MySQL server starts very slowly in Docker container

I have a MySQL server deployed in a Docker container for testing purposes.

Recently, I found that the server started extremely slowly in the container.

After investigation, it turned out that the problem was caused by the too large open-file limits…

The problem occurs recently after several versions of Archlinux rolling upgrades, and the specific symptoms are:

• The server starts extremely slowly, and it takes a very long time to start successfully;
• After the container starts, the mysqld process occupies a large amount of memory;
• With Docker container memory limits configured, the server starts to fail with a high probability;

I searched for solutions online for this problem, and occasionally found a solution: docker-library/mysql#579.

What? The problem is caused by the huge default open-file limits in the container.

You can confirm this by running the following command:

1docker run --rm mysql:5.7 /bin/bash -c 'ulimit -Hn && ulimit -Sn'

In my environment, the output is:

11073741816
21073741816

The cause of this problem is that the LimitNPROC option in docker configuration is set to infinity by default. While the infinity has been changed from 1048576 (2^20) to 1073741816 (2^30) in some systems.

So, after adding ulimit settings for the container, the problem is gone.

References:

• https://github.com/docker-library/mysql/issues/579#issuecomment-519495808
• https://gist.github.com/vicenterusso/ae6d6c903b790e5dae91c339c1ce2ba8
• https://github.com/docker/for-linux/issues/73#issuecomment-1455386927