docker如何处理多种挂载类型？ [英] How docker handles multiple mount types?

问题描述

这篇文章有点长，但请耐心等待一段时间...

This post is somewhat lenglthy but bear with me for a while...

假设您有一个位于 / app 在具有以下结构的本地（主机）文件系统中

Suppose you have an app that sits in /app in your local (host) filesystem with the following structure

app
|-- index.php
|-- foo
|   `-- file-h1
`-- bar
    `-- file-h2

现在假设我们有一个利用以下数据结构的图像（标记为 myrepo / app ）

Now suppose we have an image (tagged myrepo/app) that exploits the following data structure

opt
|-- app
|   `-- foo
|       `-- file-c1

如果我们通过挂载主机的<$ c $从该映像运行容器c> / app 到容器的 / opt / app

If we run a container from that image by mounting host's /app to container's /opt/app as follows

docker container run \
-v /app:/opt/app \
myrepo/app

容器的结果数据结构如下

The resulting data structure of the container will be the following

opt
|-- app
|   |-- index.php
|   |-- foo       
|   |   `-- file-h1          
|   `-- bar
|       `-- file-h2 

到目前为止，很好...

So far, so good...

现在，假设我们要同时使用 / strong>名为 data 的命名卷，将其安装在 / opt / app / vendor 和 bind-mount 来将 / app 装入 / opt / app

Now, lets say that we want to use both a named volume called data to be mounted on /opt/app/vendor and a bind-mount to mount /app to /opt/app

docker container run \
-v /app:/opt/app \
-v data:/opt/app/foo
myrepo/app

容器中的结果数据结构将是：

The resulting data structure inside the container will be:

opt
|-- app
|   |-- index.php
|   |-- foo       
|   |   `-- file-c1          
|   `-- bar
|       `-- file-h2 

在各种帖子中都已提到（例如此和此），按目录顺序（即最短路径优先）执行docker挂载。据此，我希望docker首先执行bind-mount（ -v / app：/ opt / app ），然后执行卷（ -v数据：/ opt / app / foo ）。

As it is stated in various posts (like this and this) the docker mounts are performed in lexicographic order (i.e shortest path first). According to this, I would expect docker first to execute the bind-mount (-v /app:/opt/app) and then the volume (-v data:/opt/app/foo).

因此，我期望主机的 / app 的内容将替换/遮盖了容器 / opt / app 的内容，因此 file-h1 放在 /之内opt / app / foo 。最后， file-h1 将是复制在新创建的 data 卷中，并且该卷将安装在 / opt / app / foo （因此应显示 file-h1 而不是 file-c1 ）

Hence, I would expect that the contents of host's /app would replace/obscure the contents of container's /opt/app and thus file-h1 to be inside /opt/app/foo. Finally, file-h1 would be copied in the newly created data volume and the volume would be mounted on /opt/app/foo (so file-h1 should be shown instead of file-c1 )

我尝试理解此时提出的问题SO的答案

My questions raised when I tried to understand this answer on SO

推荐答案

最后，在github用户 cpuguy83 中，我弄清楚了当我们尝试运行一个使用多个不同挂载的容器时docker引擎实际上是做什么的类型（例如 bind-mount 和 volume ），例如：

Finally, and with a lot of help by github user cpuguy83, I figured out what actually docker engine does when we try to run a container that uses multiple mounts of different type (e.g both a bind-mount and a volume) as, for instance:

docker container run \
-v /app:/opt/app \
-v data:/opt/app/foo
myrepo/app

这里要理解的关键是docker在两个步骤，它们按以下顺序完成：

The key point to understand here is that docker executes the process in two steps that are done in the following order:




1. 首先，它会创建一个新的存储-空间（即卷）在主机文件系统（ ... data / ）中供容器保留文件，然后（由于新创建的卷为空）将其复制容器的文件（即<$ c内的任何内容$ c> / opt / app / foo / * ）到卷的存储位置（ ... data / ）

1. First, it creates a new storage-space (i.e. volume) in the host filesystem (... data/) for the container to persist files and then (since that newly created volume is empty) it copies the container's files (i.e whatever is inside /opt/app/foo/*) to the volume's storage location (... data/)

然后，它按字典顺序执行所有挂载（绑定，卷，tmpfs挂载等...都集中在一起）（第一个挂载 /应用到 / opt / app ，然后 ... data / 到 / opt / app / foo ）

Then after that, it performs all mounts (binds, volumes, tmpfs mounts, etc... are all lumped together) in lexicographic order (first mounts /app to /opt/app then ... data/ to /opt/app/foo)

因此，我们在示例中使用装载器运行容器，泊坞窗 first 将 file-c1 复制到。主机文件系统中的.data / 位置，然后 second 将主机的 / app 的内容首先安装到con tainer的 / opt / app ，然后主机的 ... data / （其中包含文件-c1 ）到容器的 / opt / app / foo 覆盖/遮盖其内容（即覆盖 file-h1 和 file-c1 ）。因此，如果我们在挂载完成后看一下正在运行的容器，结果将是：

So, because of that, when we run the container with the mounts in our example, the docker first will copy the file-c1 to the ... data/ location in the host filesystem and second it will mount first the contents of host's /app to the container's /opt/app and then the host's ... data/ (which contains file-c1) to container's /opt/app/foo overwriting/obscuring its contents (i.e overwriting file-h1 with file-c1). Therefore, if we take a look inside the running container after the mounts are done, the result will be the following:

opt
|-- app
|   |-- index.php
|   |-- foo       
|   |   `-- file-c1          
|   `-- bar
|       `-- file-h2

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