Increasing MySQL 5.5 max_connections on RHEL 5
By Jon Jensen
December 24, 2013
Busy database-backed websites often hit scalability limits in the database first. In tuning MySQL, one of the first things to look at is the max_connections parameter, which is often too low. (Of course another thing to look at is appropriate fragment caching in your app server, HTTP object caching in your web server, and a CDN in front of it all.)
When using MySQL 5.5 from Oracle’s RPMs through cPanel (MySQL55-server-5.5.32-1.cp1136) on RHEL 5.10 x86_64, there is an interesting problem if you try to increase the max_connections setting beyond 214 in /etc/my.cnf. It will silently be ignored, and the limit remains 214:
mysql> show variables like 'max_connections';
+-----------------+-------+
| Variable_name | Value |
+-----------------+-------+
| max_connections | 214 |
+-----------------+-------+
1 row in set (0.00 sec)
The problem is that the maximum number of open files allowed is too small, by default 1024, to increase max_connections beyond 214.
There are plenty of online guides that explain how to handle this, including increasing the kernel fs.file-max setting, which may be necessary by editing /etc/sysctl.conf, in this example to double the default:
fs.file-max = 2459688
Then run sysctl -p to make the change take immediate effect. (It’ll remain after reboot too.)
There are also many guides that say you need to change /etc/security/limits.conf along these lines:
mysql soft nofile 4096
mysql hard nofile 4096
However, the /etc/security/limits.conf change does not actually work when mysqld is started via the init script in /etc/init.d/mysql or via service mysql restart.
With standard Red Hat mysql-server (5.1) package that provides /etc/init.d/mysqld (not /etc/init.d/mysql as the Oracle and Percona versions do), you could create a file /etc/sysconfig/mysqld containing ulimit -n 4096 and that setting will take effect for each restart of the MySQL daemon.
But the ulimit -n setting hacked into the init script or put into /etc/sysconfig/mysqld isn’t really needed after all, because you can simply set open_files_limit in /etc/my.cnf:
[mysqld]
open_files_limit = 8192
max_connections = 1000
# etc.
… and mysqld_safe will increase the ulimit on its own before invoking the actual mysqld daemon.
After service mysql restart you can verify the new open file limit in the running process, like this:
# cat /var/lib/mysql/*.pid
30697
# ps auxww | grep 30697
mysql 30697 97.8 9.8 6031872 1212224 pts/1 Sl 13:09 3:01 /usr/sbin/mysqld --basedir=/usr --datadir=/var/lib/mysql --plugin-dir=/usr/lib64/mysql/plugin --user=mysql --log-error=/var/lib/mysql/some.hostname.err --open-files-limit=8192 --pid-file=/var/lib/mysql/some.hostname.pid
# cat /proc/30697/limits
Limit Soft Limit Hard Limit Units
Max cpu time unlimited unlimited seconds
Max file size unlimited unlimited bytes
Max data size unlimited unlimited bytes
Max stack size 10485760 unlimited bytes
Max core file size 0 unlimited bytes
Max resident set unlimited unlimited bytes
Max processes 96086 96086 processes
Max open files 8192 8192 files
Max locked memory 32768 32768 bytes
Max address space unlimited unlimited bytes
Max file locks unlimited unlimited locks
Max pending signals 96086 96086 signals
Max msgqueue size 819200 819200 bytes
Max nice priority 0 0
Max realtime priority 0 0
And the running MySQL server will reveal the desired max_connections setting stuck this time:
mysql> show variables like 'max_connections';
+-----------------+-------+
| Variable_name | Value |
+-----------------+-------+
| max_connections | 1000 |
+-----------------+-------+
1 row in set (0.00 sec)
The relevant code in /usr/bin/mysqld_safe is here:
if test -w / -o "$USER" = "root"
then
# ... [snip] ...
if test -n "$open_files"
then
ulimit -n $open_files
fi
fi
if test -n "$open_files"
then
append_arg_to_args "--open-files-limit=$open_files"
fi
I have found that some newer versions of either MySQL55-server or cPanel or some intersection of the two has made manually specifying a higher open_files_limit in /etc/my.cnf no longer necessary, although it does not do any harm.
But in conclusion, if you find yourself hitting the mysterious max_connections = 214 limit, just add the appropriately-sized open_files_limit to the [mysqld] section of /etc/my.cnf and restart the server with service mysql restart, and your problem should be solved!
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