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geekpi
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[#]: collector: (lujun9972)
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[#]: translator: (geekpi)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (How containers work: overlayfs)
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[#]: via: (https://jvns.ca/blog/2019/11/18/how-containers-work--overlayfs/)
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[#]: author: (Julia Evans https://jvns.ca/)
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How containers work: overlayfs
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======
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I wrote a comic about overlay filesystems for a potential future container [zine][1] this morning, and then I got excited about the topic and wanted to write a blog post with more details. Here’s the comic, to start out:
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<https://jvns.ca/images/overlay.jpeg>
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### container images are big
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Container images can be pretty big (though some are really small, like [alpine linux is 2.5MB][2]). Ubuntu 16.04 is about 27MB, and [the Anaconda Python distribution is 800MB to 1.5GB][3].
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Every container you start with an image starts out with the same blank slate, as if it made a copy of the image just for that container to use. But for big container images, like that 800MB Anaconda image, making a copy would be both a waste of disk space and pretty slow. So Docker doesn’t make copies – instead it uses an **overlay**.
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### how overlays work
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Overlay filesystems, also known as “union filesystems” or “union mounts” let you mount a filesystem using 2 directories: a “lower” directory, and an “upper” directory.
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Basically:
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* the **lower** directory of the filesystem is read-only
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* the **upper** directory of the filesystem can be both read to and written from
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When a process **reads** a file, the overlayfs filesystem driver looks in the upper directory and reads the file from there if it’s present. Otherwise, it looks in the lower directory.
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When a process **writes** a file, overlayfs will just write it to the upper directory.
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### let’s make an overlay with `mount`!
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That was all a little abstract, so let’s make an overlay filesystem and try it out! This is just going to have a few files in it: I’ll make upper and lower directories, and a `merged` directory to mount the combined filesystem into:
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```
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$ mkdir upper lower merged work
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$ echo "I'm from lower!" > lower/in_lower.txt
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$ echo "I'm from upper!" > upper/in_upper.txt
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$ # `in_both` is in both directories
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$ echo "I'm from lower!" > lower/in_both.txt
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$ echo "I'm from upper!" > upper/in_both.txt
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```
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Combining the upper and lower directories is pretty easy: we can just do it with `mount!`
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```
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$ sudo mount -t overlay overlay
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-o lowerdir=/home/bork/test/lower,upperdir=/home/bork/test/upper,workdir=/home/bork/test/work
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/home/bork/test/merged
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```
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There’s was an extremely annoying error message I kept getting while doing this, that said `mount: /home/bork/test/merged: special device overlay does not exist.`. This message is a lie, and actually just means that one of the directories I specified was missing (I’d written `~/test/merged` but it wasn’t being expanded).
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Okay, let’s try to read one of the files from the overlay filesystem! The file `in_both.txt` exists in both `lower/` and `upper/`, so it should read the file from the `upper/` directory.
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```
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$ cat merged/in_both.txt
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"I'm from upper!
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```
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It worked!
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And the contents of our directories are what we’d expect:
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```
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find lower/ upper/ merged/
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lower/
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lower/in_lower.txt
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lower/in_both.txt
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upper/
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upper/in_upper.txt
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upper/in_both.txt
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merged/
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merged/in_lower.txt
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merged/in_both.txt
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merged/in_upper.txt
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```
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### what happens when you create a new file?
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```
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$ echo 'new file' > merged/new_file
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$ ls -l */new_file
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-rw-r--r-- 1 bork bork 9 Nov 18 14:24 merged/new_file
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-rw-r--r-- 1 bork bork 9 Nov 18 14:24 upper/new_file
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```
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That makes sense, the new file gets created in the `upper` directory.
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### what happens when you delete a file?
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Reads and writes seem pretty straightforward. But what happens with deletes? Let’s do it!
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```
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$ rm merged/in_both.txt
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```
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What happened? Let’s look with `ls`:
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```
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ls -l upper/in_both.txt lower/lower1.txt merged/lower1.txt
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ls: cannot access 'merged/in_both.txt': No such file or directory
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-rw-r--r-- 1 bork bork 6 Nov 18 14:09 lower/in_both.txt
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c--------- 1 root root 0, 0 Nov 18 14:19 upper/in_both.txt
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```
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So:
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* `in_both.txt` is still in the `lower` directory, and it’s unchanged
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* it’s not in the `merged` directory. So far this is all what we expected.
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* But what happened in `upper` is a little strange: there’s a file called `upper/in_both.txt`, but it’s a… character device? I guess this is how the overlayfs driver represents a file being deleted.
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What happens if we try to copy this weird character device file?
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```
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$ sudo cp upper/in_both.txt upper/in_lower.txt
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cp: cannot open 'upper/in_both.txt' for reading: No such device or address
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```
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Okay, that seems reasonable, being able to copy this weird deletion signal file doesn’t really make sense.
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### you can mount multiple “lower” directories
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Docker images are often composed of like 25 “layers”. Overlayfs supports having multiple lower directories, so you can run
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```
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mount -t overlay overlay
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-o lowerdir:/dir1:/dir2:/dir3:...:/dir25,upperdir=...
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```
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So I assume that’s how containers with many Docker layers work, it just unpacks each layer into a separate directory and then asks overlayfs to combine them all together together with an empty upper directory that the container will write its changes to it.
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### docker can also use btrfs snapshots
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Right now I’m using ext4, and Docker uses overlayfs snapshots to run containers. But I used to use btrfs, and then Docker would use btrfs copy-on-write snapshots instead. (Here’s a list of when Docker uses which [storage drivers][4])
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Using btrfs snapshots this way had some interesting consequences – at some point last year I was running hundreds of short-lived Docker containers on my laptop, and this resulted in me running out of btrfs metadata space (like [this person][5]). This was really confusing because I’d never heard of btrfs metadata before and it was tricky to figure out how to clean up my filesystem so I could run Docker containers again. ([this docker github issue][6] describes a similar problem with Docker and btrfs)
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### it’s fun to try out container features in a simple way!
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I think containers often seem like they’re doing “complicated” things and I think it’s fun to break them down like this – you can just run one `mount` incantation without actually doing anything else related to containers at all and see how overlays work!
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--------------------------------------------------------------------------------
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via: https://jvns.ca/blog/2019/11/18/how-containers-work--overlayfs/
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作者：[Julia Evans][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://jvns.ca/
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[b]: https://github.com/lujun9972
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[1]: https://wizardzines.com
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[2]: https://hub.docker.com/_/alpine?tab=tags
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[3]: https://hub.docker.com/r/continuumio/anaconda3/tags
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[4]: https://docs.docker.com/storage/storagedriver/select-storage-driver/
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[5]: https://www.reddit.com/r/archlinux/comments/5jrmfe/btrfs_metadata_and_docker/
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[6]: https://github.com/moby/moby/issues/27653
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170
translated/tech/20191118 How containers work- overlayfs.md
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170
translated/tech/20191118 How containers work- overlayfs.md
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@ -0,0 +1,170 @@
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[#]: collector: (lujun9972)
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[#]: translator: (geekpi)
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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[#]: subject: (How containers work: overlayfs)
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[#]: via: (https://jvns.ca/blog/2019/11/18/how-containers-work--overlayfs/)
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[#]: author: (Julia Evans https://jvns.ca/)
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容器如何工作：overlayfs
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======
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今天早上，我在未来潜在容器[杂志][1]上画了一幅 overlay 文件系统漫画，我对这个主题感到兴奋，想写一篇关于它的博客来提供更多详细信息。下面是漫画：
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<https://jvns.ca/images/overlay.jpeg>
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### 容器镜像很大
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容器镜像可能会很大（尽管有些很小，例如 [alpine linux 是 2.5MB][2]）。Ubuntu 16.04 约为 27 MB，[Anaconda Python 发行版为 800MB 至 1.5GB][3]。
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你以镜像启动的每个容器都是原始空白状态，仿佛它只是为使用容器而复制的一份镜像拷贝一样。但是对于大的容器镜像，像 800MB 的 Anaconda 镜像，复制一份拷贝既浪费磁盘空间也很慢。因此 Docker 不会复制，而是采用**叠加**。
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### 叠加如何工作
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overlayfs，也被称为 **Union 文件系统**或 **Union 挂载**， 它可让你使用 2 个目录挂载文件系统：“下层”目录和“上层”目录。
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基本上：
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* 文件系统的**下层**目录是只读的
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* 文件系统的**上层**目录可以读写
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当进程“读取”文件时，overlayfs 文件系统驱动将在上层目录中查找并从该目录中读取文件（如果存在）。否则，它将在下层目录中查找。
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当进程“写入”文件时，overlayfs 会将其写入上层目录。
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### 让我们使用 `mount` 制造一个叠加层！
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这有点抽象，所以让我们制作一个 overlayfs 并尝试一下！这将只包含一些文件：我将创建上，下层目录，并将合并的文件系统挂载到的`合并`目录：
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```
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$ mkdir upper lower merged work
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$ echo "I'm from lower!" > lower/in_lower.txt
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$ echo "I'm from upper!" > upper/in_upper.txt
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$ # `in_both` is in both directories
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$ echo "I'm from lower!" > lower/in_both.txt
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$ echo "I'm from upper!" > upper/in_both.txt
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```
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合并上层目录和下层目录非常容易：我们可以通过 `mount` 来完成！
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```
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$ sudo mount -t overlay overlay
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-o lowerdir=/home/bork/test/lower,upperdir=/home/bork/test/upper,workdir=/home/bork/test/work
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/home/bork/test/merged
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```
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在执行此操作时，我不断收到一条非常烦人的错误消息，内容为：`mount: /home/bork/test/merged: special device overlay does not exist.`。这条消息是错误的，实际上只是意味着我指定的一个目录缺失（我写成了 `~/test/merged`，但它没有被扩展）。
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让我们尝试从 overlayfs 中读取其中一个文件！文件 `in_both.txt` 同时存在于 `lower/` 和 `upper/` 中，因此应从 `upper/` 目录中读取该文件。
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```
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$ cat merged/in_both.txt
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"I'm from upper!
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```
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可以成功！
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目录的内容就是我们所期望的：
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```
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find lower/ upper/ merged/
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lower/
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lower/in_lower.txt
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lower/in_both.txt
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upper/
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upper/in_upper.txt
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upper/in_both.txt
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merged/
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merged/in_lower.txt
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merged/in_both.txt
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merged/in_upper.txt
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```
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### 创建新文件时会发生什么？
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```
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$ echo 'new file' > merged/new_file
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$ ls -l */new_file
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-rw-r--r-- 1 bork bork 9 Nov 18 14:24 merged/new_file
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-rw-r--r-- 1 bork bork 9 Nov 18 14:24 upper/new_file
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```
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这是有作用的，新文件会在 `upper` 目录创建。
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### 删除文件时会发生什么？
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读写似乎很简单。但是删除会发生什么？开始试试！
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```
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$ rm merged/in_both.txt
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```
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发生了什么？让我们用 `ls` 看下：
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```
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ls -l upper/in_both.txt lower/lower1.txt merged/lower1.txt
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ls: cannot access 'merged/in_both.txt': No such file or directory
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-rw-r--r-- 1 bork bork 6 Nov 18 14:09 lower/in_both.txt
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c--------- 1 root root 0, 0 Nov 18 14:19 upper/in_both.txt
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```
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所以：
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* `in_both.txt` 仍在 `lower` 目录中，并且保持不变
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* 它不在 `merged` 目录中。到目前为止，这就是我们所期望的。
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* 但是在 `upper` 中发生的事情有点奇怪：有一个名为 `upper/in_both.txt` 的文件，但是它是字符设备？我想这就是 overlayfs 驱动表示删除的文件的方式。
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如果我们尝试复制这个奇怪的字符设备文件，会发生什么？
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```
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$ sudo cp upper/in_both.txt upper/in_lower.txt
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cp: cannot open 'upper/in_both.txt' for reading: No such device or address
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```
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好吧，这似乎很合理，复制这个奇怪的删除信号文件并没有任何意义。
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### 你可以挂载多个“下层”目录
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Docker 镜像通常由 25 个“层”组成。overlayfs 支持具有多个下层目录，因此你可以运行
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```
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mount -t overlay overlay
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-o lowerdir:/dir1:/dir2:/dir3:...:/dir25,upperdir=...
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```
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因此，我假设这是有多个 Docker 层的容器的工作方式，它只是将每个层解压缩到一个单独的目录中，然后要求 overlayfs 将它们全部合并在一起，并使用一个空的上层目录，容器将对其进行更改。
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### Docker 也可以使用 btrfs 快照
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现在，我使用的是 ext4，而 Docker 使用 overlayfs 快照来运行容器。但是我曾经用过 btrfs，接着 Docker 将改为使用 btrfs 的写时复制快照。 （这是 Docker 何时使用哪种[存储驱动][4]的列表）
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以这种方式使用 btrfs 快照会产生一些有趣的结果-去年某个时候，我在笔记本上运行了数百个临时的 Docker 容器，这导致我用尽了 btrfs 元数据空间（像[这个人][5]）。这真的很令人困惑，因为我以前从未听说过 btrfs 元数据，而且弄清楚如何清理文件系统以便再次运行 Docker 容器非常棘手。（[这个 docker github 上的问题][6]描述了 Docker 和 btrfs 的类似问题）
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### 以简单的方式尝试容器功能很有趣！
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我认为容器通常看起来像是在做“复杂的”事情，我认为将它们分解成这样很有趣。你可以运行一条 `mount` 咒语，而实际上并没有做任何与容器相关的其他事情，看看叠加层是如何工作的！
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--------------------------------------------------------------------------------
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via: https://jvns.ca/blog/2019/11/18/how-containers-work--overlayfs/
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作者：[Julia Evans][a]
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选题：[lujun9972][b]
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||||
译者：[geekpi](https://github.com/geekpi)
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||||
校对：[校对者ID](https://github.com/校对者ID)
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||||
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||||
本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
|
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[a]: https://jvns.ca/
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[b]: https://github.com/lujun9972
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[1]: https://wizardzines.com
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[2]: https://hub.docker.com/_/alpine?tab=tags
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[3]: https://hub.docker.com/r/continuumio/anaconda3/tags
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[4]: https://docs.docker.com/storage/storagedriver/select-storage-driver/
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[5]: https://www.reddit.com/r/archlinux/comments/5jrmfe/btrfs_metadata_and_docker/
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[6]: https://github.com/moby/moby/issues/27653
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