Show Layers of Docker Image

Arun GuptaDecember 15th, 2015Last Updated: December 14th, 2015

0 32 2 minutes read

Every speaking opportunity is a learning lesson, and hence a new opportunity to share. This blog will address one of the questions that has been bothering me for the past few days. It is about how to show different layers of Docker image, and their sizes, after it is downloaded.

I was invited to talk about Docker for Java developers at Peru JUG this morning. The recording is now available:

The real content starts from 5:27.

Lets address the question now.

Each Docker image consists of a series of layers. Here is a quote from docs.docker.com:

Each image consists of a series of layers. Docker makes use of union file systems to combine these layers into a single image. Union file systems allow files and directories of separate file systems, known as branches, to be transparently overlaid, forming a single coherent file system.
How does a Docker image work?

The exact size of the image can be easily seen using docker images:

docker images couchbase
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
couchbase           latest              45abdd57689a        3 weeks ago         372 MB

The command to show different layers, and their size, is docker history:

docker history couchbase
IMAGE               CREATED             CREATED BY                                      SIZE                COMMENT
45abdd57689a        3 weeks ago         /bin/sh -c #(nop) VOLUME [/opt/couchbase/var]   0 B                 
dd8c5611343d        3 weeks ago         /bin/sh -c #(nop) EXPOSE 11207/tcp 11210/tcp    0 B                 
30852bbad62b        3 weeks ago         /bin/sh -c #(nop) CMD ["couchbase-server"]      0 B                 
5537747ea12f        3 weeks ago         /bin/sh -c #(nop) ENTRYPOINT &{["/entrypoint.   0 B                 
e8a83a5448df        3 weeks ago         /bin/sh -c #(nop) COPY file:cbb44c9c65b64a9dc   182 B               
18165b90fefa        3 weeks ago         /bin/sh -c #(nop) COPY file:34e32c52f0895191f   389 B               
5f37b8bdc5a6        3 weeks ago         /bin/sh -c wget -N $CB_RELEASE_URL/$CB_VERSIO   212.1 MB            
1a8da511d01b        3 weeks ago         /bin/sh -c groupadd -g 1000 couchbase && user   328.7 kB            
d9b2222c39b4        3 weeks ago         /bin/sh -c #(nop) ENV CB_VERSION=4.0.0 CB_REL   0 B                 
815f08b3c781        3 weeks ago         /bin/sh -c apt-get update &&     apt-get inst   23.57 MB            
fc38f156c0ea        3 weeks ago         /bin/sh -c #(nop) MAINTAINER Couchbase Docker   0 B                 
2a7a952931ec        3 weeks ago         /bin/sh -c #(nop) CMD ["/bin/bash"]             0 B                 
10f1b5844a9c        3 weeks ago         /bin/sh -c sed -i 's/^#\s*\(deb.*universe\)$/   1.911 kB            
23c388b926b6        3 weeks ago         /bin/sh -c echo '#!/bin/sh' > /usr/sbin/polic   156.2 kB            
b45376f323f5        3 weeks ago         /bin/sh -c #(nop) ADD file:4a9e089e81d6581a54   135.9 MB

This command shows different layers, command used for each layer, and the exact size of each image.

The corresponding Dockerfile for this image is at github.com/couchbase/docker/blob/master/enterprise/couchbase-server/4.0.0/Dockerfile.

Now I can compare the Dockerfile with the exact size and easily find which image layers are the biggest. For example, biggest contributors in this Couchbase image are shown below: