[k8s] Persistent Volumes
=================
###### tags: `kubernetes` `storage`

> The **easiest** way to create the PV/PVC pair for your Pod is to use a StorageClass object, and then ***<span style="color: red">using the storageclass to create your PV-PVC pair dynamically</span>*** whenever you need to use it.

# Persistent Volume
+ <span style="color:red; font-weight: 600">a cluster resource</span>
+ need to be there before using
+ created via yaml file
+ kind : PersistenVolume
+ spec: e.g. how much storage?
+ what types of storage do you need?
+ local storage、nfs server、cloud storage

### config
+ <kbd>accessModes</kbd>
+ `ReadWriteOnce` — the volume can be mounted as read-write by a single node
+ `ReadOnlyMany` — the volume can be mounted read-only by many nodes
+ `ReadWriteMany` — the volume can be mounted as read-write by many nodes
+ <kbd>persistentVolumeReclaimPolicy</kbd>
+ `Retain`
+ 當 PVC 刪除後(此時 PV 狀態為 Released)，cluster administrator 可以進行手動的 reclaim
+ `Recycle` - 未來將會被廢棄，若有同樣的需求，建議改用 dynamic provisioning 來取代 (storage class)
+ `Delete` - 當這個 PVC 被刪除後，相對應的 PV & 實際的 storage 空間也都會一併被刪除。
+ as default value
+ <kbd>storageClassName</kbd>
+ <span style="color: purple; font-weight: 600;">若 PV 設定了 class，那就只能讓帶有相同 class request 的 PVC 連結；反之亦然。</span>
+ <kbd>volumeMode</kbd>
+ <font color="red">???</font>: FileSystem、Block

### life cycle
+ `Available`: 可被 PVC 使用的狀態
+ `Bound`: 已經連結到特定的 PVC
+ `Released`: PVC 已經被刪除，但 cluster 尚未進行 reclaim 的工作
+ reclaim步驟
1. 刪除 PV (對應的實際資料並不會消失)
2. 清除實際資料 (optional, 如果有需求可做)
3. 若是使用已經存在的資料，那就再建立一個 PV，並指到原本的儲存空間
+ `Failed`: reclaim 失敗後會進入的狀態

## how to create PV
#### YAML example: Local storage

apiVersion: v1
kind: PersistentVolume
metadata:
  name: example-pv
spec:
  capacity:
    storage: 100Gi
  volumeMode: Filesystem
  accessModes:
    - ReadWriteOnce
  persistentVolumeReclaimPolicy: Delete
  storageClassName: local-storage
  local:
    path: /mnt/disks/ssd1
  nodeAffinity:
    required:
      nodeSelectorTerms:
        -matchExpressions:
          - key: kubernetes.io/hostname
            operator: In
            values:
              - example-node

#### YAML example: NFS storage

apiVersion: v1
kind: PersistentVolume
metadata:
  name: pv-name
spec:
  capacity:
    storage: 5Gi
  volumeMode: Filesystem
  accessModes:
    - ReadWriteOne
  persistentVolumeReclaimPolicy: Recycle
  storeageClassName: slow
  mountOptions:
    - hard
    - nfsvers=4.0
  nfs:
    path: /dir/path/on/nfs/server
    server: nfs-server-ip-address
  

#### YAML example: google cloud storage

apiVersion: v1
kind: PersistentVolume
metadata:
  name: test-volume
  labels:
    failure-domain.beta.kubernetes.io/zone: us-central1-a__us-central1-b
  spec:
    storage: 400Gi
  accessModes:
    - ReadWriteOnce
  gcePersistentDisk:
    pdName: my-data-disk
    fsType: ext4

## persisten volumes are not namespaced

# Persistent Volume Claim


## How Application to access volume

1. Pod ***requests the volume*** through the PV claim
2. Claim tries to ***find a volume*** in cluster
3. Volume has the ***actual storage*** backend
4. Claims must be in the ***same namespace***
5. Volume is mounted into the Pod
6. Volume is mounted into Container


apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: pvc-name
spec:
  storageClassName: manual
  volumeMode: Filesystem
  accessModes:
    - ReadWriteOnce
  resources:
    requests:
      storage: 10Gi

apiVersion: v1
kind: Pod
metadata:
  name: mypod
spec:
  containers:
    - name: myfronted
      image: nginx
      volumeMounts:
        - mountPath: "/var/www/html"
          name: mypd
  volumes:
    - name: mypd
      persistentVolumeClaim:
        claimName: pvc-name