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Binding Persistent Volumes by Labels

Overview

This topic provides an end-to-end example for binding persistent volume claims (PVCs) to persistent volumes (PVs), by defining labels in the PV and matching selectors in the PVC. This feature is available for all storage options. It is assumed that a OpenShift Container Platform cluster contains persistent storage resources which are available for binding by PVCs.

A Note on Labels and Selectors

Labels are an OpenShift Container Platform feature that support user-defined tags (key-value pairs) as part of an object’s specification. Their primary purpose is to enable the arbitrary grouping of objects by defining identical labels among them. These labels can then be targeted by selectors to match all objects with specified label values. It is this functionality we will take advantage of to enable our PVC to bind to our PV. For a more in-depth look at labels, see Pods and Services.

For this example, we will be using modified GlusterFS PV and PVC specifications. However, implementation of selectors and labels is generic across for all storage options. See the relevant storage option for your volume provider to learn more about its unique configuration.

Assumptions

It is assumed that you have:

  • An existing OpenShift Container Platform cluster with at least one master and one node

  • At least one supported storage volume

  • A user with cluster-admin privileges

Defining Specifications

These specifications are tailored to GlusterFS. Consult the relevant storage option for your volume provider to learn more about its unique configuration.

Persistent Volume with Labels

Example 1. glusterfs-pv.yaml
apiVersion: v1
kind: PersistentVolume
metadata:
  name: gluster-volume
  labels: (1)
    storage-tier: gold
    aws-availability-zone: us-east-1
spec:
  capacity:
    storage: 2Gi
  accessModes:
    - ReadWriteMany
  glusterfs:
    endpoints: glusterfs-cluster (2)
    path: myVol1
    readOnly: false
  persistentVolumeReclaimPolicy: Retain
1 Use labels to identify common attributes or characteristics shared among volumes. In this case, we defined the Gluster volume to have a custom attribute (key) named storage-tier with a value of gold assigned. A claim will be able to select a PV with storage-tier=gold to match this PV.
2 Endpoints define the Gluster trusted pool and are discussed below.

Persistent Volume Claim with Selectors

A claim with a selector stanza (see example below) attempts to match existing, unclaimed, and non-prebound PVs. The existence of a PVC selector ignores a PV’s capacity. However, accessModes are still considered in the matching criteria.

It is important to note that a claim must match all of the key-value pairs included in its selector stanza. If no PV matches the claim, then the PVC will remain unbound (Pending). A PV can subsequently be created and the claim will automatically check for a label match.

Example 2. glusterfs-pvc.yaml
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: gluster-claim
spec:
  accessModes:
  - ReadWriteMany
  resources:
     requests:
       storage: 1Gi
  selector: (1)
    matchLabels:
      storage-tier: gold
      aws-availability-zone: us-east-1
1 The selector stanza defines all labels necessary in a PV in order to match this claim.

Volume Endpoints

To attach the PV to the Gluster volume, endpoints should be configured before creating our objects.

Example 3. glusterfs-ep.yaml
apiVersion: v1
kind: Endpoints
metadata:
  name: glusterfs-cluster
subsets:
  - addresses:
      - ip: 192.168.122.221
    ports:
      - port: 1
  - addresses:
      - ip: 192.168.122.222
    ports:
      - port: 1

Deploy the PV, PVC, and Endpoints

For this example, run the oc commands as a cluster-admin privileged user. In a production environment, cluster clients might be expected to define and create the PVC.

# oc create -f glusterfs-ep.yaml
endpoints "glusterfs-cluster" created
# oc create -f glusterfs-pv.yaml
persistentvolume "gluster-volume" created
# oc create -f glusterfs-pvc.yaml
persistentvolumeclaim "gluster-claim" created

Lastly, confirm that the PV and PVC bound successfully.

# oc get pv,pvc
NAME              CAPACITY   ACCESSMODES      STATUS     CLAIM                     REASON    AGE
gluster-volume    2Gi        RWX              Bound      gfs-trial/gluster-claim             7s
NAME              STATUS     VOLUME           CAPACITY   ACCESSMODES               AGE
gluster-claim     Bound      gluster-volume   2Gi        RWX                       7s

PVCs are local to a project, whereas PVs are a cluster-wide, global resource. Developers and non-administrator users may not have access to see all (or any) of the available PVs.