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Updated formatting in "Persistent Volumes" & "Storage Classes" (#19575)
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* Updated formatting in "Persistent Volumes" & "Storage Classes"

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72 changes: 33 additions & 39 deletions content/en/docs/concepts/storage/persistent-volumes.md
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Expand Up @@ -17,7 +17,7 @@ weight: 20


{{% capture overview %}} {{% capture overview %}}


This document describes the current state of `PersistentVolumes` in Kubernetes. Familiarity with [volumes](/docs/concepts/storage/volumes/) is suggested. This document describes the current state of _persistent volumes_ in Kubernetes. Familiarity with [volumes](/docs/concepts/storage/volumes/) is suggested.


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Expand All @@ -26,19 +26,13 @@ This document describes the current state of `PersistentVolumes` in Kubernetes.


## Introduction ## Introduction


Managing storage is a distinct problem from managing compute instances. 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`. Managing storage is a distinct problem from managing compute instances. 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 storage in the cluster that has been provisioned by an administrator or dynamically provisioned using [Storage Classes](/docs/concepts/storage/storage-classes/). 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 PersistentVolume (PV) is a piece of storage in the cluster that has been provisioned by an administrator or dynamically provisioned using [Storage Classes](/docs/concepts/storage/storage-classes/). 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., they can be mounted once read/write or many times read-only). 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., they can be mounted once read/write or many times read-only).


While `PersistentVolumeClaims` allow a user to consume abstract storage 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.
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.


See the [detailed walkthrough with working examples](/docs/tasks/configure-pod-container/configure-persistent-volume-storage/). See the [detailed walkthrough with working examples](/docs/tasks/configure-pod-container/configure-persistent-volume-storage/).


Expand All @@ -56,9 +50,9 @@ A cluster administrator creates a number of PVs. They carry the details of the r


#### Dynamic #### Dynamic


When none of the static PVs the administrator created match a user's `PersistentVolumeClaim`, When none of the static PVs the administrator created match a user's PersistentVolumeClaim,
the cluster may try to dynamically provision a volume specially for the PVC. the cluster may try to dynamically provision a volume specially for the PVC.
This provisioning is based on `StorageClasses`: the PVC must request a This provisioning is based on StorageClasses: the PVC must request a
[storage class](/docs/concepts/storage/storage-classes/) and [storage class](/docs/concepts/storage/storage-classes/) and
the administrator must have created and configured that class for dynamic the administrator must have created and configured that class for dynamic
provisioning to occur. Claims that request the class `""` effectively disable provisioning to occur. Claims that request the class `""` effectively disable
Expand All @@ -81,10 +75,10 @@ Claims will remain unbound indefinitely if a matching volume does not exist. Cla


Pods use claims as volumes. The cluster inspects the claim to find the bound volume and mounts that volume for a Pod. For volumes that support multiple access modes, the user specifies which mode is desired when using their claim as a volume in a Pod. Pods use claims as volumes. The cluster inspects the claim to find the bound volume and mounts that volume for a Pod. For volumes that support multiple access modes, the user specifies which mode is 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](#claims-as-volumes). 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` section in a Pod's `volumes` block. See [Claims As Volumes](#claims-as-volumes) for more details on this.


### Storage Object in Use Protection ### Storage Object in Use Protection
The purpose of the Storage Object in Use Protection feature is to ensure that Persistent Volume Claims (PVCs) in active use by a Pod and Persistent Volume (PVs) that are bound to PVCs are not removed from the system, as this may result in data loss. The purpose of the Storage Object in Use Protection feature is to ensure that PersistentVolumeClaims (PVCs) in active use by a Pod and PersistentVolume (PVs) that are bound to PVCs are not removed from the system, as this may result in data loss.


{{< note >}} {{< note >}}
PVC is in active use by a Pod when a Pod object exists that is using the PVC. PVC is in active use by a Pod when a Pod object exists that is using the PVC.
Expand Down Expand Up @@ -132,19 +126,19 @@ Events: <none>


### Reclaiming ### Reclaiming


When a user is done with their volume, they can delete the PVC objects from the API that allows reclamation of the resource. 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. When a user is done with their volume, they can delete the PVC objects from the API that allows reclamation of the resource. 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.


#### Retain #### Retain


The `Retain` reclaim policy allows for manual reclamation of the resource. When the `PersistentVolumeClaim` is deleted, the `PersistentVolume` still exists and the volume is considered "released". But it is not yet available for another claim because the previous claimant's data remains on the volume. An administrator can manually reclaim the volume with the following steps. The `Retain` reclaim policy allows for manual reclamation of the resource. When the PersistentVolumeClaim is deleted, the PersistentVolume still exists and the volume is considered "released". But it is not yet available for another claim because the previous claimant's data remains on the volume. An administrator can manually reclaim the volume with the following steps.


1. Delete the `PersistentVolume`. The associated storage asset in external infrastructure (such as an AWS EBS, GCE PD, Azure Disk, or Cinder volume) still exists after the PV is deleted. 1. Delete the PersistentVolume. The associated storage asset in external infrastructure (such as an AWS EBS, GCE PD, Azure Disk, or Cinder volume) still exists after the PV is deleted.
1. Manually clean up the data on the associated storage asset accordingly. 1. Manually clean up the data on the associated storage asset accordingly.
1. Manually delete the associated storage asset, or if you want to reuse the same storage asset, create a new `PersistentVolume` with the storage asset definition. 1. Manually delete the associated storage asset, or if you want to reuse the same storage asset, create a new PersistentVolume with the storage asset definition.


#### Delete #### Delete


For volume plugins that support the `Delete` reclaim policy, deletion removes both the `PersistentVolume` object from Kubernetes, as well as the associated storage asset in the external infrastructure, such as an AWS EBS, GCE PD, Azure Disk, or Cinder volume. Volumes that were dynamically provisioned inherit the [reclaim policy of their `StorageClass`](#reclaim-policy), which defaults to `Delete`. The administrator should configure the `StorageClass` according to users' expectations; otherwise, the PV must be edited or patched after it is created. See [Change the Reclaim Policy of a PersistentVolume](/docs/tasks/administer-cluster/change-pv-reclaim-policy/). For volume plugins that support the `Delete` reclaim policy, deletion removes both the PersistentVolume object from Kubernetes, as well as the associated storage asset in the external infrastructure, such as an AWS EBS, GCE PD, Azure Disk, or Cinder volume. Volumes that were dynamically provisioned inherit the [reclaim policy of their StorageClass](#reclaim-policy), which defaults to `Delete`. The administrator should configure the StorageClass according to users' expectations; otherwise, the PV must be edited or patched after it is created. See [Change the Reclaim Policy of a PersistentVolume](/docs/tasks/administer-cluster/change-pv-reclaim-policy/).


#### Recycle #### Recycle


Expand Down Expand Up @@ -214,8 +208,8 @@ allowVolumeExpansion: true
``` ```


To request a larger volume for a PVC, edit the PVC object and specify a larger To request a larger volume for a PVC, edit the PVC object and specify a larger
size. This triggers expansion of the volume that backs the underlying `PersistentVolume`. A size. This triggers expansion of the volume that backs the underlying PersistentVolume. A
new `PersistentVolume` is never created to satisfy the claim. Instead, an existing volume is resized. new PersistentVolume is never created to satisfy the claim. Instead, an existing volume is resized.


#### CSI Volume expansion #### CSI Volume expansion


Expand All @@ -229,7 +223,7 @@ Support for expanding CSI volumes is enabled by default but it also requires a s
You can only resize volumes containing a file system if the file system is XFS, Ext3, or Ext4. You can only resize volumes containing a file system if the file system is XFS, Ext3, or Ext4.


When a volume contains a file system, the file system is only resized when a new Pod is using When a volume contains a file system, the file system is only resized when a new Pod is using
the `PersistentVolumeClaim` in ReadWrite mode. File system expansion is either done when a Pod is starting up the PersistentVolumeClaim in `ReadWrite` mode. File system expansion is either done when a Pod is starting up
or when a Pod is running and the underlying file system supports online expansion. or when a Pod is running and the underlying file system supports online expansion.


FlexVolumes allow resize if the driver is set with the `RequiresFSResize` capability to `true`. FlexVolumes allow resize if the driver is set with the `RequiresFSResize` capability to `true`.
Expand Down Expand Up @@ -262,7 +256,7 @@ Expanding EBS volumes is a time-consuming operation. Also, there is a per-volume


## Types of Persistent Volumes ## Types of Persistent Volumes


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


* GCEPersistentDisk * GCEPersistentDisk
* AWSElasticBlockStore * AWSElasticBlockStore
Expand Down Expand Up @@ -345,7 +339,7 @@ for an example on how to use a volume with `volumeMode: Block` in a Pod.


### Access Modes ### 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. 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: The access modes are:


Expand Down Expand Up @@ -521,22 +515,22 @@ by the cluster, depending on whether the
is turned on. is turned on.


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


Depending on installation method, a default StorageClass may be deployed Depending on installation method, a default StorageClass may be deployed
to a Kubernetes cluster by addon manager during installation. to a Kubernetes cluster by addon manager during installation.


When a PVC specifies a `selector` in addition to requesting a `StorageClass`, 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 requirements are ANDed together: only a PV of the requested class and with
the requested labels may be bound to the PVC. the requested labels may be bound to the PVC.


Expand All @@ -550,7 +544,7 @@ it won't be supported in a future Kubernetes release.


## Claims As Volumes ## 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. 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.


```yaml ```yaml
apiVersion: v1 apiVersion: v1
Expand All @@ -572,7 +566,7 @@ spec:


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


## Raw Block Volume Support ## Raw Block Volume Support


Expand All @@ -592,7 +586,7 @@ applicable:
* RBD (Ceph Block Device) * RBD (Ceph Block Device)
* VsphereVolume * VsphereVolume


### Persistent Volumes using a Raw Block Volume ### PersistentVolume using a Raw Block Volume {#persistent-volume-using-a-raw-block-volume}


```yaml ```yaml
apiVersion: v1 apiVersion: v1
Expand All @@ -611,7 +605,7 @@ spec:
lun: 0 lun: 0
readOnly: false readOnly: false
``` ```
### Persistent Volume Claim requesting a Raw Block Volume ### PersistentVolumeClaim requesting a Raw Block Volume {#persistent-volume-claim-requesting-a-raw-block-volume}


```yaml ```yaml
apiVersion: v1 apiVersion: v1
Expand Down Expand Up @@ -655,7 +649,7 @@ When adding a raw block device for a Pod, you specify the device path in the con


### Binding Block Volumes ### Binding Block Volumes


If a user requests a raw block volume by indicating this using the `volumeMode` field in the `PersistentVolumeClaim` spec, the binding rules differ slightly from previous releases that didn't consider this mode as part of the spec. If a user requests a raw block volume by indicating this using the `volumeMode` field in the PersistentVolumeClaim spec, the binding rules differ slightly from previous releases that didn't consider this mode as part of the spec.
Listed is a table of possible combinations the user and admin might specify for requesting a raw block device. The table indicates if the volume will be bound or not given the combinations: Listed is a table of possible combinations the user and admin might specify for requesting a raw block device. The table indicates if the volume will be bound or not given the combinations:
Volume binding matrix for statically provisioned volumes: Volume binding matrix for statically provisioned volumes:


Expand Down Expand Up @@ -684,7 +678,7 @@ Volume snapshot feature was added to support CSI Volume Plugins only. For detail
To enable support for restoring a volume from a volume snapshot data source, enable the To enable support for restoring a volume from a volume snapshot data source, enable the
`VolumeSnapshotDataSource` feature gate on the apiserver and controller-manager. `VolumeSnapshotDataSource` feature gate on the apiserver and controller-manager.


### Create Persistent Volume Claim from Volume Snapshot ### Create a PersistentVolumeClaim from a Volume Snapshot {#create-persistent-volume-claim-from-volume-snapshot}


```yaml ```yaml
apiVersion: v1 apiVersion: v1
Expand All @@ -708,7 +702,7 @@ spec:


[Volume Cloning](/docs/concepts/storage/volume-pvc-datasource/) only available for CSI volume plugins. [Volume Cloning](/docs/concepts/storage/volume-pvc-datasource/) only available for CSI volume plugins.


### Create Persistent Volume Claim from an existing pvc ### Create PersistentVolumeClaim from an existing PVC {#create-persistent-volume-claim-from-an-existing-pvc}


```yaml ```yaml
apiVersion: v1 apiVersion: v1
Expand Down Expand Up @@ -755,8 +749,8 @@ and need persistent storage, it is recommended that you use the following patter
{{% /capture %}} {{% /capture %}}
{{% capture whatsnext %}} {{% capture whatsnext %}}


* Learn more about [Creating a Persistent Volume](/docs/tasks/configure-pod-container/configure-persistent-volume-storage/#create-a-persistentvolume). * Learn more about [Creating a PersistentVolume](/docs/tasks/configure-pod-container/configure-persistent-volume-storage/#create-a-persistentvolume).
* Learn more about [Creating a Persistent Volume Claim](/docs/tasks/configure-pod-container/configure-persistent-volume-storage/#create-a-persistentvolumeclaim). * Learn more about [Creating a PersistentVolumeClaim](/docs/tasks/configure-pod-container/configure-persistent-volume-storage/#create-a-persistentvolumeclaim).
* Read the [Persistent Storage design document](https://git.k8s.io/community/contributors/design-proposals/storage/persistent-storage.md). * Read the [Persistent Storage design document](https://git.k8s.io/community/contributors/design-proposals/storage/persistent-storage.md).


### Reference ### Reference
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