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Kubernetes - Storage: Volume - emptyDir

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• Kubernetes - Storage: Volume - emptyDir
  • emptyDir
    • Declarative Manifest
    • Classic use case: work with init container
    • Lab: emptyDir - share file

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emptyDir

• a temporary volume that is created when a Pod is assigned to a node and exists as long as that Pod remains on that node.

• physical path of directory on host node
  • /var/lib/kubelet/pods/pod_uid/volumes/kubernetes.io~empty/volume_name/
• if the pod is deleted, the directory is deleteed as well.

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Declarative Manifest

• pod.spec.volumes field

  • medium field:
    • default: medium of the host node; the directory is created on one of the node’s disks
    • value:
      • Memory: use tmpfs, a virtual memory filesystem where the files are kept in memory instead of on the hard disk.
  • sizeLimit field:
    • total amount of local storage
    • e.g., 10Mi

• common spec.containers.volumeMounts configuration

  • name field: name of the volume to mount.
  • mountPath field: path within the container at which to mount the volume.
  • readOnly field:
    • Whether to mount the volume as read-only.
    • Defaults: false.

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Classic use case: work with init container

• requirements:
  • db pod is empty when created and need manually insert init data
  • db needs to insert init data automatically
  • db needs to persist data
• solution:
  • define 2 volumes:
    • init_script_vol
    • db_data_vol
  • Copy script of inserting data to the init_container_image
  • setup init container to copy the script from init_container_image to the init_script_vol
  • mount init_script_vol to regular container to automatically execute the init script
  • mount db_data_vol to persist data even db container restart.

apiVersion: v1
kind: Pod
metadata:
  name: demo-init-db
spec:
  volumes:
    - name: initdb
      emptyDir: {}
    - name: db-data
      emptyDir: {}
  # init container
  initContainers:
    - name: installer
      image: installer_image
      volumeMounts:
        - name: initdb
          mountPath: /initdb.d
  # container
  containers:
    - name: mongo
      image: mongo
      volumeMounts:
        - name: initdb
          mountPath: /docker-entrypoint-initdb.d/ # use entry point for init script
          readOnly: true
        - name: db-data
          mountPath: /data/db

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Lab: emptyDir - share file

# demo-emptydir-sharefile.yaml
apiVersion: v1
kind: Pod
metadata:
  name: demo-emptydir-sharefile
spec:
  volumes:
    - name: shared-data
      emptyDir: {}
  # no restart
  restartPolicy: Never

  containers:
    - name: file-writer
      image: busybox
      volumeMounts:
        - name: shared-data
          mountPath: /app-data
      command:
        [
          "/bin/sh",
          "-c",
          "echo '<h1>Hello from the data writer container!</h1>' > /app-data/index.html; sleep 10",
        ]

    - name: nginx
      image: nginx:alpine
      volumeMounts:
        - name: shared-data
          mountPath: /usr/share/nginx/html
      ports:
        - containerPort: 80

kubectl apply -f demo-emptydir-sharefile.yaml
# pod/demo-emptydir-sharefile created

kubectl get pod
# NAME                      READY   STATUS     RESTARTS      AGE
# demo-emptydir-sharefile   1/2     NotReady   0             23s

kubectl port-forward demo-emptydir-sharefile 8080:80
# Forwarding from 127.0.0.1:8080 -> 80
# Forwarding from [::1]:8080 -> 80

curl localhost:8080
# <h1>Hello from the data writer container!</h1>

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