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Using Read-Write-Many (RWX) volumes with Kubernetes

Introduction

The Pure Service Orchestrator Kubernetes CSI driver includes support for Read-Write-Many (RWX) block volumes on FlashArray starting with version 5.1.0. This feature allows Kubernetes end-users to create persistent block volumes that may be mounted into multiple pods simultaneously. Persistent volume claims created this way can be mounted exactly the same as a normal pod, only requiring that accessModes contains ReadWriteMany.

Restrictions

Read-Write-Many cannot be used with all combinations of storage classes. Specifically, we prohibit mounting a block volume as a filesystem as RWX.

Backend Type Mount Type Access Mode Valid?
Block Block RWO Yes
Block Block RWX Yes
Block File RWO Yes
Block File RWX No
File File RWO Yes
File File RWX Yes

Examples

To use these examples, install the Pure CSI plugin and apply the following example files.

For FlashArray/Cloud Block Store

FlashArrays can only be used for RWX volumes with raw block mounts, as shown in the following example files.

Raw Block PVC

Example Pods

To apply:

kubectl apply -f https://raw.githubusercontent.com/purestorage/helm-charts/master/docs/examples/rwx/pvc-block-many.yaml
kubectl apply -f https://raw.githubusercontent.com/purestorage/helm-charts/master/docs/examples/rwx/pod-block-many.yaml
# The raw block device will be mounted at /dev/pure-block-device

A note on caching: while testing using the above examples, it can be remarkably annoying to test writing data between pods, as it can be difficult to ensure the caches are flushed. One easy way to test the shared block storage is dd if=/dev/urandom of=/dev/pure-block-device bs=512 count=1 oflag=direct (where oflag=direct will bypass caches), and then read using dd if=/dev/pure-block-device of=/dev/stdout bs=512 count=1 iflag=direct (where iflag=direct will bypass caches again).

For FlashBlade

FlashBlade shares can be easily used for RWX volumes since they use NFS, as shown in the following example files.

File PVC

Example Pods

To apply:

kubectl apply -f https://raw.githubusercontent.com/purestorage/helm-charts/master/docs/examples/rwx/pvc-file-many.yaml
kubectl apply -f https://raw.githubusercontent.com/purestorage/helm-charts/master/docs/examples/rwx/pod-file-many.yaml
# The NFS volume will be mounted at /data