Shantanu's Blog

Corporate Consultant

March 04, 2012

 

max connections and table cache

Adjusting max connections:

As per my.cnf setting, the maximum permissible max connections are 1024 but we have used only 3 simultaneous connections so far. So there is no need of such a high value and it can be reduced to 900

[root@myserver]# mysqladmin variables | grep max_connections
| max_connections | 1024 |

[root@myserver]# mysqladmin extended-status | grep Max_used_connections
| Max_used_connections | 3 |

[root@myserver]# mysql -e"set global max_connections=600"

Now don't we have a risk of running out of available connections in the future?
Yes. we do. and we still need to keep the max connections as low as possible. why? read on.

table cache
In order to have better performance, we can set the table cache number very high. for e.g.

table_cache=1M

But it is wrong to set such a high value (one million !?), since mysql does not allow more than half a million size for this variable. Let mysql initially have the default limit of 64 that is sufficient for most usual operations. Increasing this limit will return "too many connections" error to the application that is going to be very costly. The high value of table cache conflicts with high value of max_connections and limit its usage. In other words trying to set table cache to 1 million will limit the max_connections to just around 200 or so because there is an upper limit of allowed open file descriptors set by OS.
This point will be clear if you see the following line in mysql error log ...

120201 19:40:24 [Warning] Changed limits: max_open_files: 1024 max_connections: 214 table_cache: 400

In the above case, Mysql has automatically adjusted the table_cache value from 1 million to 400. At the same time it has also brought down the max_connections value from 2000 to 214. This has been done because the max_open_files limit of 1024 is set by OS and mysql has no control over it. If we reduce the table_cache value to the default 64 then mysql will allow max_connections to be set higher than 214. Another option would be to increase the max_open_files variable using "ulimit" command from default 1024 to something like 4024. Since disabling "table_cache" option from my.cnf can easily resolve this issue, I do not recommend increasing file descriptors limit unless absolutely necessary.

In other words, disabling one million table_cache from my.cnf is the only easy option to avoid "max connections reached" error.

Default value of table cache is 64. If the number of file descriptors mysql needs to open is too high (in millions) only then we need to increase the table_cache variable.

[root@myserver]# mysqladmin variables | grep table_cache
| table_cache | 64 |

[root@myserver]# mysqladmin extended-status | grep Opened_tables
| Opened_tables | 1 million |

[root@myserver]# mysql -e"set global table_cache=512"

After restarting mysql service, always check the error log, there may be a useful warning!

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