In this blog, we’ll look at a mystery around setting the open_file_limit variable in MySQL and Percona Server for MySQL.

MySQL Server needs file descriptors to run. It uses them to open new connections, store tables in the cache, create temporary tables to resolve complicated queries and access persistent ones. If mysqld is not able to open new files when needed, it can stop functioning correctly. A common symptom of this issue is error 24: “Too many open files.”

The number of file descriptors

mysqld

 can open simultaneously is defined by the configuration

open_files_limit

 option. You would expect it to work like any other MySQL Server option: set in the configuration file, …

I recently worked with a customer who had a weird issue: when their MySQL server was started (Percona Server 5.5), if they try to run service mysql start a second time, the init script was not able to detect that an instance was already running. As a result, it tried to start a second instance with the same settings as the first one. Of course this fails and this creates a mess. What was the issue? A missing rule in SELinux. At least it looks like

Summary

If SELinux is set to enforcing and if you are using Percona Server on CentOS/RHEL 6 (other versions could be affected), service mysql start doesn’t work properly and a fix is simple to run:

# grep mysqld_safe /var/log/audit/audit.log | audit2allow -M mysqld_safe
# semodule -i mysqld_safe.pp
# service mysql restart

Other options are:

• Set SELinux to permissive
• Use the CentOS/RHEL standard MySQL init script …