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Persistent Volumes

This document describes the current state of PersistentVolumes in Kubernetes. Familiarity with volumes is suggested.

  • TOC {:toc}

Introduction

Managing storage is a distinct problem from managing compute. The PersistentVolume subsystem provides an API for users and administrators that abstracts details of how storage is provided from how it is consumed. To do this we introduce two new API resources: PersistentVolume and PersistentVolumeClaim.

A PersistentVolume (PV) is a piece of networked storage in the cluster that has been provisioned by an administrator. It is a resource in the cluster just like a node is a cluster resource. PVs are volume plugins like Volumes, but have a lifecycle independent of any individual pod that uses the PV. This API object captures the details of the implementation of the storage, be that NFS, iSCSI, or a cloud-provider-specific storage system.

A PersistentVolumeClaim (PVC) is a request for storage by a user. It is similar to a pod. Pods consume node resources and PVCs consume PV resources. Pods can request specific levels of resources (CPU and Memory). Claims can request specific size and access modes (e.g., can be mounted once read/write or many times read-only).

While PersistentVolumeClaims allow a user to consume abstract storage resources, it is common that users need PersistentVolumes with varying properties, such as performance, for different problems. Cluster administrators need to be able to offer a variety of PersistentVolumes that differ in more ways than just size and access modes, without exposing users to the details of how those volumes are implemented. For these needs there is the StorageClass resource.

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.

Please see the detailed walkthrough with working examples.

Lifecycle of a volume and claim

PVs are resources in the cluster. PVCs are requests for those resources and also act as claim checks to the resource. The interaction between PVs and PVCs follows this lifecycle:

Provisioning

There are two ways PVs may be provisioned: statically or dynamically.

Static

A cluster administrator creates a number of PVs. They carry the details of the real storage which is available for use by cluster users. They exist in the Kubernetes API and are available for consumption.

Dynamic

When none of the static PVs the administrator created matches a user's PersistentVolumeClaim, the cluster may try to dynamically provision a volume specially for the PVC. This provisioning is based on StorageClasses: the PVC must request a class and the administrator must have created and configured that class in order for dynamic provisioning to occur. Claims that request the class "" effectively disable dynamic provisioning for themselves.

Binding

A user creates, or has already created in the case of dynamic provisioning, a PersistentVolumeClaim with a specific amount of storage requested and with certain access modes. A control loop in the master watches for new PVCs, finds a matching PV (if possible), and binds them together. If a PV was dynamically provisioned for a new PVC, the loop will always bind that PV to the PVC. Otherwise, the user will always get at least what they asked for, but the volume may be in excess of what was requested. Once bound, PersistentVolumeClaim binds are exclusive, regardless of the mode used to bind them.

Claims will remain unbound indefinitely if a matching volume does not exist. Claims will be bound as matching volumes become available. For example, a cluster provisioned with many 50Gi PVs would not match a PVC requesting 100Gi. The PVC can be bound when a 100Gi PV is added to the cluster.

Using

Pods use claims as volumes. The cluster inspects the claim to find the bound volume and mounts that volume for a pod. For volumes which support multiple access modes, the user specifies which mode desired when using their claim as a volume in a pod.

Once a user has a claim and that claim is bound, the bound PV belongs to the user for as long as they need it. Users schedule Pods and access their claimed PVs by including a persistentVolumeClaim in their Pod's volumes block. See below for syntax details.

Releasing

When a user is done with their volume, they can delete the PVC objects from the API which allows reclamation of the resource. The volume is considered "released" when the claim is deleted, but it is not yet available for another claim. The previous claimant's data remains on the volume which must be handled according to policy.

Reclaiming

The reclaim policy for a PersistentVolume tells the cluster what to do with the volume after it has been released of its claim. Currently, volumes can either be Retained, Recycled or Deleted. Retention allows for manual reclamation of the resource. For those volume plugins that support it, deletion removes both the PersistentVolume object from Kubernetes, as well as deleting the associated storage asset in external infrastructure (such as an AWS EBS, GCE PD, Azure Disk, or Cinder volume). Volumes that were dynamically provisioned are always deleted.

Recycling

If supported by appropriate volume plugin, recycling performs a basic scrub (rm -rf /thevolume/*) on the volume and makes it available again for a new claim.

However, an administrator can configure a custom recycler pod templates using the Kubernetes controller manager command line arguments as described here. The custom recycler pod template must contain a volumes specification, as shown in the example below:

apiVersion: v1
kind: Pod
metadata:
  name: pv-recycler-
  namespace: default
spec:
  restartPolicy: Never
  volumes:
  - name: vol
    hostPath:
      path: /any/path/it/will/be/replaced
  containers:
  - name: pv-recycler
    image: "gcr.io/google_containers/busybox"
    command: ["/bin/sh", "-c", "test -e /scrub && rm -rf /scrub/..?* /scrub/.[!.]* /scrub/*  && test -z \"$(ls -A /scrub)\" || exit 1"]
    volumeMounts:
    - name: vol
      mountPath: /scrub

However, the particular path specified in the custom recycler pod template in the volumes part is replaced with the particular path of the volume that is being recycled.

Types of Persistent Volumes

PersistentVolume types are implemented as plugins. Kubernetes currently supports the following plugins:

  • GCEPersistentDisk
  • AWSElasticBlockStore
  • AzureFile
  • AzureDisk
  • FC (Fibre Channel)
  • NFS
  • iSCSI
  • RBD (Ceph Block Device)
  • CephFS
  • Cinder (OpenStack block storage)
  • Glusterfs
  • VsphereVolume
  • HostPath (single node testing only -- local storage is not supported in any way and WILL NOT WORK in a multi-node cluster)

Persistent Volumes

Each PV contains a spec and status, which is the specification and status of the volume.

  apiVersion: v1
  kind: PersistentVolume
  metadata:
    name: pv0003
    annotations:
      volume.beta.kubernetes.io/storage-class: "slow"
  spec:
    capacity:
      storage: 5Gi
    accessModes:
      - ReadWriteOnce
    persistentVolumeReclaimPolicy: Recycle
    nfs:
      path: /tmp
      server: 172.17.0.2

Capacity

Generally, a PV will have a specific storage capacity. This is set using the PV's capacity attribute. See the Kubernetes Resource Model to understand the units expected by capacity.

Currently, storage size is the only resource that can be set or requested. Future attributes may include IOPS, throughput, etc.

Access Modes

A PersistentVolume can be mounted on a host in any way supported by the resource provider. As shown in the table below, providers will have different capabilities and each PV's access modes are set to the specific modes supported by that particular volume. For example, NFS can support multiple read/write clients, but a specific NFS PV might be exported on the server as read-only. Each PV gets its own set of access modes describing that specific PV's capabilities.

The access modes are:

  • ReadWriteOnce -- the volume can be mounted as read-write by a single node
  • ReadOnlyMany -- the volume can be mounted read-only by many nodes
  • ReadWriteMany -- the volume can be mounted as read-write by many nodes

In the CLI, the access modes are abbreviated to:

  • RWO - ReadWriteOnce
  • ROX - ReadOnlyMany
  • RWX - ReadWriteMany

Important! A volume can only be mounted using one access mode at a time, even if it supports many. For example, a GCEPersistentDisk can be mounted as ReadWriteOnce by a single node or ReadOnlyMany by many nodes, but not at the same time.

Volume Plugin ReadWriteOnce ReadOnlyMany ReadWriteMany
AWSElasticBlockStore x - -
AzureFile x x x
AzureDisk x - -
CephFS x x x
Cinder x - -
FC x x -
FlexVolume x x -
GCEPersistentDisk x x -
Glusterfs x x x
HostPath x - -
iSCSI x x -
NFS x x x
RBD x x -
VsphereVolume x - -

Class

A PV can have a class, which is specified by setting the volume.beta.kubernetes.io/storage-class annotation to the name of a StorageClass. A PV of a particular class can only be bound to PVCs requesting that class. A PV with no annotation or its class annotation set to "" has no class and can only be bound to PVCs that request no particular class.

In the future after beta, the volume.beta.kubernetes.io/storage-class annotation will become an attribute.

Reclaim Policy

Current reclaim policies are:

  • Retain -- manual reclamation
  • Recycle -- basic scrub ("rm -rf /thevolume/*")
  • Delete -- associated storage asset such as AWS EBS, GCE PD, Azure Disk, or OpenStack Cinder volume is deleted

Currently, only NFS and HostPath support recycling. AWS EBS, GCE PD, Azure Disk, and Cinder volumes support deletion.

Phase

A volume will be in one of the following phases:

  • Available -- a free resource that is not yet bound to a claim
  • Bound -- the volume is bound to a claim
  • Released -- the claim has been deleted, but the resource is not yet reclaimed by the cluster
  • Failed -- the volume has failed its automatic reclamation

The CLI will show the name of the PVC bound to the PV.

PersistentVolumeClaims

Each PVC contains a spec and status, which is the specification and status of the claim.

kind: PersistentVolumeClaim
apiVersion: v1
metadata:
  name: myclaim
  annotations:
    volume.beta.kubernetes.io/storage-class: "slow"
spec:
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 8Gi
  selector:
    matchLabels:
      release: "stable"
    matchExpressions:
      - {key: environment, operator: In, values: [dev]}

Access Modes

Claims use the same conventions as volumes when requesting storage with specific access modes.

Resources

Claims, like pods, can request specific quantities of a resource. In this case, the request is for storage. The same resource model applies to both volumes and claims.

Selector

Claims can specify a label selector to further filter the set of volumes. Only the volumes whose labels match the selector can be bound to the claim. The selector can consist of two fields:

  • matchLabels - the volume must have a label with this value
  • matchExpressions - a list of requirements made by specifying key, list of values, and operator that relates the key and values. Valid operators include In, NotIn, Exists, and DoesNotExist.

All of the requirements, from both matchLabels and matchExpressions are ANDed together – they must all be satisfied in order to match.

Class

A claim can request a particular class by specifying the name of a StorageClassusing the annotation volume.beta.kubernetes.io/storage-class. Only PVs of the requested class, ones with the same annotation as the PVC, can be bound to the PVC.

PVCs don't necessarily have to request a class. A PVC with its annotation set equal to "" is always interpreted to be requesting a PV with no class, so it can only be bound to PVs with no class (no annotation or one set equal to ""). A PVC with no annotation is not quite the same and is treated differently by the cluster depending on whether the DefaultStorageClass admission plugin is turned on.

  • If the admission plugin is turned on, the administrator may specify a default StorageClass. All PVCs that have no annotation can be bound only to PVs of that default. Specifying a default StorageClass is done by setting the annotation storageclass.beta.kubernetes.io/is-default-class equal to "true" in a StorageClass object. If the administrator does not specify a default, the cluster responds to PVC creation as if the admission plugin were turned off. If more than one default is specified, the admission plugin forbids the creation of all PVCs.
  • If the admission plugin is turned off, there is no notion of a default StorageClass. All PVCs that have no annotation can be bound only to PVs that have no class. In this case the PVCs that have no annotation are treated the same way as PVCs that have their annotation set to "".

When a PVC specifies a selector in addition to requesting a StorageClass, the requirements are ANDed together: only a PV of the requested class and with the requested labels may be bound to the PVC. Note that currently, a PVC with a non-empty selector can't have a PV dynamically provisioned for it.

In the future after beta, the volume.beta.kubernetes.io/storage-class annotation will become an attribute.

Claims As Volumes

Pods access storage by using the claim as a volume. Claims must exist in the same namespace as the pod using the claim. The cluster finds the claim in the pod's namespace and uses it to get the PersistentVolume backing the claim. The volume is then mounted to the host and into the pod.

kind: Pod
apiVersion: v1
metadata:
  name: mypod
spec:
  containers:
    - name: myfrontend
      image: dockerfile/nginx
      volumeMounts:
      - mountPath: "/var/www/html"
        name: mypd
  volumes:
    - name: mypd
      persistentVolumeClaim:
        claimName: myclaim

A Note on Namespaces

PersistentVolumes binds are exclusive, and since PersistentVolumeClaims are namespaced objects, mounting claims with "Many" modes (ROX, RWX) is only possible within one namespace.

StorageClasses

Each StorageClass contains the fields provisioner and parameters, 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/v1beta1
metadata:
  name: standard
provisioner: kubernetes.io/aws-ebs
parameters:
  type: gp2

Provisioner

Storage classes have a provisioner that determines what volume plugin is used for provisioning PVs. This field must be specified. During beta, the available provisioner types are kubernetes.io/aws-ebs and kubernetes.io/gce-pd.

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/v1beta1
metadata:
  name: slow
provisioner: kubernetes.io/aws-ebs
parameters:
  type: io1
  zone: us-east-1d
  iopsPerGB: "10"
  • type: io1, gp2, sc1, st1. See AWS docs for details. Default: gp2.
  • zone: AWS zone. If not specified, a random zone from those where Kubernetes cluster has a node is chosen.
  • iopsPerGB: only for io1 volumes. I/O operations per second per GiB. AWS volume plugin multiplies this with size of requested volume to compute IOPS of the volume and caps it at 20 000 IOPS (maximum supported by AWS, see AWS docs. A string is expected here, i.e. "10", not 10.
  • encrypted: denotes whether the EBS volume should be encrypted or not. Valid values are "true" or "false". A string is expected here, i.e. "true", not true.
  • kmsKeyId: optional. The full Amazon Resource Name of the key to use when encrypting the volume. If none is supplied but encrypted is true, a key is generated by AWS. See AWS docs for valid ARN value.

GCE

kind: StorageClass
apiVersion: storage.k8s.io/v1beta1
metadata:
  name: slow
provisioner: kubernetes.io/gce-pd
parameters:
  type: pd-standard
  zone: us-central1-a
  • type: pd-standard or pd-ssd. Default: pd-ssd
  • zone: GCE zone. If not specified, a random zone in the same region as controller-manager will be chosen.

Glusterfs

apiVersion: storage.k8s.io/v1beta1
kind: StorageClass
metadata:
  name: slow
provisioner: kubernetes.io/glusterfs
parameters:
  resturl: "http://127.0.0.1:8081"
  restauthenabled: "true"
  restuser: "admin"
  secretNamespace: "default"
  secretName: "heketi-secret"
  • resturl: Gluster REST service/Heketi service url which provision gluster volumes on demand. The general format should be IPaddress:Port and this is a mandatory parameter for GlusterFS dynamic provisioner. If Heketi service is exposed as a routable service in openshift/kubernetes setup, this can have a format similar to http://heketi-storage-project.cloudapps.mystorage.com where the fqdn is a resolvable heketi service url.
  • restauthenabled : Gluster REST service authentication boolean that enables authentication to the REST server. If this value is 'true', restuser and restuserkey or secretNamespace + secretName have to be filled. This option is deprecated, authentication is enabled when any of restuser, restuserkey, secretName or secretNamespace is specified.
  • restuser : Gluster REST service/Heketi user who has access to create volumes in the Gluster Trusted Pool.
  • restuserkey : Gluster REST service/Heketi user's password which will be used for authentication to the REST server. This parameter is deprecated in favor of secretNamespace + secretName.
  • secretNamespace + secretName : Identification of Secret instance that containes user password to use when talking to Gluster REST service. These parameters are optional, empty password will be used when both secretNamespace and secretName are omitted. The provided secret must have type "kubernetes.io/glusterfs", e.g. created in this way: 
    $ kubectl create secret heketi-secret --type="kubernetes.io/glusterfs" --from-literal=key='opensesame' --namespace=default

OpenStack Cinder

kind: StorageClass
apiVersion: storage.k8s.io/v1beta1
metadata:
  name: gold
provisioner: kubernetes.io/cinder
parameters:
  type: fast
  availability: nova
  • type: VolumeType created in Cinder. Default is empty.
  • availability: Availability Zone. Default is empty.

vSphere

kind: StorageClass
apiVersion: storage.k8s.io/v1beta1
metadata:
  name: fast
provisioner: kubernetes.io/vsphere-volume
parameters:
  diskformat: zeroedthick
  • diskformat: thin, zeroedthick and eagerzeroedthick. Default: "thin".

Ceph RBD

  apiVersion: storage.k8s.io/v1beta1
  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
  • monitors: Ceph monitors, comma delimited. This parameter is required.
  • adminId: Ceph client ID that is capable of creating images in the pool. Default is "admin".
  • adminSecretNamespace: The namespace for adminSecret. Default is "default".
  • adminSecret: Secret Name for adminId. This parameter is required. The provided secret must have type "kubernetes.io/rbd".
  • pool: Ceph RBD pool. Default is "rbd".
  • userId: Ceph client ID that is used to map the RBD image. Default is the same as adminId.
  • userSecretName: The name of Ceph Secret for userId to map RBD image. It must exist in the same namespace as PVCs. This parameter is required. The provided secret must have type "kubernetes.io/rbd", e.g. created in this way: 
    $ kubectl create secret ceph-secret --type="kubernetes.io/rbd" --from-literal=key='QVFEQ1pMdFhPUnQrSmhBQUFYaERWNHJsZ3BsMmNjcDR6RFZST0E9PQ==' --namespace=kube-system

Quobyte

apiVersion: storage.k8s.io/v1beta1
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"
  • quobyteAPIServer: API Server of Quobyte in the format http(s)://api-server:7860
  • registry: Quobyte registry to use to mount the volume. You can specify the registry as <host>:<port> pair or if you want to specify multiple registries you just have to put a comma between them e.q. <host1>:<port>,<host2>:<port>,<host3>:<port>. The host can be an IP address or if you have a working DNS you can also provide the DNS names.
  • adminSecretNamespace: The namespace for adminSecretName. Default is "default".
  • adminSecretName: secret that holds information about the Quobyte user and the password to authenticate agains the API server. The provided secret must have type "kubernetes.io/quobyte", e.g. created in this way: 
    $ kubectl create secret quobyte-admin-secret --type="kubernetes.io/quobyte" --from-literal=key='opensesame' --namespace=kube-system
  • user: maps all access to this user. Default is "root".
  • group: maps all access to this group. Default is "nfsnobody".
  • quobyteConfig: use the specified configuration to create the volume. You can create a new configuration or modify an existing one with the Web console or the quobyte CLI. Default is "BASE".
  • quobyteTenant: use the specified tenant ID to create/delete the volume. This Quobyte tenant has to be already present in Quobyte. Default is "DEFAULT".

Azure Disk

kind: StorageClass
apiVersion: storage.k8s.io/v1beta1
metadata:
  name: slow
provisioner: kubernetes.io/azure-disk
parameters:
  skuName: Standard_LRS
  location: eastus
  storageAccount: azure_storage_account_name
  • skuName: Azure storage account Sku tier. Default is empty.
  • location: Azure storage account location. Default is empty.
  • storageAccount: Azure storage account name. If storage account is not provided, all storage accounts associated with the resource group are searched to find one that matches skuName and location. If storage account is provided, skuName and location are ignored.

Writing Portable Configuration

If you're writing configuration templates or examples that run on a wide range of clusters and need persistent storage, we recommend that you use the following pattern:

  • Do include PersistentVolumeClaim objects in your bundle of config (alongside Deployments, ConfigMaps, etc).
  • Do not include PersistentVolume objects in the config, since the user instantiating the config may not have permission to create PersistentVolumes.
  • Give the user the option of providing a storage class name when instantating the template.
    • If the user provides a storage class name, and the cluster is version 1.4 or newer, put that value into the volume.beta.kubernetes.io/storage-class annotation of the PVC. This will cause the PVC to match the right storage class if the cluster has StorageClasses enabled by the admin.
    • If the user does not provide a storage class name or the cluster is version 1.3, then instead put a volume.alpha.kubernetes.io/storage-class: default annotation on the PVC.
      • This will cause a PV to be automatically provisioned for the user with sane default characteristics on some clusters.
      • Despite the word alpha in the name, the code behind this annotation has beta level support.
      • Do not use volume.beta.kubernetes.io/storage-class: with any value including the empty string since it will prevent DefaultStorageClass admission controller from running if enabled.
  • In your tooling, do watch for PVCs that are not getting bound after some time and surface this to the user, as this may indicate that the cluster has no dynamic storage support (in which case the user should create a matching PV) or the cluster has no storage system (in which case the user cannot deploy config requiring PVCs).
  • In the future, we expect most clusters to have DefaultStorageClass enabled, and to have some form of storage available. However, there may not be any storage class names which work on all clusters, so continue to not set one by default. At some point, the alpha annotation will cease to have meaning, but the unset storageClass field on the PVC will have the desired effect.