-tree"). You install a vendor's CSI driver into your cluster as a DaemonSet, allowing Kubernetes to communicate with third-party storage arrays like NetApp, Portworx, or Ceph without modifying the core Kubernetes code.
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