PV & PVCs

rosehgal · Sep 28, 2020 · 1733a5d · 1733a5d
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diff --git a/README.md b/README.md
@@ -2,7 +2,7 @@
 </p>
 
 # K8s in 30 mins
-This is not a comprehensive guide to learn Kubernetes from scratch, rahter this is just a small guide/cheat sheet to quickly setup and run application with Kubernetes and deploy a very simple application on single workload VM. This repo can be served as quick learning manual to understand kubernetes.
+This is not a comprehensive guide to learn Kubernetes from scratch, rather this is just a small guide/cheat sheet to quickly setup and run application with Kubernetes and deploy a very simple application on single workload VM. This repo can be served as quick learning manual to understand kubernetes.
 
 #### Prerequisite
 - [Linux](https://files.fosswire.com/2007/08/fwunixref.pdf)
@@ -28,13 +28,12 @@ This is not a comprehensive guide to learn Kubernetes from scratch, rahter this
  - I will pick up the plugin called [Flannel](https://github.com/coreos/flannel#flannel).
 1. [**Stateless Workload**](#stateless-workloads)
  - Replicasets & Deployments
-1. **Stateful Workloads**
- - Persistent Volumes
- - Persistent Volume Claims
+1. [**Stateful Workloads**](#stateful-workloads)
+ - [Persistent Volumes](#persistent-volumes)
+ - [Persistent Volume Claims](#persistent-volume-claims)
 1. **Deploying a simple Java sprinboot app in kubernetes cluster**
  - Java app deployment with Mysql PV & PVC.
  - Setting up LB service to connect to Springboot application.
- - A short discussion about Cloud Config Manager(CCM)
 1. [**Understanding** advance kubernetes resources](#advance-kubernetes-resources):
  - [Namespaces](#namespaces)
  - [Context](#context)
@@ -531,4 +530,150 @@ root@vagrant:/home/vagrant/kubedata#
 - Deployments and Replicasets that we had deployed so far are stateless workloads.
 - There is no state related information stored at Pods/Service, so request from kube-proxy via serivce resource can be routed to any of the Pod in the cluster.
 - This constitutes stateless workload.
-- Next section is to create a Stateful workload.
+- Next section is to create a Stateful workload.
+
+## Stateful workloads
+- Preserve the state of data present on Pods.
+- Two situations can be possible:
+ - Multi pod stateful workload
+ - If multiple pods are connecting to stateful workload, there should be worker based synchronization
+ - Else, stateful data may go out of sync.
+ - Single pod stateful workload
+ - Create persitant volumes
+ - Create persitant volume claims to access persitant volumes in a synchronized way, just to prevent ensure data atomicity.
+
+### Persistent Volumes
+- PV are like volumes in Docker, just that their lifecycle is independent of Pods.
+- This is an API object. Captures details about storage implementation.
+- Provised by Kubernetes administrator.
+- Way to abstract storage resource.
+- Create a persistent volume for MySQL server. [File](files/pv.yml)
+ ```yaml
+ kind: PersistentVolume
+ apiVersion: v1
+ metadata:
+ name: pv
+ labels:
+ type: local
+ spec:
+ storageClassName: manual
+ capacity:
+ storage: 5Gi
+ accessModes:
+ - ReadWriteOnce
+ hostPath:
+ path: "/data"
+ ```
+ This spec specifies the volume is at `/data` on cluster's node.
+ Apply it : `kubectl apply -f pv.yml`
+ ```bash
+ root@vagrant:/home/vagrant/kubedata# kubectl get pv
+ NAME CAPACITY ACCESS MODES RECLAIM POLICY STATUS CLAIM STORAGECLASS REASON AGE
+ pv 5Gi RWO Retain Available manual 62s
+ ```
+
+
+### Persistent Volume Claims
+- Storage requeest by a user.
+- PVCs consume PV resources.
+- Way to access abstract storage.
+- PVC can request specific size and access mode: `ReadWriteOnce`, `ReadOnlyMany`, `ReadWriteMany`
+
+|Access Mode| Meaning|
+|---|---|
+|ReadWriteOnce | volume can be mounted as read-write by a single node|
+|ReadOnlyMany | volume can be mounted read-only by many nodes|
+|ReadWriteMany | volume can be mounted as read-write by many nodes|
+
+- Create a PVC spec. [File](files/pvc.yml)
+ ```yaml
+ kind: PersistentVolumeClaim
+ apiVersion: v1
+ metadata:
+ name: pv-claim
+ spec:
+ storageClassName: manual
+ accessModes:
+ - ReadWriteOnce
+ resources:
+ requests:
+ storage: 1Gi
+ ```
+ Apply it.
+ ```bash
+ root@vagrant:/home/vagrant/kubedata# kubectl apply -f pv-claim.yml
+ persistentvolumeclaim/pv-claim created
+
+ root@vagrant:/home/vagrant/kubedata# kubectl get pvc
+ NAME STATUS VOLUME CAPACITY ACCESS MODES STORAGECLASS AGE
+ pv-claim Bound pv 5Gi RWO manual 8s
+
+ root@vagrant:/home/vagrant/kubedata# kubectl describe pvc pv-claim
+ Name: pv-claim
+ Namespace: default
+ StorageClass: manual
+ Status: Bound
+ Volume: pv
+ Labels: <none>
+ Annotations: pv.kubernetes.io/bind-completed: yes
+ pv.kubernetes.io/bound-by-controller: yes
+ Finalizers: [kubernetes.io/pvc-protection]
+ Capacity: 5Gi
+ Access Modes: RWO
+ VolumeMode: Filesystem
+ Mounted By: <none>
+ Events: <none>
+ root@vagrant:/home/vagrant/kubedata#
+ ```
+ - Pods use PersistentVolumeClaims to request physical storage
+ - After creating the PersistentVolumeClaim, the Kubernetes control plane looks for a PersistentVolume that satisfies the claim's requirements. If the control plane finds a suitable PersistentVolume with the same StorageClass, it binds the claim to the volume.
+
+- Lets create a POD which will use PV as Volume using PVC. [File](files/nginx-pod-with-pv.yml)
+```yaml
+apiVersion: v1
+kind: Pod
+metadata:
+ name: nginx-pod-with-pvc
+spec:
+ volumes:
+ - name: nginx-pv-storage
+ persistentVolumeClaim:
+ claimName: pv-claim
+ containers:
+ - name: nginx-with-pv
+ image: nginx
+ volumeMounts:
+ - mountPath: "/usr/share/nginx/html"
+ name: nginx-pv-storage
+```
+```bash
+root@vagrant:/home/vagrant/kubedata# kubectl get pods nginx-pod-with-pvc
+NAME READY STATUS RESTARTS AGE
+nginx-pod-with-pvc 1/1 Running 0 16s
+
+root@vagrant:/home/vagrant/kubedata# kubectl exec -it nginx-pod-with-pvc -c nginx-with-pv -- /bin/bash
+root@nginx-pod-with-pvc:/# curl localhost
+Hi PV
+
+```
+- The file we just created in storage is made accessible to Nginx POD.
+
+
+### Summary
+```
+ +--------------------------------------+
+ | +------------+ |
+ | | POD | +--------------->
+ | +-----+------+ | | |
+ | | | | |
+ | | +-----+------+ | v
+ | | | PV | | /data
+ | | +------+-----+ |
+ | +-----v------+ ^ |
+ | | PVC +---------+ |
+ | +------------+ |
+ | |
+ +--------------------------------------+
+```
+- PV to PVC bind is automatic, based on storage class.
+- Pod/Deployment/K8s-Resource has to be done manually.
diff --git a/files/nginx-pod-with-pv.yml b/files/nginx-pod-with-pv.yml
@@ -0,0 +1,15 @@
+apiVersion: v1
+kind: Pod
+metadata:
+ name: nginx-pod-with-pvc
+spec:
+ volumes:
+ - name: nginx-pv-storage
+ persistentVolumeClaim:
+ claimName: pv-claim
+ containers:
+ - name: nginx-with-pv
+ image: nginx
+ volumeMounts:
+ - mountPath: "/usr/share/nginx/html"
+ name: nginx-pv-storage
diff --git a/files/pv.yml b/files/pv.yml
@@ -0,0 +1,14 @@
+kind: PersistentVolume
+apiVersion: v1
+metadata:
+ name: pv
+ labels:
+ type: local
+spec:
+ storageClassName: manual
+ capacity:
+ storage: 5Gi
+ accessModes:
+ - ReadWriteOnce
+ hostPath:
+ path: "/data" 
diff --git a/files/pvc.yml b/files/pvc.yml
@@ -0,0 +1,11 @@
+kind: PersistentVolumeClaim
+apiVersion: v1
+metadata:
+ name: pv-claim
+spec:
+ storageClassName: manual
+ accessModes:
+ - ReadWriteOnce
+ resources:
+ requests:
+ storage: 1Gi