What's the reason for the 42 layer limit in Do

Question

At different places I found the information that a docker image can only consist of up to 42 layers. This seems to be a limitation of the used AUFS file system.

Can anybody tell me why this limit exists or does anybody have some documentation explaining this?

Answer 1

This seems to be imposed primarily by AUFS (sfjro/aufs4-linux).
See PR 50 "Prohibit more than 42 layers in the core "

Hey, out of curiosity, what's the detailed rationale behind this?
People have been asking how to bypass the 42 layers limit, and we've always told them "wait for device mapper!" so... what should we tell them now?

We can't allow more than 42 layers just yet. Devicemapper indeed supports it, but then, if you push a 50 layer image to the registry, most people won't be able to use it. We'll enable this once we support more than 42 layers for AUFS.

PR 66 was supposed to remove that limit

This thread reported:

The 42 limit comes from the aufs code itself.
My guess is that it's because of stack depth: if you stack these kernel data structures too many levels deep, you might trigger an overflow somewhere. The hardcoded limit is probably a pragmatic way to avoid that.

Answer 2

From the Docker BP documentation:

Minimize the number of layers

You need to find the balance between readability (and thus long-term maintainability) of the Dockerfile and minimizing the number of layers it uses. Be strategic and cautious about the number of layers you use.

They also give advices on how to avoid too many layers:

That way you can delete the files you no longer need after they’ve been extracted and you won’t have to add another layer in your image

[..]

Lastly, to reduce layers and complexity, avoid switching USER back and forth frequently.

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TL;DR: the benefit of minimizing the number of layers can be likened to the benefit of minimizing the number of small files but rather have fewer bigger ones. A docker pull is also faster (try downloading 2048 files of 1kB or one file of 2MB) . and having fewer layers reduces the complexity of an image, hence the maintainability.

As for the 42 limit. Well... I guess they had to come up with a number and they pick this particular one ;)