How MySQL Opens and Closes Tables refuse connections 拒绝连接的原因 file descriptors 最大连接数
show status like '%connections%';
MySQL :: MySQL 8.0 Reference Manual :: 8.4.3.1 How MySQL Opens and Closes Tables https://dev.mysql.com/doc/refman/8.0/en/table-cache.html
8.4.3.1 How MySQL Opens and Closes Tables
When you execute a mysqladmin status command, you should see something like this:
Uptime: 426 Running threads: 1 Questions: 11082
Reloads: 1 Open tables: 12The Open tables value of 12 can be somewhat puzzling if you have fewer than 12 tables.
MySQL is multithreaded, so there may be many clients issuing queries for a given table simultaneously. To minimize the problem with multiple client sessions having different states on the same table, the table is opened independently by each concurrent session. This uses additional memory but normally increases performance. With MyISAM tables, one extra file descriptor is required for the data file for each client that has the table open. (By contrast, the index file descriptor is shared between all sessions.)
The table_open_cache and max_connections system variables affect the maximum number of files the server keeps open. If you increase one or both of these values, you may run up against a limit imposed by your operating system on the per-process number of open file descriptors. Many operating systems permit you to increase the open-files limit, although the method varies widely from system to system. Consult your operating system documentation to determine whether it is possible to increase the limit and how to do so.
table_open_cache is related to max_connections. For example, for 200 concurrent running connections, specify a table cache size of at least 200 * , where NN is the maximum number of tables per join in any of the queries which you execute. You must also reserve some extra file descriptors for temporary tables and files.
Make sure that your operating system can handle the number of open file descriptors implied by the table_open_cache setting. If table_open_cache is set too high, MySQL may run out of file descriptors and exhibit symptoms such as refusing connections or failing to perform queries.
Also take into account that the MyISAM storage engine needs two file descriptors for each unique open table. To increase the number of file descriptors available to MySQL, set the open_files_limit system variable. See Section B.3.2.16, “File Not Found and Similar Errors”.
The cache of open tables is kept at a level of table_open_cache entries. The server autosizes the cache size at startup. To set the size explicitly, set the table_open_cache system variable at startup. MySQL may temporarily open more tables than this to execute queries, as described later in this section.
MySQL :: MySQL 8.0 Reference Manual :: 5.1.8 Server System Variables https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_max_connections
| Command-Line Format | --max-connections=# |
|---|---|
| System Variable | max_connections |
| Scope | Global |
| Dynamic | Yes |
SET_VAR Hint Applies |
No |
| Type | Integer |
| Default Value | 151 |
| Minimum Value | 1 |
| Maximum Value | 100000 |
The maximum permitted number of simultaneous client connections. The maximum effective value is the lesser of the effective value of open_files_limit - 810, and the value actually set for max_connections.
