这篇文章主要介绍Ubuntu中MySQL的参数文件my.cnf有什么用，文中介绍的非常详细，具有一定的参考价值，感兴趣的小伙伴们一定要看完！
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前言
对于MySQL的理解，我认为很多性能优化工作、主从主主复制都是在调整参数，来适应不同时期不同数量级的数据。
故，理解透彻my.cnf里的参数是永恒的话题；只有理解透彻了参数设置，才能在某些方面对数据库进行调优。
前几天刚接手一个MySQL数据，操作系统为Ubuntu 16.04.5 LTS， 数据库版本为5.7.23-0ubuntu0.16.04.1（APT方式安装的MySQL）。这个操作系统下的MySQL的配置文件my.cnf很多地方都让人有点不适应（跟之前的MySQL环境有些出入，之前都是维护RHEL、CentOS等操作系统环境下的MySQL）。
遂研究总结了一下。具体如下所示：
root@mylnx12:~# find / -name "my.cnf"
/etc/alternatives/my.cnf
/etc/mysql/my.cnf
/var/lib/dpkg/alternatives/my.cnf
root@mylnx12:~# locate my.cnf
/etc/alternatives/my.cnf
/etc/mysql/my.cnf
/etc/mysql/my.cnf.fallback
/var/lib/dpkg/alternatives/my.cnf
root@mylnx12:~# mysql --help | grep my.cnf
order of preference, my.cnf, $MYSQL_TCP_PORT,
/etc/my.cnf /etc/mysql/my.cnf ~/.my.cnf
root@mylnx12:~# mysqld --verbose --help | grep -A 1 'Default options'
Default options are read from the following files in the given order:
/etc/my.cnf /etc/mysql/my.cnf ~/.my.cnf
从上面这些信息判断，MySQL的参数文件为/etc/mysql/my.cnf， 但是其他几个my.cnf又是什么情况呢？
root@mylnx12:~# ls -lrt /etc/alternatives/my.cnf
lrwxrwxrwx 1 root root 20 Sep 28 16:28 /etc/alternatives/my.cnf -> /etc/mysql/mysql.cnf
从上面信息可以看出，/etc/alternatives/my.cnf 其实是一个软连接，指向参数文件/etc/mysql/mysql.cnf
root@mylnx12:~# cat /var/lib/dpkg/alternatives/my.cnf
auto
/etc/mysql/my.cnf
/etc/mysql/my.cnf.fallback
100
/etc/mysql/mysql.cnf
200
光从上面这些信息，我们还看不出/var/lib/dpkg/alternatives/my.cnf与其它配置文件my.cnf是什么关系。那么我们先来看看参数文件/etc/mysql/my.cnf,从下面信息，可以看出“/etc/mysql/my.cnf”是全局配置，“~/.my.cnf”隐藏文件是个人用户设置.
root@mylnx12:~# cat /etc/mysql/my.cnf
#
# The MySQL database server configuration file.
#
# You can copy this to one of:
# - "/etc/mysql/my.cnf" to set global options,
# - "~/.my.cnf" to set user-specific options.
#
# One can use all long options that the program supports.
# Run program with --help to get a list of available options and with
# --print-defaults to see which it would actually understand and use.
#
# For explanations see
# http://dev.mysql.com/doc/mysql/en/server-system-variables.html
#
# * IMPORTANT: Additional settings that can override those from this file!
# The files must end with '.cnf', otherwise they'll be ignored.
#
!includedir /etc/mysql/conf.d/
!includedir /etc/mysql/mysql.conf.d/
但是/etc/mysql/my.cnf参数文件下面没有任何参数设置，只看到下面两行设置，表示导入这两个目录里面的配置文件。
!includedir /etc/mysql/conf.d/
#表示包含/etc/mysql/conf.d/这个路径下面的配置文件，前提是必须以为.cnf为后缀
!includedir /etc/mysql/mysql.conf.d/
#表示包含/etc/mysql/mysql.conf.d/这个路径下面的配置文件，前提是必须以为.cnf为后缀
其实MySQL的相关配置都位于mysqld.cnf（/etc/mysql/mysql.conf.d/mysqld.cnf）下面。使用相关参数测试了一下，确实都能生效。这种设置确实有点让刚接触的人有点不适应。暂时先总结到此！
root@mylnx12:~# cd /etc/mysql/mysql.conf.d/
root@mylnx12:/etc/mysql/mysql.conf.d# ls -lrt
total 8
-rw-r--r-- 1 root root 21 Feb 4 2017 mysqld_safe_syslog.cnf
-rw-r--r-- 1 root root 3148 Oct 6 23:34 mysqld.cnf
root@mylnx12:/etc/mysql/mysql.conf.d# cat mysqld.cnf
#
# The MySQL database server configuration file.
#
# You can copy this to one of:
# - "/etc/mysql/my.cnf" to set global options,
# - "~/.my.cnf" to set user-specific options.
#
# One can use all long options that the program supports.
# Run program with --help to get a list of available options and with
# --print-defaults to see which it would actually understand and use.
#
# For explanations see
# http://dev.mysql.com/doc/mysql/en/server-system-variables.html
# This will be passed to all mysql clients
# It has been reported that passwords should be enclosed with ticks/quotes
# escpecially if they contain "#" chars...
# Remember to edit /etc/mysql/debian.cnf when changing the socket location.
# Here is entries for some specific programs
# The following values assume you have at least 32M ram
[mysqld_safe]
socket = /var/run/mysqld/mysqld.sock
nice = 0
[mysqld]
#
# * Basic Settings
#
user = mysql
pid-file = /var/run/mysqld/mysqld.pid
socket = /var/run/mysqld/mysqld.sock
port = 3306
basedir = /usr
datadir = /var/lib/mysql
tmpdir = /tmp
lc-messages-dir = /usr/share/mysql
skip-external-locking
log_bin = mylnx12_bin
server_id = 0
character-set-server=utf8mb4
collation-server=utf8mb4_general_ci
#
# Instead of skip-networking the default is now to listen only on
# localhost which is more compatible and is not less secure.
bind-address = 10.21.6.7
#
# * Fine Tuning
#
key_buffer_size = 16M
max_allowed_packet = 100M
thread_stack = 192K
thread_cache_size = 8
# This replaces the startup script and checks MyISAM tables if needed
# the first time they are touched
myisam-recover-options = BACKUP
#max_connections = 100
#table_cache = 64
#thread_concurrency = 10
#
# * Query Cache Configuration
#
query_cache_limit = 1M
query_cache_size = 16M
#
# * Logging and Replication
#
# Both location gets rotated by the cronjob.
# Be aware that this log type is a performance killer.
# As of 5.1 you can enable the log at runtime!
#general_log_file = /var/log/mysql/mysql.log
#general_log = 1
#
# Error log - should be very few entries.
#
log_error = /var/log/mysql/error.log
#
# Here you can see queries with especially long duration
#log_slow_queries = /var/log/mysql/mysql-slow.log
#long_query_time = 2
#log-queries-not-using-indexes
#
# The following can be used as easy to replay backup logs or for replication.
# note: if you are setting up a replication slave, see README.Debian about
# other settings you may need to change.
#server-id = 1
#log_bin = /var/log/mysql/mysql-bin.log
expire_logs_days = 10
max_binlog_size = 100M
#binlog_do_db = include_database_name
#binlog_ignore_db = include_database_name
#
# * InnoDB
#
# InnoDB is enabled by default with a 10MB datafile in /var/lib/mysql/.
# Read the manual for more InnoDB related options. There are many!
#
# * Security Features
#
# Read the manual, too, if you want chroot!
# chroot = /var/lib/mysql/
#
# For generating SSL certificates I recommend the OpenSSL GUI "tinyca".
#
# ssl-ca=/etc/mysql/cacert.pem
# ssl-cert=/etc/mysql/server-cert.pem
#: ssl-key=/etc/mysql/server-key.pem
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