EN_K8s_Storage

Kubernetes Storage

7. Kubernetes Persistent Volumes (PV) and Persistent Volume Claims (PVC), StorageClass, and CSI (Container Storage Interface)

graph TD
    PV("Persistent Volume (PV)")
    PVC("Persistent Volume Claim (PVC)")
    Pod("Pod")
    Retain("Retain Policy")
    Delete("Delete Policy")
    Recycle("Recycle Policy")
    Provisioning("Provisioning")
    Binding("Binding")
    Using("Using")
    Releasing("Releasing")
    Reclaiming("Reclaiming")
    SC("Storage Class")
    Dynamic("Dynamic Provisioning")
    Static("Static Provisioning")
    CSI("CSI-Driver")
    Block("Block Storage")
    File("File Storage")
    Object("Object Storage")

    PV -- "Bound by" --> PVC
    PVC -- "Referenced by" --> Pod

    PV -- "Retain" --> Retain
    PV -- "Delete" --> Delete
    PV -- "Recycle" --> Recycle

    Provisioning -- "leads to" --> Binding
    Binding -- "leads to" --> Using
    Using -- "leads to" --> Releasing
    Releasing -- "leads to" --> Reclaiming

    SC -- "creates" --> PV
    SC -- "Defines provisioning" --> Dynamic
    SC -- "Defines provisioning" --> Static

    CSI -- "Manages" --> Block
    CSI -- "Manages" --> File
    CSI -- "Manages" --> Object

    PVC -- "Can specify" --> SC
    PV -- "Managed by" --> CSI

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Persistent Volumes (PV)

• Persistent Volumes (PV) are storage resources within a cluster, provisioned by an administrator or dynamically using a storage class.

• PVs exist as cluster resources, similar to nodes, and can be claimed by users.

• Key Features:

  • Life Cycle Independent of Pod: PVs are independent of the life cycle of individual pods that use them.
  • Storage Abstraction: Abstracts the specifics of storage provisioning and usage.
  • Supports Various Access Modes: Includes modes like ReadWriteOnce, ReadOnlyMany, and ReadWriteMany.
    • ReadWriteOnce: Mountable as read-write by a single node.
    • ReadOnlyMany: Mountable as read-only by multiple nodes.
    • ReadWriteMany: Mountable as read-write by multiple nodes.

Persistent Volume Claims (PVC)

• PVCs are requests for storage made by users, similar to how pods use node resources.

• Key Features:

  • Storage Request: Users request specific sizes and access modes.
  • Binding: Automatically binds to an appropriate PV in the cluster.
  • Usage in Pods: Used as a volume within a pod, referenced by name.

• persistentVolumeReclaimPolicy:

  • This field in the PV specifies the action on the volume post-release from the claim.
  • Policies:
    • Retain: Default policy to keep the volume and preserve data after release.
    • Delete: For dynamically provisioned PVs, deletes the volume from the underlying storage upon PVC deletion.
    • Recycle: Not recommended for dynamic provisioning. Used to clear the volume data for new claims.

PV and PVC Lifecycle

• Provisioning: PVs can be dynamically provisioned through storage classes or pre-provisioned manually.
• Binding: Users create PVCs requesting specific sizes and access modes, which then bind to available PVs.
• Using: Once bound, the PVC is used by pods, referencing and mounting the PV as a volume.
• Releasing: Users can delete the PVC after use. Depending on the reclaim policy, the PV either becomes available again or is deleted.
• Reclaiming: With Retain policy, PV remains in the cluster post-PVC deletion. With Delete, both PV and related external storage assets are deleted.

StorageClass

StorageClass allows administrators to define different "classes" of storage.

• Key Aspects:
  • Provisioning: Can be dynamic or static, defining how storage is allocated.
  • Parameters: Various parameters depending on the storage provider.
  • Binding: PVCs can specify a StorageClass. Only PVs of the same class can bind to a PVC.

Container Storage Interface (CSI)

CSI standardizes the exposure of block and file storage systems to workloads in COS like Kubernetes.

• Key Goal: Provide a consistent API for storage solutions, facilitating integration in cloud-native ecosystems.

Volume Types in CSI

CSI supports various volume types:

• Block Storage: Basic block-level storage, used mainly in performance-critical applications.
• File Storage: Shared or dedicated file access, often in shared storage systems.
• Object Storage: Stores each object separately, used mainly for unstructured data.

Volume Types Available in Pods

In addition to CSI volume types, pods can use other volume types:

• hostPath: Mounts a file/directory from the host node's filesystem into the pod.
• emptyDir: A temporary directory shared with the pod's lifespan.

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Reference

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