Access modes

Learn ReadWriteOnce, ReadOnlyMany and ReadWriteMany.

Access modes

Access modes say how a disk can be mounted at the same time:

ReadWriteOnce (RWO): a single node reads and writes. Cloud block disks (EBS, PD) are like this.
ReadOnlyMany (ROX): many nodes only read.
ReadWriteMany (RWX): many nodes read and write; file systems like NFS, CephFS and EFS are needed.
ReadWriteOncePod (RWOP): a single pod reads and writes.

$ kubectl get pv -o custom-columns=AD:.metadata.name,MOD:.spec.accessModes
AD        MOD
nfs-pv    [ReadWriteMany]
pvc-9c1d  [ReadWriteOnce]

The RWO trap

Using an RWO PVC in a multi-replica Deployment is a common mistake: if the replicas land on different nodes, the second can't attach the disk. Solutions: a separate disk per replica (StatefulSet), RWX storage, or moving the data to object storage (S3):

Warning  FailedAttachVolume  Multi-Attach error for volume "pvc-9c1d…"
         Volume is already exclusively attached to one node and can't be attached to another

Exercise A shared upload area

Write a PersistentVolumeClaim YAML: name yuklemeler; ReadWriteMany under accessModes; storageClassName: nfs; resources.requests.storage: 20Gi.

Show example solution

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: yuklemeler
spec:
  accessModes:
    - ReadWriteMany
  storageClassName: nfs
  resources:
    requests:
      storage: 20Gi

Test yourself

Which mode is needed for 3 web pods on different nodes to write to the same upload folder?
- ReadOnlyMany
- ReadWriteOncePod
- ReadWriteMany
- ReadWriteOnce
Show answer

ReadWriteMany — An RWO disk can't be attached to a second node; the pod stays Pending or ContainerCreating.

Request read-write from many nodes.

spec:
  accessModes:
    - ____

Show answer

spec:
  accessModes:
    - ReadWriteMany

Match the modes.

RWO

One node writes
ROX

Many nodes read
RWX

Many nodes write
RWOP

One pod writes

Next lesson: Mounting a PVC to a Pod

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