Come incrementare il valore max_connections di MySQL 5.5 su RHEL 5

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Il limite di Default è impostato a 214 connections.

mysql show variables like 'max_connections';
+-----------------+-------+
| Variable_name   | Value |
+-----------------+-------+
| max_connections |  214  |
+-----------------+-------+
1 row in set (0.00 sec)

Il numero massimo di file aperti consentiti è troppo piccolo, per impostazione predefinita 1024, per questo occorre aumentare la max_connections ad un numero maggiore di 214. Ci sono molte guide online che spiegano come gestire questo problema con differenti metodi:

  • aumentando l’impostazione fs.file-max del kernel, modificando /etc/sysctl.conf come in questo esempio raddoppiando l’impostazione predefinita di fs.file-max:
fs.file-max = 2459688

Fatto questo, eseguire sysctl -p per rendere immediate le modifiche. (Rimarranno fino al prossimo reboot)

  • modificando opportunatamente il file /etc/security/limits.conf:
mysql   soft   nofile    4096
mysql   hard   nofile    8192

La modifica di /etc/security/limits.conf non funziona quando il servizio mysqld viene avviato tramite lo script init /etc/init.d/mysql o tramite il 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

… 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)

and

mysql show variables like '%open_files_limit%';
+----------------------------+----------+
| Variable_name              | Value    |
+----------------------------+----------+
| open_files_limit           | 5000     |
+----------------------------+----------+

you can force ulimit -n 4096 to /etc/init.d/mysql-server start function.   Source: Increasing MySQL 5.5 max_connections on RHEL 5 | End Point Blog