Monitoring Kubernetes PVC disk usage - kubernetes

Question

I'm trying to monitor Kubernetes PVC disk usage. I need the memory that is in use for Persistent Volume Claim. I found the command:
kubectl get --raw / api / v1 / persistentvolumeclaims
Return:
"status":{
"phase":"Bound",
"accessModes":[
"ReadWriteOnce"
],
"capacity":{
"storage":"1Gi"
}
}
But it only brings me the full capacity of the disk, and as I said I need the used one
Does anyone know which command could return this information to me?

Answer 1

I don't have a definitive anwser, but I hope this will help you. Also, I would be interested if someone has a better anwser.
Get current usage
The PersistentVolume subsystem provides an API for users and administrators that abstracts details of how storage is provided from how it is consumed.
-- Persistent Volume | Kubernetes
As stated in the Kubernetes documentation, PV (PersistentVolume) and PVC (PersistentVolumeClaim) are abstractions over storage. As such, I do not think you can inspect PV or PVC, but you can inspect the storage medium.
To get the usage, create a debugging pod which will use your PVC, from which you will check the usage. This should work depending on your storage provider.
# volume-size-debugger.yaml
kind: Pod
apiVersion: v1
metadata:
name: volume-size-debugger
spec:
volumes:
- name: debug-pv
persistentVolumeClaim:
claimName: <pvc-name>
containers:
- name: debugger
image: busybox
command: ["sleep", "3600"]
volumeMounts:
- mountPath: "/data"
name: debug-pv
Apply the above manifest with kubectl apply -f volume-size-debugger.yaml, and run a shell inside it with kubectl exec -it volume-size-debugger sh. Inside the shell run du -sh to get the usage in a human readable format.
Monitoring
As I am sure you have noticed, this is not especially useful for monitoring. It may be useful for a one-time check from time to time, but not for monitoring or low disk space alerts.
One way to setup monitoring would be to have a similar sidecar pod like ours above and gather our metrics from there. One such example seems to be the node_exporter.
Another way would be to use CSI (Container Storage Interface). I have not used CSI and do not know enough about it to really explain more. But here are a couple of related issues and related Kubernetes documentation:
Monitoring Kubernetes PersistentVolumes - prometheus-operator
Volume stats missing - csi-digitalocean
Storage Capacity | Kubernetes

Answer 2

+1 to touchmarine's answer however I'd like to expand it a bit and add also my three cents.
But it only brings me the full capacity of the disk, and as I said I
need the used one
PVC is an abstraction which represents a request for a storage and simply doesn't store such information as disk usage. As a higher level abstraction it doesn't care at all how the underlying storage is used by its consumer.
#touchmarine, Instead of using a Pod whose only function is to sleep and every time you need to check the disk usage you need to attach to it maually, I would propose to use something like this:
apiVersion: apps/v1
kind: Deployment
metadata:
name: nginx-deployment
labels:
app: nginx
spec:
replicas: 1
selector:
matchLabels:
app: nginx
template:
metadata:
labels:
app: nginx
spec:
volumes:
- name: media
persistentVolumeClaim:
claimName: media
containers:
- name: nginx
image: nginx
ports:
- containerPort: 80
volumeMounts:
- mountPath: "/data"
name: media
- name: busybox
image: busybox
command: ["/bin/sh"]
args: ["-c", "while true; do du -sh /data; sleep 10;done"]
volumeMounts:
- mountPath: "/data"
name: media
It can be of course a single-container busybox Pod as in #touchmarine's example but here I decided to to show also how it can be used as a sidecar running next to nginx container within a single Pod.
As it runs a simple bash script - an infinite while loop, which prints out current disk usage to the standard output it can be read with kubectl logs without a need of using kubectl exec and attaching to the Pod:
$ kubectl logs nginx-deployment-56bb5c87f6-dqs5h busybox
20.0K /data
20.0K /data
20.0K /data
I guess it can be also used more effectively to configure some sort of monitoring of disk usage.