This is unreleased documentation for SUSE® Storage 1.11 (Dev).

Create Volumes

You can create Kubernetes persistent storage resources of persistent volumes (PVs) and persistent volume claims (PVCs) that correspond to Longhorn volumes. You will use kubectl to dynamically provision storage for workloads using a Longhorn StorageClass. For help creating volumes from the SUSE Storage UI, refer to this section.

This section assumes that you understand how Kubernetes persistent storage works. For more information, see the Kubernetes documentation.

Access modes

SUSE Storage supports the following Kubernetes PersistentVolume access modes:

  • ReadWriteOnce (RWO): The volume can be mounted as read-write by a single node. Multiple pods on the same node can access the volume. This is the default and most common access mode.

  • ReadWriteOncePod (RWOP): The volume can be mounted as read-write by a single pod in the cluster. This mode provides the strongest isolation, ensuring that only one pod can access the volume at any time. It is recommended for stateful workloads that require single-writer access.

  • ReadWriteMany (RWX): The volume can be mounted as read-write by many nodes at the same time, enabling shared access across multiple pods. For more information, see ReadWriteMany (RWX) volumes.

ReadOnlyMany (ROX) is not supported. For read-only access from multiple pods, use ReadWriteMany with read-only mount options in the pod specification.

Creating Longhorn Volumes with kubectl

First, you need to create a Longhorn StorageClass. The Longhorn StorageClass contains the parameters to provision PVs.

Next, a PersistentVolumeClaim is created that references the StorageClass. Finally, the PersistentVolumeClaim is mounted as a volume within a Pod.

When the Pod is deployed, the Kubernetes master will check the PersistentVolumeClaim to make sure the resource request can be fulfilled. If storage is available, the Kubernetes master will create the Longhorn volume and bind it to the Pod.

  1. Use following command to create a StorageClass called longhorn:

    kubectl create -f https://raw.githubusercontent.com/longhorn/longhorn/v1.11.0/examples/storageclass.yaml

    The following example StorageClass is created:

    kind: StorageClass
apiVersion: storage.k8s.io/v1
metadata:
  name: longhorn
provisioner: driver.longhorn.io
allowVolumeExpansion: true
parameters:
  numberOfReplicas: "3"
  staleReplicaTimeout: "2880" # 48 hours in minutes
  fromBackup: ""
  fsType: "ext4"
#  backupTargetName: "default"
#  mkfsParams: "-I 256 -b 4096 -O ^metadata_csum,^64bit"
#  diskSelector: "ssd,fast"
#  nodeSelector: "storage,fast"
#  recurringJobSelector: '[
#   {
#     "name":"snap",
#     "isGroup":true,
#   },
#   {
#     "name":"backup",
#     "isGroup":false,
#   }
#  ]'

    The parameter mkfsParams can be used to specify file system format options for each StorageClass.

    The parameter backupTargetName can be used to specify the backup target. The name of the default backup target (default) is used if backupTargetName is not specified.

    Parameters may be omitted from the StorageClass specification. When the StorageClass is used to create a PV and a volume, parameters that are not specified will generally be set using a default value taken from the global settings. For the full list of global settings, see StorageClass Parameters and Settings.

  2. Create a Pod that uses Longhorn volumes by running this command:

    kubectl create -f https://raw.githubusercontent.com/longhorn/longhorn/v1.11.0/examples/pod_with_pvc.yaml

    A Pod named volume-test is launched, along with a PersistentVolumeClaim named longhorn-volv-pvc. The PersistentVolumeClaim references the Longhorn StorageClass:

    apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: longhorn-volv-pvc
spec:
  accessModes:
    - ReadWriteOnce
  storageClassName: longhorn
  resources:
    requests:
      storage: 2Gi

    The persistentVolumeClaim is mounted in the Pod as a volume:

    apiVersion: v1
kind: Pod
metadata:
  name: volume-test
  namespace: default
spec:
  containers:
  - name: volume-test
    image: nginx:stable-alpine
    imagePullPolicy: IfNotPresent
    volumeMounts:
    - name: volv
      mountPath: /data
    ports:
    - containerPort: 80
  volumes:
  - name: volv
    persistentVolumeClaim:
      claimName: longhorn-volv-pvc

    More examples are available here.

Binding Workloads to PVs without a Kubernetes StorageClass

It is possible to use a Longhorn StorageClass to bind a workload to a PV without creating a StorageClass object in Kubernetes.

Since the StorageClass is also a field used to match a PVC with a PV, which does not have to be created by a Provisioner, you can create a PV manually with a custom StorageClass name, then create a PVC asking for the same StorageClass name.

When a PVC requests a StorageClass that does not exist as a Kubernetes resource, Kubernetes will try to bind your PVC to a PV with the same StorageClass name. The StorageClass will be used like a label to find the matching PV, and only existing PVs labeled with the StorageClass name will be used.

If the PVC names a StorageClass, Kubernetes will:

  1. Look for an existing PV that has the label matching the StorageClass

  2. Look for an existing StorageClass Kubernetes resource. If the StorageClass exists, it will be used to create a PV.

Creating Longhorn Volumes with the Longhorn UI

Since the Longhorn volume already exists while creating PV/PVC, a StorageClass is not needed for dynamically provisioning Longhorn volume. However, the field storageClassName should be set in PVC/PV, to be used for PVC binding purposes. It is not necessary for users to create the related StorageClass object.

By default the StorageClass for Longhorn created PV/PVC is longhorn-static. Users can modify it in Setting - General - Default Longhorn Static StorageClass Name as they need.

Users need to manually delete PVC and PV created by SUSE Storage.

PV/PVC Creation for Existing Longhorn Volume

Now users can create PV/PVC via our Longhorn UI for the existing Longhorn volumes. Only detached volume can be used by a newly created pod.

The Failure of the Longhorn Volume Creation

Creating a Longhorn volume can fail for different reasons. The issues are categorized into:

  • insufficient storage

  • disk not found

  • disks are unavailable

  • tags not fulfilled

  • node not found

  • nodes are unavailable

  • none of the node candidates contains a ready engine image

  • hard affinity cannot be satisfied

  • replica scheduling failed

  • unused failed replica is not supported

  • replica already scheduled

  • longhorn client operation failed

  • incompatible volume size

The failure results in the workload failing to use the provisioned PV and showing a warning message

# kubectl describe pod workload-test

Events:
  Type     Reason              Age                From                     Message
  ----     ------              ----               ----                     -------
  Warning  FailedAttachVolume  14s (x8 over 82s)  attachdetach-controller  AttachVolume.Attach
  failed for volume "pvc-e130e369-274d-472d-98d1-f6074d2725e8" : rpc error: code = Aborted
  desc = volume pvc-e130e369-274d-472d-98d1-f6074d2725e8 is not ready for workloads

To help users understand the error causes, SUSE Storage summarizes them in the PV annotation, longhorn.io/volume-scheduling-error. Failures are combined in this annotation and separated by a semicolon, for example, longhorn.io/volume-scheduling-error: insufficient storage;disks are unavailable. The annotation can be checked by using kubectl describe pv <pvc name>.

# kubectl describe pv pvc-e130e369-274d-472d-98d1-f6074d2725e8
Name:            pvc-e130e369-274d-472d-98d1-f6074d2725e8
Labels:          <none>
Annotations:     longhorn.io/volume-scheduling-error: insufficient storage
                 pv.kubernetes.io/provisioned-by: driver.longhorn.io

...