Docker File System Exploration

We will now have a look inside the /var/lib/docker/overlay2 folder where the image and container layers are stored.
ls /var/lib/docker/overlay2
As we do not have any images yet, there should not be anything in this folder.
Let’s pull an nginx image
docker image pull nginx
You should get something like the following where we can see that 3 layers are pulled.
Using default tag: latest
latest: Pulling from library/nginx
5040bd298390: Pull complete
d7a91cdb22f0: Pull complete
9cac4850e5df: Pull complete
Digest: sha256:33ff28a2763feccc1e1071a97960b7fef714d6e17e2d0ff573b74825d0049303
Status: Downloaded newer image for nginx:latest
If we have a look to the changes that occurs in the /var/lib/docker/overlay2 folder
ls /var/lib/docker/overlay2
we can see the following:
261fed39e3aca63326758681c96cad5bfe7eeeabafda23408bee0f5ae365d3fd
28f7998921ca5e4b28231b59b619394ba73571b5127a9c28cc9bacb3db706d2a
backingFsBlockDev
c1ae1be1c1c62dbaacf26bb9a5cde02e30d5364e06a437d0626f31c55af82a58
l
Some folders, with names that looks like hash, were created. Those are the layers which, merged together, build the image filesystem.
Let’s run a container based on nginx.
docker container run -d nginx
We can now see 2 additional folders (ID, ID-init), those ones correspond to the read-write layer of the running container.
11995e6da1dc5acab33aceacea3656d3795a4fb136c3a65b37d40b97747b5f84
11995e6da1dc5acab33aceacea3656d3795a4fb136c3a65b37d40b97747b5f84-init
261fed39e3aca63326758681c96cad5bfe7eeeabafda23408bee0f5ae365d3fd
28f7998921ca5e4b28231b59b619394ba73571b5127a9c28cc9bacb3db706d2a
backingFsBlockDev
c1ae1be1c1c62dbaacf26bb9a5cde02e30d5364e06a437d0626f31c55af82a58
l
Feel free to go into those folder and explore their filesystems.

Reference: https://training.play-with-docker.com/docker-images/

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