kv and kvs in k8s

PVC and PV

1: PVC and PV overview

1.1 what are pvc and pv

Persistent volume (PV) is a segment of network storage that has been configured by the administrator in the cluster. A resource in a cluster is like a node, which is a cluster resource. PV is a volume plug-in such as a volume, but has a lifecycle independent of any single pod using PV. This API object captures the implementation details of storage, that is, NFS, iSCSI 1 or cloud provider specific storage systems.

Persistent volume claim (PVC) is a user stored request. Usage logic of pvc: define a storage volume (the storage volume type is PVC) in the pod. When defining, directly specify the size. pvc must establish a relationship with the corresponding pv. pvc will apply for pv according to the definition, and pv is created from the storage space. pv and pvc are storage resources abstracted from kubernetes.

Although persistent volume claims allows users to use abstract storage resources, the common requirement is that users need to create PVS according to different requirements for different scenarios. At this time, the cluster administrator needs to provide PV with different requirements, not just the size and access mode of PV, but users do not need to understand the implementation details of these volumes. For such requirements, the storageclass resource can be used at this time.

PV is a resource in the cluster. PVc is a request for these resources and an index check for resources.

The interaction between PV and pvc follows this life cycle

  • Provisioning - > binding - > Using - > releasing - > recycling



1.2 two pv supply modes

PV is provided in two ways: static and dynamic

  • Static - > direct fixed storage space
    • The Cluster Administrator creates some PVS. They carry details of the formal storage available to cluster users. They exist in the kubernetes API and can be used for consumption.
  • Dynamic ----- > dynamically create space through storage classes
    • When the static PVS created by the administrator do not match the user's pvc, the cluster may try to dynamically configure volumes for pvc. This configuration is based on StorageClasses: PVC must request a storage class, and the administrator must have created and configured the class for dynamic configuration. The declaration of this class is required to effectively disable dynamic configuration for itself



Summary

PV is to create a storage resource (logically existing) from the space of the storage device

  • Static: storage resources created by k8s administrator for k8s cluster (pod) can be created from remote NFS or distributed object storage system (pv storage space size and access mode)

  • Dynamic storageclass (storage class resource): it is used to dynamically and automatically create pv resources of pvc application for use by pod

The pod uses pvc -- request -------- PV resource -------- in the storage device



2: View how pv and pvc are defined

2.1 use explain to view the definition of pv

2.1.1 viewing the definition method of pv

kubectl  explain pv  #View how pv is defined

FIELDS:
  apiVersion
  kind
  metadata
  spec



2.1.2 viewing pv defined specifications

[root@master ~]# kubectl  explain pv.spec
spec:
  nfs (Define storage type)
    path (Define mount volume path)
    server (Define server name)
  accessModes (Define access model,There are three access models,Exists as a list,That is, multiple access patterns can be defined)
    ReadwriteOnce (RWO) Single node read / write
    ReadonlyMany (ROX) Multi node read only
    ReadwriteMany (RWX) Multi node read / write
  capacity (definition PV Size of space)
    storage (Specify size)



2.2 use explain to view the definition of pvc

2.2.1 view the definition of pvc

kubectl  explain  pvc  #View how pvc is defined
KIND:  PersistentVolumeClaim
VERSION:  v1
FIELDS:
    apiVersion: <string>
    kind <string>
    metadata <Object>
    spec <Object>


2.2.2 check the specification of pvc

kubectl  explain pvc.spec  #View pvc specifications

spec:
	accessModes (Define access mode,Must be pv Subset of access patterns)
	resources (Define the size of the requested resource)
	  requests:  
	  storage:



3: Configure nfs to use pv and pvc

3.1 configuring nfs storage  

[root@nfs ~]# yum -y install nfs-utils rpcbind
[root@nfs ~]# mkdir -p /data/volumes/v{1..5}
[root@nfs ~]# ls -R /data/
[root@nfs ~]# chmod  -R 777 /data/*


#Configure directory for nfs share
[root@nfs ~]# for i in {1..5}
do 
echo "/data/volumes/v$1 192.168.23.0/24(rw,no_root_squash,sync)" >> /etc/exports
done 


#Write page content
[root@nfs ~]# for i in {1..5}
do
echo "this is pv00$i" > /data/volumes/v$i/index.html
done


[root@nfs ~]# systemctl  start rpcbind
[root@nfs ~]# systemctl start nfs
[root@nfs ~]# exportfs  -arv
[root@nfs ~]# showmount  -e



3.2 definition pv

Define five PVS, and define the mounting path and access mode. PVS are divided into sizes

[root@master ~]# vim pv-demo.yaml
apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv001
  labels:
    name: pv001
spec:
  nfs:
    path: /data/volumes/v1
    server: stor01
  accessModes: 
    - ReadWriteMany
    - ReadWriteOnce
  capacity:
    storage: 1Gi
---
apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv002
  labels:
    name: pv002
spec:
  nfs:
    path: /data/volumes/v2
    server: stor01
  accessModes: 
    - ReadWriteOnce
  capacity:
    storage: 2Gi
---
apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv003
  labels:
    name: pv003
spec:
  nfs:
    path: /data/volumes/v3
    server: stor01
  accessModes: 
    - ReadWriteMany
    - ReadWriteOnce
  capacity:
    storage: 2Gi
---
apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv004
  labels:
    name: pv004
spec:
  nfs:
    path: /data/volumes/v4
    server: stor01
  accessModes: 
    - ReadWriteMany
    - ReadWriteOnce
  capacity:
    storage: 4Gi
---
apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv005
  labels:
    name: pv005
spec:
  nfs:
    path: /data/volumes/v5
    server: stor01
  accessModes: 
    - ReadWriteMany
    - ReadWriteOnce
  capacity:
    storage: 5Gi


[root@master ~]# kubectl  apply  -f pv-demo.yaml 
[root@master ~]# kubectl  get pv



3.3 definition of pvc

3.3.1 situation 1  

The access mode, access mode and storage size (capacity column) of the pvc request are fully consistent

[root@master ~]# vim pod-vol-pvc.yaml
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: mypvc
  namespace: default
spec:
  accessModes:
  - ReadWriteMany
  resources:
    requests:
      storage: 2Gi
---
apiVersion: v1
kind: Pod
metadata:
  name: pod-vo1-pvc
  namespace: default
spec:
  containers:
  - name: myapp
    image: ikubernetes/myapp:v1
    volumeMounts:
    - name: html
      mountPath: /usr/share/nginx/html
  volumes:
  - name: html
    persistentVolumeClaim:
       claimName: mypvc


[root@master ~]# kubectl  apply -f  pod-vol-pvc.yaml 
persistentvolumeclaim/mypvc created
pod/pod-vo1-pvc created
[root@master ~]# kubectl  get pods,pv -o wide

[root@master ~]# curl 10.244.1.151
this is pv003


3.3.2   Case 2

If the access mode matches and the size does not match, the closest size will be selected among the PVs larger than the requested size

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: mypvc-test02
  namespace: default
spec:
  accessModes:
  - ReadWriteMany
  resources:
    requests:
      storage: 2Gi
---
apiVersion: v1
kind: Pod
metadata:
  name: pod-vo2-pvc
  namespace: default
spec:
  containers:
  - name: myapp
    image: ikubernetes/myapp:v1
    volumeMounts:
    - name: html
      mountPath: /usr/share/nginx/html
  volumes:
  - name: html
    persistentVolumeClaim:
       claimName: mypvc-test02


[root@master ~]# kubectl  apply  -f  pod-vol-pvc.yaml 
persistentvolumeclaim/mypvc-test02 created
pod/pod-vo2-pvc created
[root@master ~]# kubectl  get pods,pv,pvc  -o wide
[root@master ~]# curl 10.244.2.117
this is pv004


3.3.3 situation 3  

If the access mode does not match or the size is not satisfied (both valid), both pod and pvc are in pending state

[root@master ~]# vim   pod-vol-pvc.yaml 
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: mypvc-test03
  namespace: default
spec:
  accessModes:
  - ReadWriteMany
  resources:
    requests:
      storage: 7Gi
---
apiVersion: v1
kind: Pod
metadata:
  name: pod-vo3-pvc
  namespace: default
spec:
  containers:
  - name: myapp
    image: ikubernetes/myapp:v1
    volumeMounts:
    - name: html
      mountPath: /usr/share/nginx/html
  volumes:
  - name: html
    persistentVolumeClaim:
       claimName: mypvc-test03


[root@master ~]# kubectl  apply  -f  pod-vol-pvc.yaml 
persistentvolumeclaim/mypvc-test03 created
pod/pod-vo3-pvc created
[root@master ~]# kubectl  get pods,pv,pvc  -o wide

[root@master ~]# kubectl  get pods,pv,pvc  -o wide

[root@master ~]# kubectl  describe  pod pod-vo3-pvc


3.3.4 situation 4  

Use the multi host read / write RWX (ReadWriteMany) mode to add the newly created pod to the existing pvc

[root@master ~]# vim pod-vol-pvc.yaml
apiVersion: v1
kind: Pod
metadata:
  name: pod-vo4-pvc
  namespace: default
spec:
  containers:
  - name: myapp
    image: ikubernetes/myapp:v1
    volumeMounts:
    - name: html
      mountPath: /usr/share/nginx/html
  volumes:
  - name: html
    persistentVolumeClaim:
       claimName: mypvc-test02


[root@master ~]# kubectl  apply  -f  pod-vol-pvc.yaml 
pod/pod-vo4-pvc created
[root@master ~]# kubectl  get pods,pv,pvc  -o wide
[root@master ~]# curl  10.244.1.152
this is pv004



3.3.4 summary of PVC binding and multi node reading and writing

When the type of pvc request, accessModes, and storage size do not exactly match the pv

  • When the accessModes type is the same

    • Select storage to store pv larger than the requested,
    • When more than one is greater than, the closest one will be selected.

  • When the types of accessModes do not match, or the storage size is smaller than the request

    • pod and pvc will be in pnding state

Multi node read / write:

When creating a pod, the value of pod.spec.volumes.claimName uses the existing pvc name. It can be that pod uses the existing pvc to use pv


3.4 delete pvc binding

[root@master ~]# kubectl  describe  persistentvolumeclaims mypvc-test02
....
Mounted By:    pod-vo2-pvc
               pod-vo4-pvc

.....

#Delete all pod s using this pvc first
[root@master ~]# kubectl delete  pod pod-vo{2,4}-pvc
pod "pod-vo2-pvc" deleted
pod "pod-vo4-pvc" deleted


#Delete pvc again
[root@master ~]# kubectl delete  persistentvolumeclaims mypvc-test02
persistentvolumeclaim "mypvc-test02" deleted


#It is found that the pvc has indeed been deleted, but the corresponding pv is in the Released state. At this time, the pv cannot be bound by the new pvc
[root@master ~]# kubectl  get pods,pv,pvc  -o wide
NAME              READY   STATUS    RESTARTS   AGE   IP       NODE     NOMINATED NODE   READINESS GATES
persistentvolume/pv004   4Gi        RWO,RWX        Retain           Released    default/mypvc-test02                           73m   Filesystem


Use Edit to edit the PV resource online and delete the claimef paragraph. After saving, view it through the command, its status will automatically change to Available, and PV can be reused

[root@master ~]# kubectl  edit  persistentvolume pv004
...
 #delete
  claimRef:
    apiVersion: v1
    kind: PersistentVolumeClaim
    name: mypvc-test02
    namespace: default
    resourceVersion: "242922"
    uid: 95ef0c00-754e-4a8e-81c3-f8ee4d5f9824
.....


[root@master ~]# kubectl  get pods,pv,pvc  -o wide

NAME                     CAPACITY   ACCESS MODES   RECLAIM POLICY   STATUS      CLAIM           STORAGECLASS   REASON   AGE   VOLUMEMODE

persistentvolume/pv004   4Gi        RWO,RWX        Retain           Available                                           81m   Filesystem

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4: storageClass  

4.1 why storageClass

There are problems in the use of pvc and pvc. When applying for storage space for pvc, there may not be ready-made pv to meet the requirements of pvc application. The reason why nfs can succeed in pvc is that we have done the specified requirements processing.

So how to handle when the storage space applied for by PVC does not necessarily have a PV that meets the PVC requirements??? Therefore, Kubernetes provides the administrator with a method (StorageClass) to describe the storage "class".

For example, divide a 1TB storage space in the storage system for Kubernetes to use. When users need a 10G PVC, they will immediately send a request through restful to create a 10G image in the storage space, and then define 10G PV in our cluster to supply it to the current PVC for mounting. Before that, our storage system must support restful interface, which is better than that For example, ceph distributed storage, while glusterfs needs a third-party interface to complete such requests.



4.2 example  

kubectl explain storageclass #Storageclass is also the resource kind: storageclass on k8s
VERSION: storage.k8s.io/v1
FIELDS:
  allowVolumeExpansion <boolean>
  allowedTopologies  <[]object>
  apiversion <string>
  kind <string>
  metadata <object>
  mountOptions <[string> #Mount options
  parameters <map [string]string> #Parameters, depending on the allocator, can accept different parameters. For example, the value io1 of parameter type and the parameter iopsPerGB are specific to EBS PV. When the parameters are omitted, the default values are used.
  provisioner <string>-required- #Storage allocator, used to determine which volume plug-in is used to allocate pv. This field must refer to
  reclaimPolicy  <string> #The recycling policy can be Delete or Retain. If the storageclass object is created without specifying the reclaimPolicy, it will default to Delete
  volumeBindingMode  <string> #Binding mode of volume


storageclass Contains provisioner, parameters and reclaimPolicy field,When class Dynamic allocation required persistentvolume Will be used when. because storageclass A separate storage system is required,Not shown here.
View definitions from other materials storageclass The method is as follows:
kind: storageclass
apiversion: storage.k8s.io/vl
metadata:
	name: standard
provisioner: kubernetes.io/aws-ebs
parameters:
  type: gp2
reclaimPolicy: Retain
mountOptions:
  - debug

Tags: Kubernetes

Posted on Thu, 11 Nov 2021 06:16:41 -0500 by remal

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