[#]: collector: (lujun9972) [#]: translator: (geekpi) [#]: reviewer: ( ) [#]: publisher: ( ) [#]: url: ( ) [#]: subject: (How containers work: overlayfs) [#]: via: (https://jvns.ca/blog/2019/11/18/how-containers-work--overlayfs/) [#]: author: (Julia Evans https://jvns.ca/) How containers work: overlayfs ====== 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: ### container images are big 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]. 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**. ### how overlays work 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. Basically: * the **lower** directory of the filesystem is read-only * the **upper** directory of the filesystem can be both read to and written from 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. When a process **writes** a file, overlayfs will just write it to the upper directory. ### let’s make an overlay with `mount`! 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: ``` $ mkdir upper lower merged work $ echo "I'm from lower!" > lower/in_lower.txt $ echo "I'm from upper!" > upper/in_upper.txt $ # `in_both` is in both directories $ echo "I'm from lower!" > lower/in_both.txt $ echo "I'm from upper!" > upper/in_both.txt ``` Combining the upper and lower directories is pretty easy: we can just do it with `mount!` ``` $ sudo mount -t overlay overlay -o lowerdir=/home/bork/test/lower,upperdir=/home/bork/test/upper,workdir=/home/bork/test/work /home/bork/test/merged ``` 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). 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. ``` $ cat merged/in_both.txt "I'm from upper! ``` It worked! And the contents of our directories are what we’d expect: ``` find lower/ upper/ merged/ lower/ lower/in_lower.txt lower/in_both.txt upper/ upper/in_upper.txt upper/in_both.txt merged/ merged/in_lower.txt merged/in_both.txt merged/in_upper.txt ``` ### what happens when you create a new file? ``` $ echo 'new file' > merged/new_file $ ls -l */new_file -rw-r--r-- 1 bork bork 9 Nov 18 14:24 merged/new_file -rw-r--r-- 1 bork bork 9 Nov 18 14:24 upper/new_file ``` That makes sense, the new file gets created in the `upper` directory. ### what happens when you delete a file? Reads and writes seem pretty straightforward. But what happens with deletes? Let’s do it! ``` $ rm merged/in_both.txt ``` What happened? Let’s look with `ls`: ``` ls -l upper/in_both.txt lower/lower1.txt merged/lower1.txt ls: cannot access 'merged/in_both.txt': No such file or directory -rw-r--r-- 1 bork bork 6 Nov 18 14:09 lower/in_both.txt c--------- 1 root root 0, 0 Nov 18 14:19 upper/in_both.txt ``` So: * `in_both.txt` is still in the `lower` directory, and it’s unchanged * it’s not in the `merged` directory. So far this is all what we expected. * 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. What happens if we try to copy this weird character device file? ``` $ sudo cp upper/in_both.txt upper/in_lower.txt cp: cannot open 'upper/in_both.txt' for reading: No such device or address ``` Okay, that seems reasonable, being able to copy this weird deletion signal file doesn’t really make sense. ### you can mount multiple “lower” directories Docker images are often composed of like 25 “layers”. Overlayfs supports having multiple lower directories, so you can run ``` mount -t overlay overlay -o lowerdir:/dir1:/dir2:/dir3:...:/dir25,upperdir=... ``` 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. ### docker can also use btrfs snapshots 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]) 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) ### it’s fun to try out container features in a simple way! 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! -------------------------------------------------------------------------------- via: https://jvns.ca/blog/2019/11/18/how-containers-work--overlayfs/ 作者：[Julia Evans][a] 选题：[lujun9972][b] 译者：[译者ID](https://github.com/译者ID) 校对：[校对者ID](https://github.com/校对者ID) 本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出 [a]: https://jvns.ca/ [b]: https://github.com/lujun9972 [1]: https://wizardzines.com [2]: https://hub.docker.com/_/alpine?tab=tags [3]: https://hub.docker.com/r/continuumio/anaconda3/tags [4]: https://docs.docker.com/storage/storagedriver/select-storage-driver/ [5]: https://www.reddit.com/r/archlinux/comments/5jrmfe/btrfs_metadata_and_docker/ [6]: https://github.com/moby/moby/issues/27653