Number of commands in Dockerfile

Question

I noticed that each line in the Dockerfile creates a separate image. Is there any limit on the number of images that are created?

Should we try to do a oneliner of RUN cmd1 && cmd2 && cmd3 instead?

How would this differ if we use a service like Quay?

Thanks!

Answer 1

As Alister said, there is an upper limit on the number of layers in a Docker image if you are using the AUFS file system. At Docker version 0.7.2 the limit was raised to 127 layers (changelog).

Since this a limitation of the underlying union file system (in the case of AUFS), using Quay or other private registries won't change the outcome. But you could use a different file system.

The current alternative filesystem is to use devicemapper (see CLI docs). These other filesystems may have different limitations on the number of layers -- I don't think devicemapper has an upper limit.

You're right, by RUNning multiple commands in a single RUN statement, you can reduce the number of layers.

Alternatively, if you really need a lot of layers to build your image, you could build an image until it reaches the maximum and then use docker export to create an un-layered copy of the image's file system. Then docker import to turn it back into an image again, this time with just one layer, and continue building. You lose the history that way though.

Answer 2

There is a limit, of 42 - apparently a hard limit imposed by AUFS.

It can help to be somewhat avoided by putting what would be done in individual RUN commands into a script, and then running that script. You would then end up with a single, larger image layer, rather than a number of smaller files to merge. Smaller images (with multiple RUN lines) make initial testing easier (since a new addition on the end of the RUNlist can re-use the previous image), so it's typical to wait until your Dockerfile has stabilised before merging the lines.

You can also reduce the potential number of images when ADDing a number of files, by adding a directory-full, rather than a number of individual files.