Kubernetes-subpath的使用

一、什么是subpath

为了支持单一个pod多次使用同一个volume而设计，subpath翻译过来是子路径的意思，如果是数据卷挂载在容器，指的是存储卷目录的子路径，如果是配置项configMap/Secret，则指的是挂载在容器的子路径。

二、subpath的使用场景

1、 1个pod中可以拉起多个容器，有时候希望将不同容器的路径挂载在存储卷volume的子路径，这个时候需要用到subpath

2、volume支持将configMap/Secret挂载在容器的路径，但是会覆盖掉容器路径下原有的文件，如何支持选定configMap/Secret的每个key-value挂载在容器中，且不会覆盖掉原目录下的文件，这个时候也可以用到subpath

三、subpath的使用

1、存储卷

    采用hostpath的方式创建PV，宿主机的映射目录为/data/pod/volume5

[root@k8s-master zhanglei]# cat pv-subpath.yaml 
kind: PersistentVolume
apiVersion: v1
metadata:
  name: pv-subpath-05
  labels:
    release: stable
spec:
  capacity:
    storage: 0.1Gi
  accessModes:
    - ReadWriteOnce
  persistentVolumeReclaimPolicy: Recycle
  hostPath:
    path: /data/pod/volume5                 # 宿主机的目录

[root@k8s-master zhanglei]# kubectl create -f pv-subpath.yaml  

PV创建成功后，再创建PVC

[root@k8s-master zhanglei]# cat pvc-subpath.yaml 
apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: pvc-subpath
  namespace: default
spec:
 accessModes: ["ReadWriteOnce"]
 resources:
   requests: 
     storage: 0.05Gi
[root@k8s-master zhanglei]# kubectl create -f pvc-subpath.yaml 

在pod中声明并使用subpath

[root@k8s-master zhanglei]# cat pod-subpath.yaml 
apiVersion: v1
kind: Pod
metadata:
  name: pod-subpath-zltest
spec:
    containers:
    - name: ubuntu-subpath-container
      image: ubuntu
      volumeMounts:
      - mountPath: /var/lib/ubuntu            # 容器1的挂载目录
        name: subpath-vol
        subPath: ubuntutest                   # 宿主机volume5的子目录1
    - name: nginx-subpath-container
      image: nginx
      volumeMounts:
      - mountPath: /var/www/nginx             # 容器2的挂载目录
        name: subpath-vol
        subPath: nginxtest                   # 宿主机volume5的子目录2 
    volumes:
    - name: subpath-vol
      persistentVolumeClaim:
        claimName: pvc-subpath               # PVC的名字

  [root@k8s-master zhanglei]# kubectl create -f pod-subpath.yaml

[root@k8s-master zhanglei]# kubectl describe pod  pod-subpath-zltest 
Name:         pod-subpath-zltest
Namespace:    default
Priority:     0
Node:         k8s-master/192.168.126.129
Start Time:   Fri, 29 May 2020 16:45:49 +0800
Labels:       <none>
Annotations:  cni.projectcalico.org/podIP: 10.122.235.235/32
              cni.projectcalico.org/podIPs: 10.122.235.235/32
Status:       Running
IP:           10.122.235.235
IPs:
  IP:  10.122.235.235
Containers:
  ubuntu-subpath-container:
    Container ID:   docker://6e5cb30ee7e03b77d2ca22e4cd818ff326fa40836427fe17b1584646b4388dce
    Image:          ubuntu
    Image ID:       docker-pullable://ubuntu@sha256:747d2dbbaaee995098c9792d99bd333c6783ce56150d1b11e333bbceed5c54d7
    Port:           <none>
    Host Port:      <none>
    State:          Waiting
      Reason:       CrashLoopBackOff
    Last State:     Terminated
      Reason:       Completed
      Exit Code:    0
      Started:      Sun, 14 Jun 2020 22:38:11 +0800
      Finished:     Sun, 14 Jun 2020 22:38:11 +0800
    Ready:          False
    Restart Count:  558
    Environment:    <none>
    Mounts:
      /var/lib/ubuntu from subpath-vol (rw,path="ubuntutest")
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-74s86 (ro)
  nginx-subpath-container:
    Container ID:   docker://95101741eb1b6aa4c1e53d8fc4ab8006e74fd2eb923eca211ca20a01edcd7630
    Image:          nginx
    Image ID:       docker-pullable://nginx@sha256:30dfa439718a17baafefadf16c5e7c9d0a1cde97b4fd84f63b69e13513be7097
    Port:           <none>
    Host Port:      <none>
    State:          Running
      Started:      Fri, 29 May 2020 16:47:14 +0800
    Ready:          True
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-74s86 (ro)
      /var/www/nginx from subpath-vol (rw,path="nginxtest")
Conditions:
  Type              Status
  Initialized       True 
  Ready             False 
  ContainersReady   False 
  PodScheduled      True 
Volumes:
  subpath-vol:
    Type:       PersistentVolumeClaim (a reference to a PersistentVolumeClaim in the same namespace)
    ClaimName:  pvc-subpath
    ReadOnly:   false
  default-token-74s86:
    Type:        Secret (a volume populated by a Secret)
    SecretName:  default-token-74s86
    Optional:    false
QoS Class:       BestEffort
Node-Selectors:  <none>
Tolerations:     node.kubernetes.io/not-ready:NoExecute for 300s
                 node.kubernetes.io/unreachable:NoExecute for 300s
Events:
  Type     Reason   Age                    From                 Message
  ----     ------   ----                   ----                 -------
  Normal   Pulled   21m (x555 over 16d)    kubelet, k8s-master  Successfully pulled image "ubuntu"
  Normal   Created  21m (x555 over 16d)    kubelet, k8s-master  Created container ubuntu-subpath-container
  Normal   Started  21m (x555 over 16d)    kubelet, k8s-master  Started container ubuntu-subpath-container
  Normal   Pulling  6m10s (x562 over 16d)  kubelet, k8s-master  Pulling image "ubuntu"
  Warning  BackOff  71s (x11744 over 16d)  kubelet, k8s-master  Back-off restarting failed container

现在来验证下在宿主机存储卷的目录下是否有2个子目录，1个是ubuntutest用来挂载容器1的，另外1个是nginxtest用来挂载容器2的

[root@k8s-master /]# cd data/pod/volume5
[root@k8s-master volume5]# ls
nginxtest ubuntutest
[root@k8s-master volume5]# cd nginxtest/     # 可以看到是1个目录，非文件
[root@k8s-master nginxtest]#

进入到容器中，挂载一个文件，验证是否可以同步到存储卷

[root@k8s-master nginxtest]# kubectl exec -it pod-subpath-zltest -c nginx-subpath-container -- bash
root@pod-subpath-zltest:/# cd /var/www/nginx
root@pod-subpath-zltest:/var/www/nginx# ls
nginx-test-subpath.txt

[root@k8s-master volume5]# cd nginxtest/
[root@k8s-master nginxtest]# ls
nginx-test-subpath.txt

可以看到容器1的目录/var/www/nginx 和存储卷的子目录 nginxtest完成了映射，容器2类似，这里不再赘述。

2、配置项-configMap

1）创建configMap

[root@k8s-master consecret]# cat conf-subpath.yaml 
apiVersion: v1
kind: ConfigMap
metadata:
  name: conf-subpath-zltest
  namespace: default
data:
  example.property.1: hello      # key-value键值对
  example.property.2: world
  example.property.file: |-
    property.1=value-1
    property.2=value-2
    property.3=value-3

2）在Pod中使用configMap

[root@k8s-master consecret]# cat pod-conf-subpath.yaml 
apiVersion: v1
kind: Pod
metadata:
  labels:
    purpose: test-configmap-volume
  name: pod-conf-testvolume
spec:
  containers:
    - name: test-configmap-volume
      image: nginx
      volumeMounts:
        - name: config-volume
          mountPath: /etc/nginx/example.property.1       # 容器挂载目录
          subPath: example.property.1                    # 将key名称作为文件名，hello作为文件内容
  volumes:
    - name: config-volume
      configMap:
         name: conf-subpath-zltest      # 指定使用哪个CM
        
[root@k8s-master consecret]# kubectl create -f pod-conf-subpath.yaml 

[root@k8s-master consecret]# kubectl describe pod  pod-conf-testvolume 
Name:         pod-conf-testvolume
Namespace:    default
Priority:     0
Node:         k8s-master/192.168.126.129
Start Time:   Wed, 03 Jun 2020 11:46:36 +0800
Labels:       purpose=test-configmap-volume
Annotations:  cni.projectcalico.org/podIP: 10.122.235.249/32
              cni.projectcalico.org/podIPs: 10.122.235.249/32
Status:       Running
IP:           10.122.235.249
IPs:
  IP:  10.122.235.249
Containers:
  test-configmap-volume:
    Container ID:   docker://e2cf37cb24af32023eb5d22389545c3468104a4344c47363b5330addc40cb914
    Image:          nginx
    Image ID:       docker-pullable://nginx@sha256:883874c218a6c71640579ae54e6952398757ec65702f4c8ba7675655156fcca6
    Port:           <none>
    Host Port:      <none>
    State:          Running
      Started:      Wed, 03 Jun 2020 11:46:53 +0800
    Ready:          True
    Restart Count:  0
    Environment:    <none>
    Mounts:
      /etc/nginx/example.property.1 from config-volume (rw,path="example.property.1")  
      /var/run/secrets/kubernetes.io/serviceaccount from default-token-74s86 (ro)
Conditions:
  Type              Status
  Initialized       True 
  Ready             True 
  ContainersReady   True 
  PodScheduled      True 
Volumes:
  config-volume:
    Type:      ConfigMap (a volume populated by a ConfigMap)
    Name:      conf-subpath-zltest
    Optional:  false
  default-token-74s86:
    Type:        Secret (a volume populated by a Secret)
    SecretName:  default-token-74s86
    Optional:    false
QoS Class:       BestEffort
Node-Selectors:  <none>
Tolerations:     node.kubernetes.io/not-ready:NoExecute for 300s
                 node.kubernetes.io/unreachable:NoExecute for 300s
Events:          <none>

在容器挂载路径验证下是否将configMap中example.property.1挂载在容器中，且是否会覆盖掉原有的目录

root@pod-conf-testvolume:/# cd  /etc/nginx 
root@pod-conf-testvolume:/etc/nginx# ls
conf.d            fastcgi_params  koi-win    modules     scgi_params   win-utf
example.property.1  koi-utf        mime.types    nginx.conf  uwsgi_params

从上可以看到example.property.1已经挂载到容器中，且未对目录原有的文件进行覆盖

root@pod-conf-testvolume:/etc/nginx# cd example.property.1 
bash: cd: example.property.1: Not a directory
root@pod-conf-testvolume:/etc/nginx# cat example.property.1 
helloroot@pod-conf-testvolume:/etc/nginx# 

从上可以验证configMap的subpath用法支持将configMap中的每对key-value以key名称作为文件名，value作为文件内容挂载到容器的目录中。

四、总结

本文介绍了subpath分别在持久化存储卷和配置项configMap中的使用，丰富了volume在pod中的使用场景。