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Docker CoW

"Docker - Copy on Write"

This is just intended as a quick and easy playground to test out the overlay filesystem. See: https://docs.kernel.org/filesystems/overlayfs.html.

Why is this useful?

In this case it's to test out how you might want to design a CoW filesystem in docker.

For example you can see that creating a 10MB modification to the contentful file that originated from our "read-only" lower layer performs a copy of the file, appends the modification in the work directory and then stores the result in the upper layer which is exposed to the container: Appending to contentful

What I just said was a over abstraction (and a bit of a lie), please read: https://docs.kernel.org/filesystems/overlayfs.html to understand how this actually works.

In short this means you can mount a read-only filesystem to the container and still transparently allow ephemeral writes. Think staging environment sharing production's assets (hopefully with some level of data washing inbetween).

Requirements

  • Docker
  • The overlay kernel module (often default)
  • Root or Linux CAPS to mount (hopefully not sysadmin since you're a security focused person, right?)
  • Tree(1) (optional - if you want a sane output, will fallback to find(1) if missing)

Check kernel modules requirements:

modprobe overlay || echo 'You need to do some setup!'

Usage

To run:

./run.bash

To jump into the container:

docker exec -it -w /cowfs cowfs bash

Stop with C-c/^C/Ctrl-c/SIGINT or from any other terminal:

docker stop cowfs

Note that the mount will be torn down automatically for you.

To reset the filesystem (while stopped):

rm -rf ./fs

Debugging

You should looking into linux's tracing utilities such as ftrace and debugfs.