Sunday, September 20, 2015

Is it a privilege to run a container in Docker?

Recently while working with various applications in a docker container, we came across few containers that will not run properly unless privileged mode is enabled. The privileged mode gives the container the same rights as host which means it can make changes on host where the container runs. (Huge difference compared to VM - Imagine your VM making changes to the hypervisor directly.)

Of course privileged mode has its uses and I am definitely glad that it is available. However it is not a general purpose option to be used lightly. So imagine my surprise that one of the most common tools that is used in many enterprises now Chef server when running in a docker container also required privileged mode to run. There are various versions available but they all required the mode.

While investigating Chef Server to see why it requires the mode I found it primarily requires it to set some ulimit parameters and a specific kernel parameter inside the container.

sysctl -w kernel.shmmax=17179869184

Now before you say, aha simple lets change the value in the host itself and let the container pick up the value from the host itself.. Let me say been there .. it ain't gonna work.  The reason it does not work is due to how Linux namespaces work with CLONEIPC. The net result is everytime a container is created a new namespace of System V IPC is setup with the default  shmmax of 32MB.  The default will be changed in a later Linux kernel to 4GB but of course like most companies there will not be patience to wait for the Linux kernel to show up let alone a certified Linux distro for production setups.

There are few hacks to work it out as Jerome indicates in a mailing list.  But of course none of them was something that was suitable.

Now lets go back to the original command that needed to be executed which required. I have worked with those commands for years always to increase shared memory for databases that uses Sys V style of shared memory like Oracle, PostgreSQL (well till 9.2), etc.

Guess what doing a little digging I did find PostgreSQL used as an embedded database in $CHEF_SERVER_INSTALL/embedded/bin/postgres. Checking the version of "postgres" binary confirmed it to be 9.2.

Checking latest version of Chef server found it to be still using Postgres 9.2. Eventually ended up creating a custom image using Postgres 9.4 and voila got the container running without privileged mode. Thanks Robert Haas.

It also means that as more and more PostgreSQL based containers are being used in containers, it is better to move to the latest version of PostgreSQL for a better experience.



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