K8S PersistenVolume and PersistentVolumeClaim - kubernetes

Question

I am practicing k8s on storage topic. I don't understand why step2: PersistentVolume has different storage size when the tutorial configures PersistenVolumeClaim in step3
For example nfs-0001.yaml, nfs-0002.yaml. storages are 2Gi and 5Gi
apiVersion: v1
kind: PersistentVolumemetadata:
name: nfs-0001
spec:
capacity:
storage: 2Gi
accessModes:
- ReadWriteOnce
- ReadWriteMany
persistentVolumeReclaimPolicy: Recycle
nfs:
server: 172.17.0.7
path: /exports/data-0001
apiVersion: v1
kind: PersistentVolume
metadata: name: nfs-0002
spec:
capacity:
storage: 5Gi
accessModes:
- ReadWriteOnce
- ReadWriteMany
persistentVolumeReclaimPolicy: Recycle
nfs:
server: 172.17.0.7
path: /exports/data-0002
Example in step3 : pvc-mysql.yaml, pvc-http.yaml
kind: PersistentVolumeClaim
apiVersion: v1metadata:
name: claim-mysql
spec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 3Gi
kind: PersistentVolumeClaim
apiVersion: v1
metadata:
name: claim-http
spec:
accessModes:
- ReadWriteOnce
resources:
requests:
storage: 1Gi
And when I check the pv and pvc
master $ kubectl get pvc
NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE
claim-http Bound nfs-0001 2Gi RWO,RWX 17m
claim-mysql Bound nfs-0002 5Gi RWO,RWX 17m
master $ kubectl get pv
NAME CAPACITY ACCESS MODES RECLAIM POLICY STATUS CLAIM STORAGECLASS REASON AGE
nfs-0001 2Gi RWO,RWX Recycle Bound default/claim-http 19m
nfs-0002 5Gi RWO,RWX Recycle Bound default/claim-mysql 19m
Neither 1Gi and 3Gi shown up in the terminal.
Question:
Where are the 1Gi and 3Gi?
If they are not used. Is it safe to put arbitrary number to storage in PersistenVolumeClaim yaml?

Answer 1

You need to understand the difference between PV and PVC's. PVC is a declaration of stroage which that at some point become available for application to use and that is not the actual size of volume allocated.
PV's are the actual volume allocated at the time on the disk and ready to use. In order to use these PVs user needs to create PersistentVolumeClaims which is nothing but a request for PVs. A claim must specify the access mode and storage capacity, once a claim is created PV is automatically bound to this claim.
In your case, you have PV size of 5 and 3 GB respectively and you have started two PVC's with 3 and 1 GB respectively with accessmode: ReadWriteOnce that means there can be only one PV is attached to the one PVC.
Now the capacity of the PV available is the larger than requested and hence it allocated the larger size PV to the PVC.
PVC.spec.capacity is user's request for storage, "I want 10 GiB volume". PV.spec.capacity is actual size of the PV. PVC can bind to a bigger PV when there is no smaller available PV, so the user can get actually more than he wants.
Similarly, dynamic provisioning works typically in bigger chunks. So if user asks for 0.5GiB in a PVC, he will get 1 GiB PV because that's the smallest one that AWS can provision.
There is nothing wrong about it. Also, you should not put any random number in PVC size, it should be well calculated according to your application need and scaling.