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Merge pull request #26908 from pmorie/pv-dynprov
Automatic merge from submit-queue dynamic provisioning proposal Proposal for dynamic provisioning using storage classes; supercedes #17056 @kubernetes/sig-storage
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docs/proposals/volume-provisioning.md
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docs/proposals/volume-provisioning.md
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<!-- BEGIN MUNGE: UNVERSIONED_WARNING -->
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<!-- BEGIN STRIP_FOR_RELEASE -->
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<img src="http://kubernetes.io/img/warning.png" alt="WARNING"
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width="25" height="25">
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<h2>PLEASE NOTE: This document applies to the HEAD of the source tree</h2>
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If you are using a released version of Kubernetes, you should
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refer to the docs that go with that version.
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Documentation for other releases can be found at
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[releases.k8s.io](http://releases.k8s.io).
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</strong>
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--
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<!-- END STRIP_FOR_RELEASE -->
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<!-- END MUNGE: UNVERSIONED_WARNING -->
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## Abstract
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Real Kubernetes clusters have a variety of volumes which differ widely in
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size, iops performance, retention policy, and other characteristics.
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Administrators need a way to dynamically provision volumes of these different
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types to automatically meet user demand.
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A new mechanism called 'storage classes' is proposed to provide this
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capability.
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## Motivation
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In Kubernetes 1.2, an alpha form of limited dynamic provisioning was added
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that allows a single volume type to be provisioned in clouds that offer
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special volume types.
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In Kubernetes 1.3, a label selector was added to persistent volume claims to
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allow administrators to create a taxonomy of volumes based on the
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characteristics important to them, and to allow users to make claims on those
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volumes based on those characteristics. This allows flexibility when claiming
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existing volumes; the same flexibility is needed when dynamically provisioning
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volumes.
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After gaining experience with dynamic provisioning after the 1.2 release, we
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want to create a more flexible feature that allows configuration of how
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different storage classes are provisioned and supports provisioning multiple
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types of volumes within a single cloud.
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### Out-of-tree provisioners
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One of our goals is to enable administrators to create out-of-tree
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provisioners, that is, provisioners whose code does not live in the Kubernetes
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project. Our experience since the 1.2 release with dynamic provisioning has
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shown that it is impossible to anticipate every aspect and manner of
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provisioning that administrators will want to perform. The proposed design
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should not prevent future work to allow out-of-tree provisioners.
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## Design
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This design represents the minimally viable changes required to provision based on storage classe configuration. Additional incremental features may be added as a separte effort.
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We propose that:
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1. For the base impelementation storage class and volume selectors are mutually exclusive.
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2. An api object will be incubated in extensions/v1beta1 named `storage` to hold the a `StorageClass`
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API resource. Each StorageClass object contains parameters required by the provisioner to provision volumes of that class. These parameters are opaque to the user.
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3. `PersistentVolume.Spec.Class` attribute is added to volumes. This attribute
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is optional and specifies which `StorageClass` instance represents
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storage characteristics of a particular PV.
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During incubation, `Class` is an annotation and not
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actual attribute.
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4. `PersistentVolume` instances do not require labels by the provisioner.
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5. `PersistentVolumeClaim.Spec.Class` attribute is added to claims. This
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attribute specifies that only a volume with equal
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`PersistentVolume.Spec.Class` value can satisfy a claim.
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During incubation, `Class` is just an annotation and not
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actual attribute.
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6. The existing provisioner plugin implementations be modified to accept
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parameters as specified via `StorageClass`.
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7. The persistent volume controller modified to invoke provisioners using `StorageClass` configuration and bind claims with `PersistentVolumeClaim.Spec.Class` to volumes with equivilant `PersistentVolume.Spec.Class`
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8. The existing alpha dynamic provisioning feature be phased out in the
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next release.
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### Controller workflow for provisioning volumes
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1. When a new claim is submitted, the controller attempts to find an existing
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volume that will fulfill the claim.
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1. If the claim has non-empty `claim.Spec.Class`, only PVs with the same
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`pv.Spec.Class` are considered.
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2. If the claim has empty `claim.Spec.Class`, all existing PVs are
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considered.
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All "considered" volumes are evaluated and the
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smallest matching volume is bound to the claim.
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2. If no volume is found for the claim and `claim.Spec.Class` is not set or is
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empty string dynamic provisioning is disabled.
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3. If `claim.Spec.Class` is set the controller tries to find instance of StorageClass with this name. If no
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such StorageClass is found, the controller goes back to step 1. and
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periodically retries finding a matching volume or storage class again until
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a match is found. The claim is `Pending` during this period.
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4. With StorageClass instance, the controller finds volume plugin specified by
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StorageClass.ProvisionerType.
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5. All provisioners are in-tree; they implement an interface called
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`ProvisionableVolumePlugin`, which has a method called `NewProvisioner`
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that returns a new provisioner.
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6. The controller calls volume plugin `Provision` with ProvisionerParameters from the `StorageClass` configuration object.
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7. If `Provision` returns an error, the controller generates an event on the
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claim and goes back to step 1., i.e. it will retry provisioning periodically
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8. If `Provision` returns no error, the controller creates the returned
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`api.PersistentVolume`, fills its `Class` attribute with `claim.Spec.Class`
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and makes it already bound to the claim
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1. If the create operation for the `api.PersistentVolume` fails, it is
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retried
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2. If the create operation does not succeed in reasonable time, the
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controller attempts to delete the provisioned volume and creates an event
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on the claim
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Existing behavior is un-changed for claims that do not specify `claim.Spec.Class`.
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### `StorageClass` API
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A new API group should hold the API for storage classes, following the pattern
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of autoscaling, metrics, etc. To allow for future storage-related APIs, we
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should call this new API group `storage` and incubate in extensions/v1beta1.
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Storage classes will be represented by an API object called `StorageClass`:
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```go
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package storage
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// StorageClass describes the parameters for a class of storage for
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// which PersistentVolumes can be dynamically provisioned.
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//
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// StorageClasses are non-namespaced; the name of the storage class
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// according to etcd is in ObjectMeta.Name.
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type StorageClass struct {
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unversioned.TypeMeta `json:",inline"`
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ObjectMeta `json:"metadata,omitempty"`
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// ProvisionerType indicates the type of the provisioner.
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ProvisionerType string `json:"provisionerType,omitempty"`
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// Parameters for dynamic volume provisioner.
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ProvisionerParameters map[string]string `json:"provisionerParameters,omitempty"`
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}
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```
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`PersistentVolumeClaimSpec` and `PersistentVolumeSpec` both get Class attribute
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(the existing annotation is used during incubation):
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```go
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type PersistentVolumeClaimSpec struct {
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// Name of requested storage class. If non-empty, only PVs with this
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// pv.Spec.Class will be considered for binding and if no such PV is
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// available, StorageClass with this name will be used to dynamically
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// provision the volume.
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Class string
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...
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}
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type PersistentVolumeSpec struct {
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// Name of StorageClass instance that this volume belongs to.
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Class string
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...
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}
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```
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Storage classes are natural to think of as a global resource, since they:
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1. Align with PersistentVolumes, which are a global resource
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2. Are administrator controlled
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### Provisioning configuration
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With the scheme outlined above the provisioner creates PVs using parameters specified in the `StorageClass` object.
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### Provisioner interface changes
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`struct volume.VolumeOptions` (containing parameters for a provisioner plugin)
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will be extended to contain StorageClass.ProvisionerParameters.
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The existing provisioner implementations will be modified to accept the StorageClass configuration object.
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### PV Controller Changes
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The persistent volume controller will be modified to implement the new
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workflow described in this proposal. The changes will be limited to the
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`provisionClaimOperation` method, which is responsible for invoking the
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provisioner and to favor existing volumes before provisioning a new one.
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## Examples
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### AWS provisioners with distinct QoS
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This example shows two storage classes, "aws-fast" and "aws-slow".
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```
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apiVersion: v1
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kind: StorageClass
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metadata:
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name: aws-fast
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provisionerType: kubernetes.io/aws-ebs
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provisionerParameters:
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zone: us-east-1b
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type: ssd
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apiVersion: v1
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kind: StorageClass
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metadata:
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name: aws-slow
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provisionerType: kubernetes.io/aws-ebs
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provisionerParameters:
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zone: us-east-1b
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type: spinning
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```
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# Additional Implementation Details
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0. Annotation `volume.alpha.kubernetes.io/storage-class` is used instead of `claim.Spec.Class` and `volume.Spec.Class` during incubation.
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1. `claim.Spec.Selector` and `claim.Spec.Class` are mutually exclusive. User can either match existing volumes with `Selector` XOR match existing volumes with `Class` and get dynamic provisioning by using `Class`. This simplifies initial PR and also provisioners.
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# Cloud Providers
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Since the `volume.alpha.kubernetes.io/storage-class` is in use a `StorageClass` must be defined to support provisioning. No default is assumed as before.
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<!-- BEGIN MUNGE: GENERATED_ANALYTICS -->
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[]()
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<!-- END MUNGE: GENERATED_ANALYTICS -->
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