Specialized applications can need to read/write directly to a block device. The main use case are databases. A filesystem is more “human-readable” and it adds extra overhead. Instead of the filesystem, block devices are chosen more often. More information about Kubernetes can be found HERE.
Storage Class YAML File
I am using the default Storage Class of the HX-CSI example directory. Nothing fancy.
kind: StorageClass apiVersion: storage.k8s.io/v1 metadata: name: csi-hxcsi-default provisioner: csi-hxcsi parameters: datastore: default-ds datastoreSize: "-1"
Persistent Volume Claim YAML File.
After creating the Storage Class, create a PVC as a Block Device with ReadWriteMany because different containers wants to read and write to the same shared storage.
apiVersion: v1 kind: PersistentVolumeClaim metadata: name: demo-block-rwx spec: storageClassName: csi-hxcsi-default volumeMode: Block accessModes: - ReadWriteMany resources: requests: storage: 10Gi
Deployement YAML File
In this example I’m using the Ubuntu container image. Because it’s a block device, devicePath is now used instead of volumeMount.
apiVersion: apps/v1 kind: Deployment metadata: name: demo-block labels: app: demo-block spec: replicas: 1 selector: matchLabels: app: demo-block template: metadata: labels: app: demo-block name: demo-block spec: volumes: - name: blockvolume persistentVolumeClaim: claimName: demo-block-rwx containers: - name: demo-block image: ubuntu command: [ "/bin/bash", "-c", "--" ] args: [ "while true; do sleep 30; done;" ] ports: - containerPort: 22 volumeDevices: - devicePath: /dev/block name: blockvolume
When the SC, PVC and the container are deployed, verification if everything is working can be done.
To verify if the block device connected in the container:
kubectl exec demo-block-757cfdc75c-59dbz -- bash -c "ls -l /dev/"
total 0 brw-rw---- 1 root disk 8, 32 Mar 11 19:48 block lrwxrwxrwx 1 root root 11 Mar 11 19:21 core -> /proc/kcore lrwxrwxrwx 1 root root 13 Mar 11 19:21 fd -> /proc/self/fd crw-rw-rw- 1 root root 1, 7 Mar 11 19:21 full drwxrwxrwt 2 root root 40 Mar 11 19:21 mqueue crw-rw-rw- 1 root root 1, 3 Mar 11 19:21 null lrwxrwxrwx 1 root root 8 Mar 11 19:21 ptmx -> pts/ptmx drwxr-xr-x 2 root root 0 Mar 11 19:21 pts crw-rw-rw- 1 root root 1, 8 Mar 11 19:21 random drwxrwxrwt 2 root root 40 Mar 11 19:21 shm lrwxrwxrwx 1 root root 15 Mar 11 19:21 stderr -> /proc/self/fd/2 lrwxrwxrwx 1 root root 15 Mar 11 19:21 stdin -> /proc/self/fd/0 lrwxrwxrwx 1 root root 15 Mar 11 19:21 stdout -> /proc/self/fd/1 -rw-rw-rw- 1 root root 0 Mar 11 19:21 termination-log crw-rw-rw- 1 root root 5, 0 Mar 11 19:21 tty crw-rw-rw- 1 root root 1, 9 Mar 11 19:21 urandom crw-rw-rw- 1 root root 1, 5 Mar 11 19:21 zero
There is a /dev/block and you can see it’s a block device, because the first letter on the line is a “b”
Let’s write some data on the block device:
kubectl exec demo-block-757cfdc75c-59dbz -- bash -c "echo "I_Am_Joost_._com" | dd conv=unblock of=/dev/block"
Verify what is written on the storage device:
kubectl exec demo-block-757cfdc75c-59dbz -- bash -c "od -An -c -N 20 /dev/block"
jvdmade@K8S-M:~$ kubectl exec demo-block-757cfdc75c-59dbz -- bash -c "od -An -c -N 20 /dev/block" I _ A m _ J o o s t _ . _ c o m
It is possible to create some Replica’s, but I want to have a complete “new” server connect to this RWX Block device, so I’ve created another deployment. Basically it’s the same YAML file, but with different names.
Here you see the Server2-demo pod is running:
jvdmade@K8S-M:~/block$ kubectl get pods
NAME READY STATUS RESTARTS AGE
csi-attacher-hxcsi-0 2/2 Running 0 3d21h
csi-nodeplugin-hxcsi-2w4cg 2/2 Running 2 3d21h
csi-nodeplugin-hxcsi-h2pfz 2/2 Running 2 3d21h
csi-nodeplugin-hxcsi-vcbjq 2/2 Running 2 3d21h
csi-provisioner-hxcsi-0 2/2 Running 0 3d21h
csi-resizer-hxcsi-7979549b8f-wh58n 2/2 Running 0 3d21h
demo-block-757cfdc75c-59dbz 1/1 Running 0 2d23h
server2-block-fcd48df5-4tctb 1/1 Running 0 15s
test-647b4c8874-l5g5d 1/1 Running 0 3d20h
To see if the block device is really has the same content of the other container:
kubectl exec server2-block-fcd48df5-4tctb -- bash -c "od -An -c -N 16 /dev/block"
And the result is what the other container wrote to the device.
I _ A m _ J o o s t _ . _ c o m
Via Server2 we’re going to overwrite this data:
kubectl exec server2-block-fcd48df5-4tctb -- bash -c "echo "This_Is_Server_2_Writing_Something_To_The_Block_Device" | dd conv=unblock of=/dev/block"
Lets see if the content can be read from the other server with the command:
kubectl exec demo-block-757cfdc75c-59dbz -- bash -c "od -An -c -N 75 /dev/block"
And here is the result:
T h i s _ I s _ S e r v e r _ 2 _ W r i t i n g _ S o m e t h i n g _ T o _ T h e _ B l o c k _ D e v i c e \n \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0 \0
Hooray! It’s working.
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