節錄線上課程Docker Mastery Section 5內容


Container Lifetime & Persistent Data

前情提要: docker image是由多個layer組成(read-only)，啟動一個container時，Docker會再掛一層Read-write!

如果在這個執行中的container中異動A文件，其實是從Read-only layer複製到Read-write layer，實際改的是複製出來的副本。

刪除container時，更改也就一起消失。 p.s. read-only layer 和最頂層的Read-write layer 合稱 Union File System

UFS

圖摘自:Virtualization-fizalihsan


  • containers 通常是 immutable, ephemeral
  • 只能re-deploy container, 才有可能改變
  • 遇到database or data 想存取時….
  • 於是Persistent data出現
  • 兩種方式- Volumes & Bind Mounts
  • volumes: make special location outside of container UFS
  • Bind Mounts: link container path to host path


Persistent Data: Data Volumes

先看看Docker hub»> mysql image»> Dockerfile

check mysql Dockerfile

docker pull mysql 載mysql

docker image inspect mysql 這包image資訊

 "Volumes": {
                "/var/lib/mysql": {}
            },

docker container run -d --name mysql -e MYSQL_ALLOW_EMPTY_PASSWORD=True mysql 啟動container

run container

docker container ls 確定running

docker container inspect mysql

   "Mounts": [
            {
                "Type": "volume",
                "Name": "46c9a803db3082250ebb375bb02dfcd3893945e85a9386b38741bd5ab4285756",
                "Source": "/mnt/sda1/var/lib/docker/volumes/46c9a803db3082250ebb375bb02dfcd3893945e85a9386b38741bd5ab4285756/_data",
                "Destination": "/var/lib/mysql",
                "Driver": "local",
                "Mode": "",
                "RW": true,
                "Propagation": ""
            }
        ],

docker volume ls

DRIVER              VOLUME NAME
local               46c9a803db3082250ebb375bb02dfcd3893945e85a9386b38741bd5ab4285756

docker volume inspect 46c9a803db3082250ebb375bb02dfcd3893945e85a9386b38741bd5ab4285756

mounts是本機位置; volume是container中的位置

volume inspect

再啟動一個 container mysql2

docker container rum -d --name mysql2 -e MYSQL_ALLOW_EMPTY_PASSWORD=True mysql

docker volume ls

DRIVER              VOLUME NAME
local               46c9a803db3082250ebb375bb02dfcd3893945e85a9386b38741bd5ab4285756
local               53b07a654678b5af510210a501f7570dd03a8ec9af7059860a471f1828e16f5d

docker container stop mysql mysql2 停止執行container

docker container rm mysql mysql2 刪除container

volume 不會隨著container 刪除而消失

volume ls

named volumes (friendly way to assign vols to containers)

docker container run -d --name mysql -e MYSQL_ALLOW_EMPTY_PASSWORD=True -v mysql-db:/var/lib/mysql mysql 在run的同時 給-v 替volume取名

volume named

docker volume create- required to do this before ‘docker run ‘ to use custom drivers and labels


Persistent Data: Bind Mounting

  • can’t use in Dockerfile, must be at container run
  • ... run -v /users/bret/stuff:/path/container (mac/linux)
  • ... run -v //c/users/bret/stuff:/path/container (windows)


docker container run -d --name nginx -p 80:80 -v $(pwd):/usr/share/nginx/html nginx

docker container run -d --name nginx2 -p 8080:80 nginx


-v pathA:pathB 用來設定主機的A目錄要同步對應到container的B目錄

試驗: 開另一個terminal, 進到nginx container中的/usr/share/nginx/html

隨意新增一檔案 touch test.txt

到本機mount的路徑，也可以看到多了一個test.txt檔案。


小結

volume & bind mount

圖摘自官方文件

  • volume會將data存到預設的docker area下
  • bind mount可以指定filesystem的位置


延伸閱讀&參考圖文

深入理解Docker Volume（一）

Virtualization-fizalihsan