官方解释
mysqld进程能使用的最大文件描述符数量，mysql实际的取值会从下面四个值当中获取最大的。
- 10 + max_connections + (table_open_cache * 2)
- max_connections * 5
- operating system limit if positive
- if operating system limit is Infinity:
open_files_limit value specified at startup, 5000 if none
我们接下来测试让mysql分别取上面四种值为实际的取值。
测试mysql版本和系统版本
操作系统版本为CentOS Linux release 7.4.1708 (Core)，全新安装，没有任何配置。
mysql版本 Percona-Server-5.7.21-20-Linux.x86_64
接下来开始测试了。
没有配置open_files_limit参数（root用户登陆操作）
mysql配置文件如
[mysqld]
user = mysql
port = 5721
socket = /tmp/mysql_sandbox5721.sock
basedir = /root/opt/mysql/5.7.21
datadir = /opt/msb_5_7_21/data
tmpdir = /opt/msb_5_7_21/tmp
pid-file = /opt/msb_5_7_21/data/mysql_sandbox5721.pid
bind-address = 127.0.0.1
table_open_cache默认值是2000,max_connections默认值是151+1，因为还有一个extra_max_connections，ulimit -n 的值为1024。
结果如下
[root@mysqlmaster ~]# ps -ef | grep mysqld
root 3047 1 0 09:52 pts/2 00:00:00 /bin/sh bin/mysqld_safe --defaults-file=/opt/msb_5_7_21/my.sandbox.cnf
mysql 3253 3047 0 09:52 pts/2 00:00:02 /root/opt/mysql/5.7.21/bin/mysqld --defaults-file=/opt/msb_5_7_21/my.sandbox.cnf --basedir=/root/opt/mysql/5.7.21 --datadir=/opt/msb_5_7_21/data --plugin-dir=/root/opt/mysql/5.7.21/lib/mysql/plugin --user=mysql --log-error=/opt/msb_5_7_21/data/msandbox.err --pid-file=/opt/msb_5_7_21/data/mysql_sandbox5721.pid --socket=/tmp/mysql_sandbox5721.sock --port=5721
root 10583 5177 0 09:57 pts/4 00:00:00 grep --color=auto mysqld
[root@mysqlmaster ~]# cat /proc/3253/limits |grep files
Max open files 5000 5000 files
--------------------------------------------------------------------------------
mysql [localhost:5721] {root} ((none)) > SELECT @@open_files_limit ;
+--------------------+
| @@open_files_limit |
+--------------------+
| 5000 |
+--------------------+
1 row in set (0.00 sec)
不管是系统层面还是mysql，取值都是5000，符合了第四种情况
- if operating system limit is Infinity:
open_files_limit value specified at startup, 5000 if none
其他几种公式的值为：
- select @@max_connections + @@extra_max_connections + 10 + @@table_open_cache*2 = 4162
- select (@@max_connections+@@extra_max_connections)*5 = 760
- ulimit -n = 1024
那我们接下来依次增大其他几种公式的值，看看结果会有什么变化。
10 + max_connections + (table_open_cache * 2)最大
设置table_open_cache=3000，其他不变，取值如下：
- select @@max_connections + @@extra_max_connections + 10 + @@table_open_cache*2 = 6162
- select (@@max_connections+@@extra_max_connections)*5 = 760
- ulimit -n = 1024
- 5000
结果为
[root@mysqlmaster msb_5_7_21]# ps -ef | grep mysqld
root 80009 1 0 10:43 pts/2 00:00:00 /bin/sh bin/mysqld_safe --defaults-file=/opt/msb_5_7_21/my.sandbox.cnf
mysql 80227 80009 11 10:43 pts/2 00:00:01 /root/opt/mysql/5.7.21/bin/mysqld --defaults-file=/opt/msb_5_7_21/my.sandbox.cnf --basedir=/root/opt/mysql/5.7.21 --datadir=/opt/msb_5_7_21/data --plugin-dir=/root/opt/mysql/5.7.21/lib/mysql/plugin --user=mysql --log-error=/opt/msb_5_7_21/data/msandbox.err --pid-file=/opt/msb_5_7_21/data/mysql_sandbox5721.pid --socket=/tmp/mysql_sandbox5721.sock --port=5721
root 80555 1129 0 10:43 pts/2 00:00:00 grep --color=auto mysqld
[root@mysqlmaster msb_5_7_21]#
[root@mysqlmaster msb_5_7_21]# cat /proc/80227/limits |grep files
Max open files 6162 6162 files
------------------------------------------------------------------------------------------------
mysql [localhost:5721] {root} ((none)) > SELECT @@open_files_limit ;
+--------------------+
| @@open_files_limit |
+--------------------+
| 6162 |
+--------------------+
1 row in set (0.00 sec)
max_connections * 5最大
设置max_connections的值为2000，其他值默认，取值如下：
- select @@max_connections + @@extra_max_connections + 10 + @@table_open_cache*2 =6011
- select (@@max_connections+@@extra_max_connections)*5 = 10005
- ulimit -n = 1024
- 5000
结果如下
[root@mysqlmaster msb_5_7_21]# ps -ef | grep mysqld
root 87914 1 0 10:48 pts/2 00:00:00 /bin/sh bin/mysqld_safe --defaults-file=/opt/msb_5_7_21/my.sandbox.cnf
mysql 88132 87914 21 10:48 pts/2 00:00:01 /root/opt/mysql/5.7.21/bin/mysqld --defaults-file=/opt/msb_5_7_21/my.sandbox.cnf --basedir=/root/opt/mysql/5.7.21 --datadir=/opt/msb_5_7_21/data --plugin-dir=/root/opt/mysql/5.7.21/lib/mysql/plugin --user=mysql --log-error=/opt/msb_5_7_21/data/msandbox.err --pid-file=/opt/msb_5_7_21/data/mysql_sandbox5721.pid --socket=/tmp/mysql_sandbox5721.sock --port=5721
root 88300 1129 0 10:48 pts/2 00:00:00 grep --color=auto mysqld
[root@mysqlmaster msb_5_7_21]# cat /proc/88132/limits | grep files
Max open files 10005 10005 files
----------------------------------------------------------------------------------------------
mysql [localhost:5721] {root} ((none)) > SELECT @@open_files_limit ;
+--------------------+
| @@open_files_limit |
+--------------------+
| 10005 |
+--------------------+
1 row in set (0.00 sec)
operating system limit if positive最大
我的理解是ulimit -n显示的值。max_connections和table_open_cache取默认值，然后ulimit -n 6000。重启mysql。
公式取值如下：
- select @@max_connections + @@extra_max_connections + 10 + @@table_open_cache*2 = 4162
- select (@@max_connections+@@extra_max_connections)*5 = 760
- ulimit -n = 6000
结果如下：
[root@mysqlmaster msb_5_7_21]# ps -ef | grep mysql
root 93825 1 0 10:52 pts/2 00:00:00 /bin/sh bin/mysqld_safe --defaults-file=/opt/msb_5_7_21/my.sandbox.cnf
mysql 94031 93825 16 10:52 pts/2 00:00:01 /root/opt/mysql/5.7.21/bin/mysqld --defaults-file=/opt/msb_5_7_21/my.sandbox.cnf --basedir=/root/opt/mysql/5.7.21 --datadir=/opt/msb_5_7_21/data --plugin-dir=/root/opt/mysql/5.7.21/lib/mysql/plugin --user=mysql --log-error=/opt/msb_5_7_21/data/msandbox.err --pid-file=/opt/msb_5_7_21/data/mysql_sandbox5721.pid --socket=/tmp/mysql_sandbox5721.sock --port=5721
root 94254 1129 0 10:52 pts/2 00:00:00 grep --color=auto mysql
[root@mysqlmaster msb_5_7_21]# cat /proc/94031/limits | grep files
Max open files 6000 6000 files
---------------------------------------------------------------------------------------
mysql [localhost:5721] {root} ((none)) > SELECT @@open_files_limit ;
+--------------------+
| @@open_files_limit |
+--------------------+
| 6000 |
+--------------------+
1 row in set (0.00 sec)
配置open_files_limit参数（root用户登陆操作）
配置了open_files_limit参数，还是会取最大值。但是不一样的是第三种公式的取值，也就是ulimit -n不会被考虑进来。而是从另外三种取值当中获取最大的值。
执行ulimit -n 6000 增大限制,然后max_connections,table_open_cache都是默认。open_files_limit设置为4200。重启mysql
结果如下：
[root@mysqlmaster msb_5_7_21]# ps -ef | grep mysqld
root 104048 1 0 10:58 pts/2 00:00:00 /bin/sh bin/mysqld_safe --defaults-file=/opt/msb_5_7_21/my.sandbox.cnf
mysql 104269 104048 19 10:58 pts/2 00:00:01 /root/opt/mysql/5.7.21/bin/mysqld --defaults-file=/opt/msb_5_7_21/my.sandbox.cnf --basedir=/root/opt/mysql/5.7.21 --datadir=/opt/msb_5_7_21/data --plugin-dir=/root/opt/mysql/5.7.21/lib/mysql/plugin --user=mysql --log-error=/opt/msb_5_7_21/data/msandbox.err --open-files-limit=4200 --pid-file=/opt/msb_5_7_21/data/mysql_sandbox5721.pid --socket=/tmp/mysql_sandbox5721.sock --port=5721
root 104460 1129 0 10:58 pts/2 00:00:00 grep --color=auto mysqld
[root@mysqlmaster msb_5_7_21]# cat /proc/104269/limits | grep files
Max open files 4200 4200 files
------------------------------------------------------------------
mysql [localhost:5721] {root} ((none)) > SELECT @@open_files_limit ;
+--------------------+
| @@open_files_limit |
+--------------------+
| 4200 |
+--------------------+
1 row in set (0.00 sec)
显示的值就变成了4200,明显6000没有被考虑进来，就算是值比4200大。其他的情况和没有配置open_files_limit相同，获取最大的值，篇幅原因就不做赘述了。
总结
在没有配置的open_files_limit的情况下，会获取下面公式的最大值。
- 10 + max_connections + (table_open_cache * 2)
- max_connections * 5
- operating system limit if positive
- if operating system limit is Infinity:
open_files_limit value specified at startup, 5000 if none
在配置了open_files_limit的情况下，第三个值的大小会被忽略。
这里还有一个疑问就是为啥ulimit -n显示1024的时候没有起到限制作用呢。我认为的原因是启动mysqld_safe的用户为root，不受这个值的限制。经过测试不管mysqld的启动用户是什么，只要mysqld_safe启动的用户为root，就不受这个限制。只有在
mysqld_safe由非root用户启动的时候，才会受到这个限制。