MySQL Bugs: #32066: max_connections may be changed online

Bug #32066	max_connections may be changed online
Submitted:	2 Nov 2007 22:33	Modified:	7 Nov 2007 12:17
Reporter:	Grigory Rubtsov	Email Updates:
Status:	Closed	Impact on me:	None
Category:	MySQL Server: Documentation	Severity:	S3 (Non-critical)
Version:	all	OS:	Any
Assigned to:	Jon Stephens	CPU Architecture:	Any
Tags:	max_connections, too many connections

Description:
Manual says "If you need to support more connections, you should restart mysqld with a larger value for this variable."

http://dev.mysql.com/doc/refman/5.1/en/too-many-connections.html

But actually the number of connections may be changed online via global variable.

How to repeat:
mysql> set @@global.max_connections=111; SELECT VERSION();
Query OK, 0 rows affected (0.00 sec)

+-------------------------+
| VERSION()               |
+-------------------------+
| 5.1.22-rc-community-log | 
+-------------------------+
1 row in set (0.00 sec)

Reviewer:

Please verify that:

1. The new number of connections actually takes effect without restarting the server.

2. That the observed behaviour is expected, and not a server bug.

I've changed the category to Server pending the outcome.

mysql> select version();
+-----------+
| version() |
+-----------+
| 5.0.22    | 
+-----------+

mysql> select @@global.max_connections;
+--------------------------+
| @@global.max_connections |
+--------------------------+
|                      100 | 
+--------------------------+
1 row in set (0.01 sec)

mysql> set @@global.max_connections=2;
Query OK, 0 rows affected (0.00 sec)

[rgbeast@elf ~]$ mysql
[rgbeast@elf ~]$ mysql

mysql> show processlist;
+----+------+-----------+------+---------+------+-------+------------------+
| Id | User | Host      | db   | Command | Time | State | Info             |
+----+------+-----------+------+---------+------+-------+------------------+
|  2 | root | localhost | NULL | Query   |    0 | NULL  | show processlist | 
|  3 | root | localhost | NULL | Sleep   |   36 | NULL  | NULL             | 
|  4 | root | localhost | NULL | Sleep   |   28 | NULL  | NULL             | 
+----+------+-----------+------+---------+------+-------+------------------+
3 rows in set (0.00 sec)

[rgbeast@elf ~]$ mysql
ERROR 1040 (00000): Too many connections

mysql> set @@global.max_connections=3;
Query OK, 0 rows affected (0.00 sec)

[rgbeast@elf ~]$ mysql

mysql> show processlist;
+----+------+-----------+------+---------+------+-------+------------------+
| Id | User | Host      | db   | Command | Time | State | Info             |
+----+------+-----------+------+---------+------+-------+------------------+
|  2 | root | localhost | NULL | Query   |    0 | NULL  | show processlist | 
|  3 | root | localhost | NULL | Sleep   |   49 | NULL  | NULL             | 
|  4 | root | localhost | NULL | Sleep   |   41 | NULL  | NULL             | 
|  5 | root | localhost | NULL | Sleep   |    4 | NULL  | NULL             | 
+----+------+-----------+------+---------+------+-------+------------------+
4 rows in set (0.00 sec)

[rgbeast@elf ~]$ mysql
ERROR 1040 (00000): Too many connections

The behavior is like expected (one thing is that is allows max_connections+1 connections)

Other manual page, http://dev.mysql.com/doc/refman/5.1/en/dynamic-system-variables.html, clearly states that max_connection is dynamic and global system variable. So, this is definitely a reasonable documentation request in any case.

Thank you for your bug report. This issue has been addressed in the documentation. The updated documentation will appear on our website shortly, and will be included in the next release of the relevant products.