mysql的配置文件是/etc/my.cnf，通过修改它来配置mysql。
/etc/my.cnf来自以下文件：
如果你的内存≤64M，则复制/usr/local/share/mysql/my-small.cnf为/etc/my.cnf
# This is for a system with little memory (<= 64M) where MySQL is only used
# from time to time and it’s important that the mysqld daemon
# doesn’t use much resources.
如果内存是128M，则复制/usr/local/share/mysql/my-medium.cnf为/etc/my.cnf
# This is for a system with little memory (32M – 64M) where MySQL plays
# an important part, or systems up to 128M where MySQL is used together with
# other programs (such as a web server)
如果内存是512M，则复制/usr/local/share/mysql/my-large.cnf为/etc/my.cnf
# This is for a large system with memory = 512M where the system runs mainly
# MySQL.
如果内存是1-2G，则复制/usr/local/share/mysql/my-huge.cnf为/etc/my.cnf
# This is for a large system with memory of 1G-2G where the system runs mainly
# MySQL.
如果内存是4G，则复制/usr/local/share/mysql/my-innodb-heavy-4G.cnf为/etc/my.cnf
# This is a MySQL example config file for systems with 4GB of memory
# running mostly MySQL using InnoDB only tables and performing complex
# queries with few connections.
不过MySQL参数那么多，很多时候我们还是要知道他们具体的含义才能根据实际问题做出具体的调整。
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我们可以通过SHOW VARIABLES;来查看系统参数，通过SHOW STATUS;来判断系统状态。
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先来看看table_cache参数对性能的影响。摘录my-innodb-heavy-4G.cnf中对它的描述：
# The number of open tables for all threads. Increasing this value
# increases the number of file descriptors that mysqld requires.
# Therefore you have to make sure to set the amount of open files
# allowed to at least 4096 in the variable “open-files-limit” in
# section [mysqld_safe]
table_cache = 2048
比如：当系统比较繁忙的时候，我们show variables;查到table_cache的值，再show
status;发现open_tables的值和table_cache差不多，而且opened_tables还一直再增加，就说明我们的
table_cache设置的太小了。