Why ReadWriteOnce is working on different nodes?

It's quite misleading concept regarding accessMode, especially in NFS.

In Kubernetes Persistent Volume docs it's mentioned that NFS supports all types of Access. RWO, RXX and RWX.

However accessMode is something like matching criteria, same as storage size. It's described better in OpenShift Access Mode documentation

A PersistentVolume 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.

In the next paragraph:

A volume’s AccessModes are descriptors of the volume’s 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.

Another example is that you can choose a few AccessModes as it is not constraint but a matching criteria.

$ cat <<EOF | kubectl create -f -> apiVersion: v1> kind: PersistentVolumeClaim> metadata:>   name: exmaple-pvc> spec:>   accessModes:>     - ReadOnlyMany>     - ReadWriteMany>     - ReadWriteOnce>   resources:>     requests:>       storage: 1Gi> EOF

or as per GKE example:

$ cat <<EOF | kubectl create -f -apiVersion: v1kind: PersistentVolumeClaimmetadata:  name: exmaple-pvc-rwo-romspec:  accessModes:    - ReadOnlyMany    - ReadWriteOnce  resources:    requests:      storage: 1GiEOF               persistentvolumeclaim/exmaple-pvc-rwo-rom created

PVC Output

$ kubectl get pvcNAME                  STATUS    VOLUME                                     CAPACITY   ACCESS MODES   STORAGECLASS   AGEexmaple-pvc           Pending                                                                        standard       2m18sexmaple-pvc-rwo-rom   Bound     pvc-d704d346-42b3-4090-af96-aebeee3053f5   1Gi        RWO,ROX        standard       6spersistentvolumeclaim/exmaple-pvc created

exmaple-pvc is in Pending state as default GKE GCEPersistentDisk its not supporting RreadWriteMany.

Warning  ProvisioningFailed  10s (x5 over 69s)  persistentvolume-controller  Failed to provision volume with StorageClass "standard": invalid AccessModes [ReadOnlyMany ReadWriteMany ReadWriteOnce]: only AccessModes [ReadWriteOnce ReadOnlyMany] are supported

However second pvc exmaple-pvc-rwo-rom were created and you can see it have 2 access mode RWO, ROX.

In short accessMode is more like requirement for PVC/PV to Bind. If NFS which is providing all access modes binds with RWO it fulfill requirement, however it will work as RWM as NFS providing that capability.

Hope it answered cleared a bit.

In addition you can check other StackOverflow threads regarding accessMode