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Detailed explanations of Kubernetes system concepts and abstractions.

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Storage Classes

This document describes the concept of 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.

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

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
AzureFile ✓ Azure File
AzureDisk ✓ Azure Disk
CephFS - -
Cinder ✓ OpenStack Cinder
FC - -
FlexVolume - -
Flocker ✓ -
GCEPersistentDisk ✓ GCE
Glusterfs ✓ Glusterfs
iSCSI - -
PhotonPersistentDisk ✓ -
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-incubator/external-storage houses a library for writing external provisioners that implements the bulk of the specification plus various community-maintained external provisioners.

For example, NFS doesn’t provide an internal provisioner, but an external provisioner can be used. Some external provisioners are listed under the repository kubernetes-incubator/external-storage. 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.

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 neither the class nor PV, so mount of the PV will simply fail if one is invalid.

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

kind: StorageClass
apiVersion: storage.k8s.io/v1
metadata:
  name: slow
provisioner: kubernetes.io/aws-ebs
parameters:
  type: io1
  zones: us-east-1d, us-east-1c
  iopsPerGB: "10"

GCE

kind: StorageClass
apiVersion: storage.k8s.io/v1
metadata:
  name: slow
provisioner: kubernetes.io/gce-pd
parameters:
  type: pd-standard
  zones: us-central1-a, us-central1-b

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

kind: StorageClass
apiVersion: storage.k8s.io/v1
metadata:
  name: gold
provisioner: kubernetes.io/cinder
parameters:
  type: fast
  availability: nova

vSphere

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

     kind: StorageClass
 apiVersion: storage.k8s.io/v1
 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.

     kind: StorageClass
 apiVersion: storage.k8s.io/v1
 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

kind: StorageClass
apiVersion: storage.k8s.io/v1
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
  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

kind: StorageClass
apiVersion: storage.k8s.io/v1
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)

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

Azure File

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

During provision, a secret is created for 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.

Portworx Volume

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

ScaleIO

kind: StorageClass
apiVersion: storage.k8s.io/v1
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

kind: StorageClass
apiVersion: storage.k8s.io/v1
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 v1.10 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.
kind: StorageClass
apiVersion: storage.k8s.io/v1
metadata:
  name: local-storage
provisioner: kubernetes.io/no-provisioner
volumeBindingMode: WaitForFirstConsumer

Local volumes do not support dynamic provisioning yet, 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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