> For the complete documentation index, see [llms.txt](https://xiaohanliang.gitbook.io/xiaohanliang/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://xiaohanliang.gitbook.io/xiaohanliang/docker/storage-driver/union-mounting.md).

# 1. 什么叫联合挂载

> Unifying filesystems with union mounts
>
> 本文译自 → [这里](https://lwn.net/Articles/312641/) , 并加上了一些我的理解

联合挂载的概念是指, 想要在同一个挂载点上挂载好几个文件系统, 呈现出来的效果是: 一个挂载点下同时出现了不同文件系统里的东西. 显然, 这说的是Docker, 所有人都知道镜像分成好多层, 将这些层做一次"融合"(merge)就形成了我们在容器里看到的东西. 我们从这个现象开始讨论.

在一次普通的挂载过程中, 比如你把目录A挂到挂载点B上, 那么B目录下的东西直接就会被隐藏起来, 进入B目录只能看到A的东西, B的东西不是说不存在了, 而是说被**隐藏起来了**. 只有在解除A挂载以后才能看到了.

在内核里, 几个文件系统按照他们被挂载的顺序被一层层的摞(stack)起来, 最先被挂载的文件系统会被放在最下层, 最后被挂载的则处在最上层, 很不幸, 在普通挂载模式下, 你只能访问最上层的文件系统. 但如果你用了联合挂载的功能, 那么上上下下所有文件你都是能看到的. (如果出现了重名文件, 只展示上面的)

我提一个有意思的场景, 我们都知道远古时代的DVD是一次性写入的, 没有修改的道理, 写错了这批货就废了. 假设你DVD里是红警的安装包. 过段时间游戏更新成v1.1了, 这种情况下, 你手上的v1.0该怎么卖呢? 如果用联合挂载的办法, 我们在碟片空白的地方写上一些重名文件作为更新包, 读碟的时候把v1.0与更新包摞起来, v1.0放在下面, 更新包放在上面, 这样重名文件就能覆盖老文件, 达到更新的效果


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