1. Find current value of open_tables and opened_tables
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mysql> show global status like 'open%'; +--------------------------+---------+ | Variable_name | Value | +--------------------------+---------+ | Open_files | 1583 | | Open_streams | 0 | | Open_table_definitions | 1400 | | Open_tables | 2000 | | Opened_files | 2619222 | | Opened_table_definitions | 110583 | | Opened_tables | 482099 | +--------------------------+---------+ 7 rows in set (0.00 sec) |
2. Find out Table cache hit rate
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Table cache hit rate = table_open_cache*100/Opened_tables. = 2000*100/482099 = 0.41% |
In general it should be more than 50%. So you need to increase value of table_open_cache, though there are lots of reasons to have a high value of Opened_tables. Like FLUSH TABLES will close all open tables and reopen it which significantly increases Opened_tables value.
At this stage you are almost sure table_open_cache system variable is not tuned properly.
Now you have to optimize MySQL table_open_cache and find out perfect value for this. To find tuned value of table_open_cache value follow the steps:
1. Find out total tables of your database
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mysql> SELECT COUNT(*) FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_TYPE='BASE TABLE'; +----------+ | COUNT(*) | +----------+ | 211 | +----------+ 1 row in set (0.05 sec) Find threads currently connected to your database. mysql> show global status like '%Threads_connected%'; +-------------------+-------+ | Variable_name | Value | +-------------------+-------+ | Threads_connected | 6 | +-------------------+-------+ 1 row in set (0.00 sec) |
It would be best if you take threads connected at busiest time of your database or take several times at different time and make an average.
3. Calculate the tune value of table_open_cache and set it
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Table_open_cache = total_tables*Threads_connected = 211*6 = 633 |
As all the threads (user) are not generally access all tables. I think you should set 50% of the value calculated. Because too big value of this variable has some other side effects. So the formula becomes
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Table_open_cache = total_tables*Threads_connected*.50 |
4. Along with table_open_cache you should also tune open_files_limit system variable.
In general it is 2x of table_open_cache.
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open_files_limit= Table_open_cache*2 |
open_files_limit is not a dynamic variable. So you should set it in my.cnf file and restart MySQL.
*Make sure that your operating system can cope with the number of open file descriptors required by the table_open_cache setting.
Go to your Mysql configuration file (in linux it is /etc/my.cnf) and set the table_open_cache and open_files_limit
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nano /etc/my.cnf |
add
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table_open_cache=633 open_files_limit=65535 |
5. LimitNOFILE
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nano /etc/systemd/system/multi-user.target.wants/mysql.service |
Add line Ubuntu/Debian
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# MySQL systemd service file [Unit] Description=MySQL Community Server After=network.target [Install] WantedBy=multi-user.target [Service] User=mysql Group=mysql PermissionsStartOnly=true ExecStartPre=/usr/share/mysql/mysql-systemd-start pre ExecStart=/usr/sbin/mysqld ExecStartPost=/usr/share/mysql/mysql-systemd-start post TimeoutSec=600 Restart=on-failure RuntimeDirectory=mysqld RuntimeDirectoryMode=755 <strong>LimitNOFILE=65536</strong> |
OR
Centos/RHEL/Fedora
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nano /etc/systemd/system/multi-user.target.wants/mysql.service |
add line
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LimitNOFILE=65536 |
Reload and Restart
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systemctl daemon-reload service mysql restart |
6. Restart the MySQL ( In Linux it is like)
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systemctl daemon-reload systemctl restart mysql |
or old Ubuntu 14.04/ Debian 7/ CentOS 62020
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/etc/init.d/mysqld restart or /etc/init.d/mysql restart |
The database I have taken has 211 tables so the value of table_open_cache is little big. For your case it may be significantly small.
