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Storage Classes

This document describes the concept of a StorageClass in Kubernetes. Familiarity with volumes and persistent volumes is suggested.

Introduction

A StorageClass provides a way for administrators to describe the “classes” of storage they offer. Different classes might map to quality-of-service levels, or to backup policies, or to arbitrary policies determined by the cluster administrators. Kubernetes itself is unopinionated about what classes represent. This concept is sometimes called “profiles” in other storage systems.

The StorageClass Resource

Each StorageClass contains the fields provisioner, parameters, and reclaimPolicy, which are used when a PersistentVolume belonging to the class needs to be dynamically provisioned.

The name of a StorageClass object is significant, and is how users can request a particular class. Administrators set the name and other parameters of a class when first creating StorageClass objects, and the objects cannot be updated once they are created.

Administrators can specify a default StorageClass just for PVCs that don’t request any particular class to bind to: see the PersistentVolumeClaim section for details.

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: standard
provisioner: kubernetes.io/aws-ebs
parameters:
  type: gp2
reclaimPolicy: Retain
allowVolumeExpansion: true
mountOptions:
  - debug
volumeBindingMode: Immediate

Provisioner

Storage classes have a provisioner that determines what volume plugin is used for provisioning PVs. This field must be specified.

Volume Plugin Internal Provisioner Config Example
AWSElasticBlockStore ✓ AWS EBS
AzureFile ✓ Azure File
AzureDisk ✓ Azure Disk
CephFS - -
Cinder ✓ OpenStack Cinder
FC - -
FlexVolume - -
Flocker ✓ -
GCEPersistentDisk ✓ GCE PD
Glusterfs ✓ Glusterfs
iSCSI - -
Quobyte ✓ Quobyte
NFS - -
RBD ✓ Ceph RBD
VsphereVolume ✓ vSphere
PortworxVolume ✓ Portworx Volume
ScaleIO ✓ ScaleIO
StorageOS ✓ StorageOS
Local - Local

You are not restricted to specifying the “internal” provisioners listed here (whose names are prefixed with “kubernetes.io” and shipped alongside Kubernetes). You can also run and specify external provisioners, which are independent programs that follow a specification defined by Kubernetes. Authors of external provisioners have full discretion over where their code lives, how the provisioner is shipped, how it needs to be run, what volume plugin it uses (including Flex), etc. The repository kubernetes-sigs/sig-storage-lib-external-provisioner houses a library for writing external provisioners that implements the bulk of the specification. Some external provisioners are listed under the repository kubernetes-incubator/external-storage.

For example, NFS doesn’t provide an internal provisioner, but an external provisioner can be used. There are also cases when 3rd party storage vendors provide their own external provisioner.

Reclaim Policy

Persistent Volumes that are dynamically created by a storage class will have the reclaim policy specified in the reclaimPolicy field of the class, which can be either Delete or Retain. If no reclaimPolicy is specified when a StorageClass object is created, it will default to Delete.

Persistent Volumes that are created manually and managed via a storage class will have whatever reclaim policy they were assigned at creation.

Allow Volume Expansion

FEATURE STATE: Kubernetes v1.11 beta
This feature is currently in a beta state, meaning:

  • The version names contain beta (e.g. v2beta3).
  • Code is well tested. Enabling the feature is considered safe. Enabled by default.
  • Support for the overall feature will not be dropped, though details may change.
  • The schema and/or semantics of objects may change in incompatible ways in a subsequent beta or stable release. When this happens, we will provide instructions for migrating to the next version. This may require deleting, editing, and re-creating API objects. The editing process may require some thought. This may require downtime for applications that rely on the feature.
  • Recommended for only non-business-critical uses because of potential for incompatible changes in subsequent releases. If you have multiple clusters that can be upgraded independently, you may be able to relax this restriction.
  • Please do try our beta features and give feedback on them! After they exit beta, it may not be practical for us to make more changes.

Persistent Volumes can be configured to be expandable. This feature when set to true, allows the users to resize the volume by editing the corresponding PVC object.

The following types of volumes support volume expansion, when the underlying Storage Class has the field allowVolumeExpansion set to true.

FEATURE STATE: Kubernetes v1.14 alpha
This feature is currently in a alpha state, meaning:

  • The version names contain alpha (e.g. v1alpha1).
  • Might be buggy. Enabling the feature may expose bugs. Disabled by default.
  • Support for feature may be dropped at any time without notice.
  • The API may change in incompatible ways in a later software release without notice.
  • Recommended for use only in short-lived testing clusters, due to increased risk of bugs and lack of long-term support.
Note: This feature cannot be used to shrink volumes.

Mount Options

Persistent Volumes that are dynamically created by a storage class will have the mount options specified in the mountOptions field of the class.

If the volume plugin does not support mount options but mount options are specified, provisioning will fail. Mount options are not validated on either the class or PV, so mount of the PV will simply fail if one is invalid.

Volume Binding Mode

The volumeBindingMode field controls when volume binding and dynamic provisioning should occur.

By default, the Immediate mode indicates that volume binding and dynamic provisioning occurs once the PersistentVolumeClaim is created. For storage backends that are topology-constrained and not globally accessible from all Nodes in the cluster, PersistentVolumes will be bound or provisioned without knowledge of the Pod’s scheduling requirements. This may result in unschedulable Pods.

A cluster administrator can address this issue by specifying the WaitForFirstConsumer mode which will delay the binding and provisioning of a PersistentVolume until a Pod using the PersistentVolumeClaim is created. PersistentVolumes will be selected or provisioned conforming to the topology that is specified by the Pod’s scheduling constraints. These include, but are not limited to, resource requirements, node selectors, pod affinity and anti-affinity, and taints and tolerations.

The following plugins support WaitForFirstConsumer with dynamic provisioning:

The following plugins support WaitForFirstConsumer with pre-created PersistentVolume binding:

FEATURE STATE: Kubernetes 1.14 beta
This feature is currently in a beta state, meaning:

  • The version names contain beta (e.g. v2beta3).
  • Code is well tested. Enabling the feature is considered safe. Enabled by default.
  • Support for the overall feature will not be dropped, though details may change.
  • The schema and/or semantics of objects may change in incompatible ways in a subsequent beta or stable release. When this happens, we will provide instructions for migrating to the next version. This may require deleting, editing, and re-creating API objects. The editing process may require some thought. This may require downtime for applications that rely on the feature.
  • Recommended for only non-business-critical uses because of potential for incompatible changes in subsequent releases. If you have multiple clusters that can be upgraded independently, you may be able to relax this restriction.
  • Please do try our beta features and give feedback on them! After they exit beta, it may not be practical for us to make more changes.

CSI volumes are also supported with dynamic provisioning and pre-created PVs, but you’ll need to look at the documentation for a specific CSI driver to see its supported topology keys and examples. The CSINodeInfo feature gate must be enabled.

Allowed Topologies

When a cluster operator specifies the WaitForFirstConsumer volume binding mode, it is no longer necessary to restrict provisioning to specific topologies in most situations. However, if still required, allowedTopologies can be specified.

This example demonstrates how to restrict the topology of provisioned volumes to specific zones and should be used as a replacement for the zone and zones parameters for the supported plugins.

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: standard
provisioner: kubernetes.io/gce-pd
parameters:
  type: pd-standard
volumeBindingMode: WaitForFirstConsumer
allowedTopologies:
- matchLabelExpressions:
  - key: failure-domain.beta.kubernetes.io/zone
    values:
    - us-central1-a
    - us-central1-b

Parameters

Storage classes have parameters that describe volumes belonging to the storage class. Different parameters may be accepted depending on the provisioner. For example, the value io1, for the parameter type, and the parameter iopsPerGB are specific to EBS. When a parameter is omitted, some default is used.

AWS EBS

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: slow
provisioner: kubernetes.io/aws-ebs
parameters:
  type: io1
  iopsPerGB: "10"
  fsType: ext4
Note: zone and zones parameters are deprecated and replaced with allowedTopologies

GCE PD

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: slow
provisioner: kubernetes.io/gce-pd
parameters:
  type: pd-standard
  replication-type: none

If replication-type is set to none, a regular (zonal) PD will be provisioned.

If replication-type is set to regional-pd, a Regional Persistent Disk will be provisioned. In this case, users must use zones instead of zone to specify the desired replication zones. If exactly two zones are specified, the Regional PD will be provisioned in those zones. If more than two zones are specified, Kubernetes will arbitrarily choose among the specified zones. If the zones parameter is omitted, Kubernetes will arbitrarily choose among zones managed by the cluster.

Note: zone and zones parameters are deprecated and replaced with allowedTopologies

Glusterfs

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: slow
provisioner: kubernetes.io/glusterfs
parameters:
  resturl: "http://127.0.0.1:8081"
  clusterid: "630372ccdc720a92c681fb928f27b53f"
  restauthenabled: "true"
  restuser: "admin"
  secretNamespace: "default"
  secretName: "heketi-secret"
  gidMin: "40000"
  gidMax: "50000"
  volumetype: "replicate:3"

OpenStack Cinder

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: gold
provisioner: kubernetes.io/cinder
parameters:
  availability: nova
Note:
FEATURE STATE: Kubernetes 1.11 deprecated
This feature is deprecated. For more information on this state, see the Kubernetes Deprecation Policy.

This internal provisioner of OpenStack is deprecated. Please use the external cloud provider for OpenStack.

vSphere

  1. Create a StorageClass with a user specified disk format.

    apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: fast
provisioner: kubernetes.io/vsphere-volume
parameters:
  diskformat: zeroedthick

    diskformat: thin, zeroedthick and eagerzeroedthick. Default: "thin".

  2. Create a StorageClass with a disk format on a user specified datastore.

    apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: fast
provisioner: kubernetes.io/vsphere-volume
parameters:
    diskformat: zeroedthick
    datastore: VSANDatastore

    datastore: The user can also specify the datastore in the StorageClass. The volume will be created on the datastore specified in the storage class, which in this case is VSANDatastore. This field is optional. If the datastore is not specified, then the volume will be created on the datastore specified in the vSphere config file used to initialize the vSphere Cloud Provider.

  3. Storage Policy Management inside kubernetes

    • Using existing vCenter SPBM policy

      One of the most important features of vSphere for Storage Management is policy based Management. Storage Policy Based Management (SPBM) is a storage policy framework that provides a single unified control plane across a broad range of data services and storage solutions. SPBM enables vSphere administrators to overcome upfront storage provisioning challenges, such as capacity planning, differentiated service levels and managing capacity headroom.

      The SPBM policies can be specified in the StorageClass using the storagePolicyName parameter.

    • Virtual SAN policy support inside Kubernetes

      Vsphere Infrastructure (VI) Admins will have the ability to specify custom Virtual SAN Storage Capabilities during dynamic volume provisioning. You can now define storage requirements, such as performance and availability, in the form of storage capabilities during dynamic volume provisioning. The storage capability requirements are converted into a Virtual SAN policy which are then pushed down to the Virtual SAN layer when a persistent volume (virtual disk) is being created. The virtual disk is distributed across the Virtual SAN datastore to meet the requirements.

      You can see Storage Policy Based Management for dynamic provisioning of volumes for more details on how to use storage policies for persistent volumes management.

There are few vSphere examples which you try out for persistent volume management inside Kubernetes for vSphere.

Ceph RBD

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: fast
provisioner: kubernetes.io/rbd
parameters:
  monitors: 10.16.153.105:6789
  adminId: kube
  adminSecretName: ceph-secret
  adminSecretNamespace: kube-system
  pool: kube
  userId: kube
  userSecretName: ceph-secret-user
  userSecretNamespace: default
  fsType: ext4
  imageFormat: "2"
  imageFeatures: "layering"

Quobyte

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
   name: slow
provisioner: kubernetes.io/quobyte
parameters:
    quobyteAPIServer: "http://138.68.74.142:7860"
    registry: "138.68.74.142:7861"
    adminSecretName: "quobyte-admin-secret"
    adminSecretNamespace: "kube-system"
    user: "root"
    group: "root"
    quobyteConfig: "BASE"
    quobyteTenant: "DEFAULT"

Azure Disk

Azure Unmanaged Disk Storage Class

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: slow
provisioner: kubernetes.io/azure-disk
parameters:
  skuName: Standard_LRS
  location: eastus
  storageAccount: azure_storage_account_name

New Azure Disk Storage Class (starting from v1.7.2)

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: slow
provisioner: kubernetes.io/azure-disk
parameters:
  storageaccounttype: Standard_LRS
  kind: Shared

Azure File

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: azurefile
provisioner: kubernetes.io/azure-file
parameters:
  skuName: Standard_LRS
  location: eastus
  storageAccount: azure_storage_account_name

During storage provisioning, a secret named by secretName is created for the mounting credentials. If the cluster has enabled both RBAC and Controller Roles, add the create permission of resource secret for clusterrole system:controller:persistent-volume-binder.

In a multi-tenancy context, it is strongly recommended to set the value for secretNamespace explicitly, otherwise the storage account credentials may be read by other users.

Portworx Volume

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: portworx-io-priority-high
provisioner: kubernetes.io/portworx-volume
parameters:
  repl: "1"
  snap_interval:   "70"
  io_priority:  "high"

ScaleIO

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: slow
provisioner: kubernetes.io/scaleio
parameters:
  gateway: https://192.168.99.200:443/api
  system: scaleio
  protectionDomain: pd0
  storagePool: sp1
  storageMode: ThinProvisioned
  secretRef: sio-secret
  readOnly: false
  fsType: xfs

The ScaleIO Kubernetes volume plugin requires a configured Secret object. The secret must be created with type kubernetes.io/scaleio and use the same namespace value as that of the PVC where it is referenced as shown in the following command:

kubectl create secret generic sio-secret --type="kubernetes.io/scaleio" \
--from-literal=username=sioadmin --from-literal=password=d2NABDNjMA== \
--namespace=default

StorageOS

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: fast
provisioner: kubernetes.io/storageos
parameters:
  pool: default
  description: Kubernetes volume
  fsType: ext4
  adminSecretNamespace: default
  adminSecretName: storageos-secret

The StorageOS Kubernetes volume plugin can use a Secret object to specify an endpoint and credentials to access the StorageOS API. This is only required when the defaults have been changed. The secret must be created with type kubernetes.io/storageos as shown in the following command:

kubectl create secret generic storageos-secret \
--type="kubernetes.io/storageos" \
--from-literal=apiAddress=tcp://localhost:5705 \
--from-literal=apiUsername=storageos \
--from-literal=apiPassword=storageos \
--namespace=default

Secrets used for dynamically provisioned volumes may be created in any namespace and referenced with the adminSecretNamespace parameter. Secrets used by pre-provisioned volumes must be created in the same namespace as the PVC that references it.

Local

FEATURE STATE: Kubernetes v1.14 stable
This feature is stable, meaning:

  • The version name is vX where X is an integer.
  • Stable versions of features will appear in released software for many subsequent versions.
apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: local-storage
provisioner: kubernetes.io/no-provisioner
volumeBindingMode: WaitForFirstConsumer

Local volumes do not currently support dynamic provisioning, however a StorageClass should still be created to delay volume binding until pod scheduling. This is specified by the WaitForFirstConsumer volume binding mode.

Delaying volume binding allows the scheduler to consider all of a pod’s scheduling constraints when choosing an appropriate PersistentVolume for a PersistentVolumeClaim.

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