AKS k8s doesn't supports new keyword ReadWriteOncePod for access mode for Volumes and Volume claims

Question

AKS k8s doesn't supports new keyword ReadWriteOncePod for access mode for Volumes and Volume claims

Artem 21

Hi.

I recently tried to create PersistentVolumeClaim with accessModes set to ReadWriteOncePod, but no lone listed version (even 24.0_preview) does nto supported it. Support chat wouldn't be able to resolve this issu either.

PersistentVolumeClaim is a new keyword, anounced in previous year (https://kubernetes.io/blog/2021/09/13/read-write-once-pod-access-mode-alpha/) and supported from Kubernetes 1.22 by the Google Docs. But Azure portal or kubectl both displaying error message:

The PersistentVolumeClaim "xnode-geth-data-volume-claim" is invalid: spec.accessModes: Unsupported value: "ReadWriteOncePod": supported values: "ReadOnlyMany", "ReadWriteMany", "ReadWriteOnce"

Google docs just says that it is available to use https://kubernetes.io/docs/concepts/storage/persistent-volumes/#access-modes

About the reason:
I have an application which exclusively using data folder and cannot share it with any another instances so this new access mode ReadWriteOncePod is exactly what I need. I already have an issue when AKS scheduled 2 pods per node and it was result in constnat looping restart of 2nd Pod in a same node.

Best Regards

shiva patpi 13,366 Reputation points Microsoft Employee Moderator

2022-08-19T19:03:24.783+00:00

Hello @Artem ,
Thanks for reaching out to us, this seems to be a valid use case scenario for your requirement. We are following up internally with AKS Product team - will keep you posted.

(I was able to repro the same)

For your next query: In order to deploy pods in different nodes - you can use the concept of affinity / podAntiAffinity in combination with requiredDuringSchedulingIgnoredDuringExecution , topology spreads !
reference doc: https://kubernetes.io/docs/concepts/scheduling-eviction/topology-spread-constraints/

Example deployment YAML file to spread 3 pods in 3 different Nodes:

apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx
spec:
replicas: 3
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx
ports:
- containerPort: 8080
affinity:
podAntiAffinity:
requiredDuringSchedulingIgnoredDuringExecution:
- labelSelector:
matchExpressions:
- key: "app"
operator: In
values:
- nginx
topologyKey: "kubernetes.io/hostname"

regards,
Shiva.

Accepted answer

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Your answer

shiva patpi 13,366 Reputation points Microsoft Employee Moderator

2022-08-19T19:03:24.783+00:00

Hello @Artem ,
Thanks for reaching out to us, this seems to be a valid use case scenario for your requirement. We are following up internally with AKS Product team - will keep you posted.

(I was able to repro the same)

For your next query: In order to deploy pods in different nodes - you can use the concept of affinity / podAntiAffinity in combination with requiredDuringSchedulingIgnoredDuringExecution , topology spreads !
reference doc: https://kubernetes.io/docs/concepts/scheduling-eviction/topology-spread-constraints/

Example deployment YAML file to spread 3 pods in 3 different Nodes:

apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx
spec:
replicas: 3
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
containers:
- name: nginx
image: nginx
ports:
- containerPort: 8080
affinity:
podAntiAffinity:
requiredDuringSchedulingIgnoredDuringExecution:
- labelSelector:
matchExpressions:
- key: "app"
operator: In
values:
- nginx
topologyKey: "kubernetes.io/hostname"

regards,
Shiva.

Answer 1

shiva patpi 13,366 Microsoft Employee Moderator

Hello @Artem ,
Got an update from Product team that, as the ReadWriteOncePod is still an alpha feature even in k8s 1.25 release, it’s disabled by default on AKS. In most cases, AKS only supports beta feature which is enabled by default , we will check whether this feature would go beta in 1.26 which is end of this year.

Regards,
Shiva.

superggn 5 Reputation points

2023-04-03T07:50:39.9666667+00:00

Hi @shiva patpi is there any updates? Is ReadWriteOncePod still an alpha feature in k8s 1.26 and the following 1.27?

Answer 2

Helllo @shiva patpi .

Thank you for the response.

After reviewing all the information that I have I ended up with the idea that ReadWriteOncePod in my case wouldn't be working as well, because I will have only a single claim.

Next I switched the way of how I create the application from Deployment to StatefulSet, since it has templates of volume claims. Plus my current application has to be in relation 1 Pod - 1 Node, so I applied affinity rules and it seems like it working well. But I guess if not the latest I probably will have a problems further.

Thanks,
Artem

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AKS k8s doesn't supports new keyword ReadWriteOncePod for access mode for Volumes and Volume claims

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