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实验环境：kubernetes Version v1.9.6 网络CNI：fannel

本地卷：

更好的利用本地高性能介质（SSD，Flash）提升数据库服务能力 QPS/TPS 更闭环的运维成本，现在越来越多的数据库支持基于Replicated的技术实现数据多副本和数据一致性（比如MySQL Group Replication / MariaDB Galera Cluster / Percona XtraDB Cluster的），DBA可以处理所有问题，而不在依赖存储工程师或者SA的支持。

为了使用本地存储需要启动FeatureGate:PersistentLocalVolumes支持本地存储，1.9是alpha版本，1.10是beta版，默认开启, v1.9版本需要api-server, controller-manager, scheduler, and all kubelets 开启 feature-gates的功能

--feature-gates=PersistentLocalVolumes=true
--VolumeScheduling=true
--MountPropagation=true

实战示例：

一、创建PersistentVolume

apiVersion: v1
kind: PersistentVolume
metadata:
name: local-pv
annotations:
    "volume.alpha.kubernetes.io/node-affinity": '{
        "requiredDuringSchedulingIgnoredDuringExecution": {
            "nodeSelectorTerms": [
                { "matchExpressions": [
                    { "key": "kubernetes.io/hostname",
                      "operator": "In",
                      "values": ["k8s-node1-product"]
                    }
                ]}
             ]}
          }'
spec:
capacity:
  storage: 10Gi
accessModes:
- ReadWriteOnce
persistentVolumeReclaimPolicy: Delete
storageClassName: local-storage
local:
  path: /mnt/disks/ssd1

二、创建Storage Class

kind: StorageClass
apiVersion: storage.k8s.io/v1
metadata:
name: local-storage
provisioner: kubernetes.io/no-provisioner
volumeBindingMode: WaitForFirstConsumer

三、创建Statefulset

apiVersion: apps/v1beta1 
kind: StatefulSet 
metadata: 
name: local-test 
spec: 
  serviceName: "local-service" 
  replicas: 1 
  template: 
    metadata: 
      labels: 
        app: local-test 
    spec: 
      containers: 
      - name: test-container 
        image: [gcr.io/google_containers/busybox:1.24]
        command: 
        - "/bin/sh" 
        args: 
        - "-c" 
        - "count=0; count_file=\"/usr/test-pod/count\"; test_file=\"/usr/test-pod/test_file\"; if [ -e $count_file ]; then count=$(cat $count_file); fi; echo $((count+1)) > $count_file; while [ 1 ]; do date >> $test_file; echo \"This is $MY_POD_NAME, count=$(cat $count_file)\" >> $test_file; sleep 10; done" 
        volumeMounts: 
        - name: local-vol 
          mountPath: /usr/test-pod 
        env: 
        - name: MY_POD_NAME 
         valueFrom: 
           fieldRef: 
             fieldPath: metadata.name 
     securityContext: 
       fsGroup: 1234 
 volumeClaimTemplates: 
 - metadata:
   annotations:
     volume.alpha.kubernetes.io/storage-class: local-storage
   name: local-vol 
   spec: 
     accessModes: [ "ReadWriteOnce" ] 
     storageClassName: "local-storage" 
     resources: 
       requests: 
         storage: 10Gi

该Statefulset的Pod将会调度到k8s-node1-product，并使用本地存储“local-pv”

“PersistentLocalVolumes”和“VolumeScheduling”的局限

使用局限需要考虑：
具体部署时,针对PersistentLocalVolumes 只能应用在特定的有状态服务的场景下
高可用风险需要考虑：

当Pod调度到某个节点后，将会跟该节点产生亲和，一旦Node发生故障，Pod不能调度到其他节点，只能等待该节点恢复，你能做的就是等待“Node恢复”，如果部署3节点MySQL集群，再挂一个Node，集群将无法提供服务，你能做的还是“等待Node恢复”。这么设计也是合理的，社区认为该Node为Stateful节点，Pod被调度到其他可用Node会导致数据丢失

最后，借用Google工程师Kelsey Hightower的一句话: “We very receptive this Kubernetes can’t be everything to everyone.”

参考文档： http://dockone.io/article/5260 https://blog.zhoulouzi.com/2018/03/kubernetes-local/ https://www.kubernetes.org.cn/2280.html?tdsourcetag=s_pctim_aiomsg https://kubernetes.io/docs/concepts/storage/volumes/#local

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