Docker is well-known, is used everywhere, is used by everybody and is a nice piece of technology, there is nothing to say about that.
Now, before moving on with the real stuff, note that for the sake of this ticket all the experiments done are made on a Raspberry PI 2, to increase a bit the difficulty of the exercise and to grab a wider understanding of how to manipulate Docker containers and images at a rather low level per the reasons of the next paragraph.
So, to move back to Docker… It is a bit sad to see that there are not many container images based on ARM architectures even if there are many machines around. And also, the size of a single container image can reach easily a couple of hundred megabytes in its most simple shape (it does not change the fact that some of those images are very popular, so perhaps the author of this blog should not do experimentations on such small-scale machines to begin with).
Not all the container images are that large though, there is for example one container based on the minimalistic distribution [Alpine Linux] (http://alpinelinux.org/), with a size of less than 5MB. Many packages are available as well for it so it makes it a nice base image for more extended operations. Now, the fact is that even if Alpine Linux does publish deliverables for ARM, there are no Docker container around that make use of it, and trying to use a container image that has been compiled for example x86_64 would just result on an epic failure.
Hence, extending a bit a script from the upstream Docker facility of Alpine Linux, it is actually easily possible to create from scratch a container image able to run on ARM architectures (the trick has been to consider the fact that Alpine Linux publishes its ARM deliverables with the alias armhf). Note in any case the following things about this script:
- root rights are needed
- ARM environment needs to be used to generate an ARM container
- the script is here Roughtly, what this script does is fetching a minimal base image of Alpine Linux and then importing it in an image using “docker import”.
Once run simply as follows, it will register a new container image:
$ ./mkimage-alpine.sh [...] $ docker images REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE alpine-armv7l edge 448a4f53f4df About an hour ago 4.937 MB alpine-armv7l latest 448a4f53f4df About an hour ago 4.937 MB
The size is drastically small, and comparable to the container image already available in the Docker registry. Now, moving on to things regarding directly Postgres: how much would it cost to have a container image able to run Postgres? Let’s use the following Dockerfile and get a look at it then (file needs to be named as Dockerfile):
$ cat Dockerfile_postgres FROM alpine-armv7l:edge RUN echo http://nl.alpinelinux.org/alpine/edge/testing >> /etc/apk/repositories && \ apk --update && \ apk add shadow postgresql bash
Note that here the package shadow is included to have pam-related utilities like useradd and usermod as Postgres cannot run as root, and it makes life simpler (and shadow is only available in the repository testing). After building the new container image, let’s look at its size:
$ docker build -t alpine-postgres . [...] $ docker images REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE alpine-postgres latest 3bcc06a7ce79 2 hours ago 23.46 MB alpine-armv7l edge 448a4f53f4df 2 hours ago 4.937 MB alpine-armv7l latest 448a4f53f4df 2 hours ago 4.937 MB
Without bash this gets down to 22.55 MB, and without shadow + bash its size is 20.86 MB. This container image includes only the necessary binaries and libraries to be able to run a PostgreSQL server, and does nothing to initialize it or configure it. Let’s use it then and create a server:
$ docker run -t -i alpine-postgres /bin/bash # useradd -m -g wheel postgres # su - postgres $ initdb -D data [...] $ pg_ctl start -D data $ psql -At -c 'SELECT version();' PostgreSQL 9.4.4 on armv6-alpine-linux-muslgnueabihf, compiled by gcc (Alpine 5.1.0) 5.1.0, 32-bit
And things are visibly working fine. Now let’s look at how much space would consume a development box for Postgres as a container image, and let’s use the following Dockerfile spec for this purpose with some packages needed to compile and work on the code:
FROM alpine-armv7l:edge RUN echo http://nl.alpinelinux.org/alpine/edge/testing >> /etc/apk/repositories && \ apk update && \ apk add shadow bash gcc bison flex git make autoconf
Once built, this gets larger to 125MB, but that’s not really a surprise…
$ docker build -t alpine-dev . [...] $ docker images REPOSITORY TAG IMAGE ID CREATED VIRTUAL SIZE alpine-dev latest 15dc9934cc36 16 minutes ago 125 MB alpine-postgres latest 3bcc06a7ce79 About an hour ago 23.46 MB alpine-armv7l edge 448a4f53f4df About an hour ago 4.937 MB alpine-armv7l latest 448a4f53f4df About an hour ago 4.937 MB
All the files and Dockerfile specs have been pushed [here] (https://github.com/michaelpq/pg_plugins/tree/master/docker). Feel free to use them and play with them.