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Chapter 3. Understanding persistent storage
3.1. Persistent storage overview
Managing storage is a distinct problem from managing compute resources. OpenShift Container Platform uses the Kubernetes persistent volume (PV) framework to allow cluster administrators to provision persistent storage for a cluster. Developers can use persistent volume claims (PVCs) to request PV resources without having specific knowledge of the underlying storage infrastructure.
PVCs are specific to a project, and are created and used by developers as a means to use a PV. PV resources on their own are not scoped to any single project; they can be shared across the entire OpenShift Container Platform cluster and claimed from any project. After a PV is bound to a PVC, that PV can not then be bound to additional PVCs. This has the effect of scoping a bound PV to a single namespace, that of the binding project.
PVs are defined by a PersistentVolume
API object, which represents a piece of existing storage in the cluster that was either statically provisioned by the cluster administrator or dynamically provisioned using a StorageClass
object. 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 that is independent of any individual pod that uses the PV. PV objects capture the details of the implementation of the storage, be that NFS, iSCSI, or a cloud-provider-specific storage system.
High availability of storage in the infrastructure is left to the underlying storage provider.
PVCs are defined by a PersistentVolumeClaim
API object, which represents a request for storage by a developer. It is similar to a pod in that pods consume node resources and PVCs consume PV resources. For example, pods can request specific levels of resources, such as CPU and memory, while PVCs can request specific storage capacity and access modes. For example, they can be mounted once read-write or many times read-only.
3.2. 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 have the following lifecycle.
3.2.1. Provision storage
In response to requests from a developer defined in a PVC, a cluster administrator configures one or more dynamic provisioners that provision storage and a matching PV.
Alternatively, a cluster administrator can create a number of PVs in advance that carry the details of the real storage that is available for use. PVs exist in the API and are available for use.
3.2.2. Bind claims
When you create a PVC, you request a specific amount of storage, specify the required access mode, and create a storage class to describe and classify the storage. The control loop in the master watches for new PVCs and binds the new PVC to an appropriate PV. If an appropriate PV does not exist, a provisioner for the storage class creates one.
The size of all PVs might exceed your PVC size. This is especially true with manually provisioned PVs. To minimize the excess, OpenShift Container Platform binds to the smallest PV that matches all other criteria.
Claims remain unbound indefinitely if a matching volume does not exist or can not be created with any available provisioner servicing a storage class. Claims are bound as matching volumes become available. For example, a cluster with many manually provisioned 50Gi volumes would not match a PVC requesting 100Gi. The PVC can be bound when a 100Gi PV is added to the cluster.
3.2.3. Use pods and claimed PVs
Pods use claims as volumes. The cluster inspects the claim to find the bound volume and mounts that volume for a pod. For those volumes that support multiple access modes, you must specify which mode applies when you use the claim as a volume in a pod.
Once you have a claim and that claim is bound, the bound PV belongs to you for as long as you need it. You can schedule pods and access claimed PVs by including persistentVolumeClaim
in the pod’s volumes block.
If you attach persistent volumes that have high file counts to pods, those pods can fail or can take a long time to start. For more information, see When using Persistent Volumes with high file counts in OpenShift, why do pods fail to start or take an excessive amount of time to achieve "Ready" state?.
3.2.4. Storage Object in Use Protection
The Storage Object in Use Protection feature ensures that PVCs in active use by a pod and PVs that are bound to PVCs are not removed from the system, as this can result in data loss.
Storage Object in Use Protection is enabled by default.
A PVC is in active use by a pod when a Pod
object exists that uses the PVC.
If a user deletes a PVC that is in active use by a pod, the PVC is not removed immediately. PVC removal is postponed until the PVC is no longer actively used by any pods. Also, if a cluster admin deletes a PV that is bound to a PVC, the PV is not removed immediately. PV removal is postponed until the PV is no longer bound to a PVC.
3.2.5. Release a persistent volume
When you are finished with a volume, you can delete the PVC object 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 and must be handled according to policy.
3.2.6. Reclaim policy for persistent volumes
The reclaim policy of a persistent volume tells the cluster what to do with the volume after it is released. A volume’s reclaim policy can be Retain
, Recycle
, or Delete
.
-
Retain
reclaim policy allows manual reclamation of the resource for those volume plugins that support it. -
Recycle
reclaim policy recycles the volume back into the pool of unbound persistent volumes once it is released from its claim.
The Recycle
reclaim policy is deprecated in OpenShift Container Platform 4. Dynamic provisioning is recommended for equivalent and better functionality.
-
Delete
reclaim policy deletes both thePersistentVolume
object from OpenShift Container Platform and the associated storage asset in external infrastructure, such as AWS EBS or VMware vSphere.
Dynamically provisioned volumes are always deleted.
3.2.7. Reclaiming a persistent volume manually
When a persistent volume claim (PVC) is deleted, the persistent volume (PV) still exists and is considered "released". However, the PV is not yet available for another claim because the data of the previous claimant remains on the volume.
Procedure
To manually reclaim the PV as a cluster administrator:
Delete the PV.
$ oc delete pv <pv-name>
The associated storage asset in the external infrastructure, such as an AWS EBS, GCE PD, Azure Disk, or Cinder volume, still exists after the PV is deleted.
- Clean up the data on the associated storage asset.
- Delete the associated storage asset. Alternately, to reuse the same storage asset, create a new PV with the storage asset definition.
The reclaimed PV is now available for use by another PVC.
3.2.8. Changing the reclaim policy of a persistent volume
To change the reclaim policy of a persistent volume:
List the persistent volumes in your cluster:
$ oc get pv
Example output
NAME CAPACITY ACCESSMODES RECLAIMPOLICY STATUS CLAIM STORAGECLASS REASON AGE pvc-b6efd8da-b7b5-11e6-9d58-0ed433a7dd94 4Gi RWO Delete Bound default/claim1 manual 10s pvc-b95650f8-b7b5-11e6-9d58-0ed433a7dd94 4Gi RWO Delete Bound default/claim2 manual 6s pvc-bb3ca71d-b7b5-11e6-9d58-0ed433a7dd94 4Gi RWO Delete Bound default/claim3 manual 3s
Choose one of your persistent volumes and change its reclaim policy:
$ oc patch pv <your-pv-name> -p '{"spec":{"persistentVolumeReclaimPolicy":"Retain"}}'
Verify that your chosen persistent volume has the right policy:
$ oc get pv
Example output
NAME CAPACITY ACCESSMODES RECLAIMPOLICY STATUS CLAIM STORAGECLASS REASON AGE pvc-b6efd8da-b7b5-11e6-9d58-0ed433a7dd94 4Gi RWO Delete Bound default/claim1 manual 10s pvc-b95650f8-b7b5-11e6-9d58-0ed433a7dd94 4Gi RWO Delete Bound default/claim2 manual 6s pvc-bb3ca71d-b7b5-11e6-9d58-0ed433a7dd94 4Gi RWO Retain Bound default/claim3 manual 3s
In the preceding output, the volume bound to claim
default/claim3
now has aRetain
reclaim policy. The volume will not be automatically deleted when a user deletes claimdefault/claim3
.
3.3. Persistent volumes
Each PV contains a spec
and status
, which is the specification and status of the volume, for example:
PersistentVolume
object definition example
apiVersion: v1 kind: PersistentVolume metadata: name: pv0001 1 spec: capacity: storage: 5Gi 2 accessModes: - ReadWriteOnce 3 persistentVolumeReclaimPolicy: Retain 4 ... status: ...
3.3.1. Types of PVs
OpenShift Container Platform supports the following persistent volume plugins:
- AliCloud Disk
- AWS Elastic Block Store (EBS)
- AWS Elastic File Store (EFS)
- Azure Disk
- Azure File
- Cinder
- Fibre Channel
- GCE Persistent Disk
- IBM VPC Block
- HostPath
- iSCSI
- Local volume
- NFS
- OpenStack Manila
- Red Hat OpenShift Data Foundation
- VMware vSphere
3.3.2. Capacity
Generally, a persistent volume (PV) has a specific storage capacity. This is set by using the capacity
attribute of the PV.
Currently, storage capacity is the only resource that can be set or requested. Future attributes may include IOPS, throughput, and so on.
3.3.3. Access modes
A persistent volume can be mounted on a host in any way supported by the resource provider. Providers 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.
Claims are matched to volumes with similar access modes. The only two matching criteria are access modes and size. A claim’s access modes represent a request. Therefore, you might be granted more, but never less. For example, if a claim requests RWO, but the only volume available is an NFS PV (RWO+ROX+RWX), the claim would then match NFS because it supports RWO.
Direct matches are always attempted first. The volume’s modes must match or contain more modes than you requested. The size must be greater than or equal to what is expected. If two types of volumes, such as NFS and iSCSI, have the same set of access modes, either of them can match a claim with those modes. There is no ordering between types of volumes and no way to choose one type over another.
All volumes with the same modes are grouped, and then sorted by size, smallest to largest. The binder gets the group with matching modes and iterates over each, in size order, until one size matches.
The following table lists the access modes:
Access Mode | CLI abbreviation | Description |
---|---|---|
ReadWriteOnce |
| The volume can be mounted as read-write by a single node. |
ReadOnlyMany |
| The volume can be mounted as read-only by many nodes. |
ReadWriteMany |
| The volume can be mounted as read-write by many nodes. |
Volume access modes are descriptors of volume capabilities. They are not enforced constraints. The storage provider is responsible for runtime errors resulting from invalid use of the resource.
For example, NFS offers ReadWriteOnce
access mode. You must mark the claims as read-only
if you want to use the volume’s ROX capability. Errors in the provider show up at runtime as mount errors.
iSCSI and Fibre Channel volumes do not currently have any fencing mechanisms. You must ensure the volumes are only used by one node at a time. In certain situations, such as draining a node, the volumes can be used simultaneously by two nodes. Before draining the node, first ensure the pods that use these volumes are deleted.
Volume plugin | ReadWriteOnce [1] | ReadOnlyMany | ReadWriteMany |
---|---|---|---|
AliCloud Disk |
✅ |
- |
- |
AWS EBS [2] |
✅ |
- |
- |
AWS EFS |
✅ |
✅ |
✅ |
Azure File |
✅ |
✅ |
✅ |
Azure Disk |
✅ |
- |
- |
Cinder |
✅ |
- |
- |
Fibre Channel |
✅ |
✅ |
- |
GCE Persistent Disk |
✅ |
- |
- |
HostPath |
✅ |
- |
- |
IBM VPC Disk |
✅ |
- |
- |
iSCSI |
✅ |
✅ |
- |
Local volume |
✅ |
- |
- |
NFS |
✅ |
✅ |
✅ |
OpenStack Manila |
- |
- |
✅ |
Red Hat OpenShift Data Foundation |
✅ |
- |
✅ |
VMware vSphere |
✅ |
- |
✅ [3] |
- ReadWriteOnce (RWO) volumes cannot be mounted on multiple nodes. If a node fails, the system does not allow the attached RWO volume to be mounted on a new node because it is already assigned to the failed node. If you encounter a multi-attach error message as a result, force delete the pod on a shutdown or crashed node to avoid data loss in critical workloads, such as when dynamic persistent volumes are attached.
- Use a recreate deployment strategy for pods that rely on AWS EBS.
-
If the underlying vSphere environment supports the vSAN file service, then the vSphere Container Storage Interface (CSI) Driver Operator installed by OpenShift Container Platform supports provisioning of ReadWriteMany (RWX) volumes. If you do not have vSAN file service configured, and you request RWX, the volume fails to get created and an error is logged. For more information, see "Using Container Storage Interface"
"VMware vSphere CSI Driver Operator".
3.3.4. Phase
Volumes can be found in one of the following phases:
Phase | Description |
---|---|
Available | A free resource not yet bound to a claim. |
Bound | The volume is bound to a claim. |
Released | The claim was deleted, but the resource is not yet reclaimed by the cluster. |
Failed | The volume has failed its automatic reclamation. |
You can view the name of the PVC bound to the PV by running:
$ oc get pv <pv-claim>
3.3.4.1. Mount options
You can specify mount options while mounting a PV by using the attribute mountOptions
.
For example:
Mount options example
apiVersion: v1
kind: PersistentVolume
metadata:
name: pv0001
spec:
capacity:
storage: 1Gi
accessModes:
- ReadWriteOnce
mountOptions: 1
- nfsvers=4.1
nfs:
path: /tmp
server: 172.17.0.2
persistentVolumeReclaimPolicy: Retain
claimRef:
name: claim1
namespace: default
- 1
- Specified mount options are used while mounting the PV to the disk.
The following PV types support mount options:
- AWS Elastic Block Store (EBS)
- Azure Disk
- Azure File
- Cinder
- GCE Persistent Disk
- iSCSI
- Local volume
- NFS
- Red Hat OpenShift Data Foundation (Ceph RBD only)
- VMware vSphere
Fibre Channel and HostPath PVs do not support mount options.
Additional resources
3.4. Persistent volume claims
Each PersistentVolumeClaim
object contains a spec
and status
, which is the specification and status of the persistent volume claim (PVC), for example:
PersistentVolumeClaim
object definition example
kind: PersistentVolumeClaim apiVersion: v1 metadata: name: myclaim 1 spec: accessModes: - ReadWriteOnce 2 resources: requests: storage: 8Gi 3 storageClassName: gold 4 status: ...
3.4.1. Storage classes
Claims can optionally request a specific storage class by specifying the storage class’s name in the storageClassName
attribute. Only PVs of the requested class, ones with the same storageClassName
as the PVC, can be bound to the PVC. The cluster administrator can configure dynamic provisioners to service one or more storage classes. The cluster administrator can create a PV on demand that matches the specifications in the PVC.
The Cluster Storage Operator might install a default storage class depending on the platform in use. This storage class is owned and controlled by the operator. It cannot be deleted or modified beyond defining annotations and labels. If different behavior is desired, you must define a custom storage class.
The cluster administrator can also set a default storage class for all PVCs. When a default storage class is configured, the PVC must explicitly ask for StorageClass
or storageClassName
annotations set to ""
to be bound to a PV without a storage class.
If more than one storage class is marked as default, a PVC can only be created if the storageClassName
is explicitly specified. Therefore, only one storage class should be set as the default.
3.4.2. Access modes
Claims use the same conventions as volumes when requesting storage with specific access modes.
3.4.3. Resources
Claims, such as pods, can request specific quantities of a resource. In this case, the request is for storage. The same resource model applies to volumes and claims.
3.4.4. 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 mounted to the host and into the pod, for example:
Mount volume to the host and into the pod example
kind: Pod apiVersion: v1 metadata: name: mypod spec: containers: - name: myfrontend image: dockerfile/nginx volumeMounts: - mountPath: "/var/www/html" 1 name: mypd 2 volumes: - name: mypd persistentVolumeClaim: claimName: myclaim 3
- 1
- Path to mount the volume inside the pod.
- 2
- Name of the volume to mount. Do not mount to the container root,
/
, or any path that is the same in the host and the container. This can corrupt your host system if the container is sufficiently privileged, such as the host/dev/pts
files. It is safe to mount the host by using/host
. - 3
- Name of the PVC, that exists in the same namespace, to use.
3.5. Block volume support
OpenShift Container Platform can statically provision raw block volumes. These volumes do not have a file system, and can provide performance benefits for applications that either write to the disk directly or implement their own storage service.
Raw block volumes are provisioned by specifying volumeMode: Block
in the PV and PVC specification.
Pods using raw block volumes must be configured to allow privileged containers.
The following table displays which volume plugins support block volumes.
Volume Plugin | Manually provisioned | Dynamically provisioned | Fully supported |
---|---|---|---|
AliCloud Disk | ✅ | ✅ | ✅ |
AWS EBS | ✅ | ✅ | ✅ |
AWS EFS | |||
Azure Disk | ✅ | ✅ | ✅ |
Azure File | |||
Cinder | ✅ | ✅ | ✅ |
Fibre Channel | ✅ | ✅ | |
GCP | ✅ | ✅ | ✅ |
HostPath | |||
IBM VPC Disk | ✅ | ✅ | ✅ |
iSCSI | ✅ | ✅ | |
Local volume | ✅ | ✅ | |
NFS | |||
Red Hat OpenShift Data Foundation | ✅ | ✅ | ✅ |
VMware vSphere | ✅ | ✅ | ✅ |
Using any of the block volumes that can be provisioned manually, but are not provided as fully supported, is a Technology Preview feature only. Technology Preview features are not supported with Red Hat production service level agreements (SLAs) and might not be functionally complete. Red Hat does not recommend using them in production. These features provide early access to upcoming product features, enabling customers to test functionality and provide feedback during the development process.
For more information about the support scope of Red Hat Technology Preview features, see Technology Preview Features Support Scope.
3.5.1. Block volume examples
PV example
apiVersion: v1
kind: PersistentVolume
metadata:
name: block-pv
spec:
capacity:
storage: 10Gi
accessModes:
- ReadWriteOnce
volumeMode: Block 1
persistentVolumeReclaimPolicy: Retain
fc:
targetWWNs: ["50060e801049cfd1"]
lun: 0
readOnly: false
- 1
volumeMode
must be set toBlock
to indicate that this PV is a raw block volume.
PVC example
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
name: block-pvc
spec:
accessModes:
- ReadWriteOnce
volumeMode: Block 1
resources:
requests:
storage: 10Gi
- 1
volumeMode
must be set toBlock
to indicate that a raw block PVC is requested.
Pod
specification example
apiVersion: v1 kind: Pod metadata: name: pod-with-block-volume spec: containers: - name: fc-container image: fedora:26 command: ["/bin/sh", "-c"] args: [ "tail -f /dev/null" ] volumeDevices: 1 - name: data devicePath: /dev/xvda 2 volumes: - name: data persistentVolumeClaim: claimName: block-pvc 3
- 1
volumeDevices
, instead ofvolumeMounts
, is used for block devices. OnlyPersistentVolumeClaim
sources can be used with raw block volumes.- 2
devicePath
, instead ofmountPath
, represents the path to the physical device where the raw block is mapped to the system.- 3
- The volume source must be of type
persistentVolumeClaim
and must match the name of the PVC as expected.
Value | Default |
---|---|
Filesystem | Yes |
Block | No |
PV volumeMode | PVC volumeMode | Binding result |
---|---|---|
Filesystem | Filesystem | Bind |
Unspecified | Unspecified | Bind |
Filesystem | Unspecified | Bind |
Unspecified | Filesystem | Bind |
Block | Block | Bind |
Unspecified | Block | No Bind |
Block | Unspecified | No Bind |
Filesystem | Block | No Bind |
Block | Filesystem | No Bind |
Unspecified values result in the default value of Filesystem
.
3.6. Using fsGroup to reduce pod timeouts
If a storage volume contains many files (~1,000,000 or greater), you may experience pod timeouts.
This can occur because, by default, OpenShift Container Platform recursively changes ownership and permissions for the contents of each volume to match the fsGroup
specified in a pod’s securityContext
when that volume is mounted. For large volumes, checking and changing ownership and permissions can be time consuming, slowing pod startup. You can use the fsGroupChangePolicy
field inside a securityContext
to control the way that OpenShift Container Platform checks and manages ownership and permissions for a volume.
fsGroupChangePolicy
defines behavior for changing ownership and permission of the volume before being exposed inside a pod. This field only applies to volume types that support fsGroup
-controlled ownership and permissions. This field has two possible values:
-
OnRootMismatch
: Only change permissions and ownership if permission and ownership of root directory does not match with expected permissions of the volume. This can help shorten the time it takes to change ownership and permission of a volume to reduce pod timeouts. -
Always
: Always change permission and ownership of the volume when a volume is mounted.
fsGroupChangePolicy
example
securityContext:
runAsUser: 1000
runAsGroup: 3000
fsGroup: 2000
fsGroupChangePolicy: "OnRootMismatch" 1
...
- 1
OnRootMismatch
specifies skipping recursive permission change, thus helping to avoid pod timeout problems.
The fsGroupChangePolicyfield has no effect on ephemeral volume types, such as secret, configMap, and emptydir.