假如你是一名 web 开发人员，如果你想调试你的应用或提升其性能的话，那你需要去参考各种日志文件。日志是开始故障排除最好的选择。就著名的 MySql 数据库服务器而言，你需要参考以下日志文件：
- 错误日志：它包含了服务器运行时(当然也包括服务启动和停止时)所发生的错误信息
- 普通查询日志：这是一个记录 mysqld 在做什么(连接，断开，查询)的通用日志
- 慢查询日志：正如其名，它记录了 "慢" 的查询 SQL 语句
关于 MySql 日志的官方文档参考 http://dev.mysql.com/doc/refman/5.7/en/server-logs.html。
通过 MySql 配置启用日志
日志相关参数位于 [mysqld] 部分。编辑 MySql 配置文件：
nano /etc/mysql/my.cnf
以上是 Debian 下的默认安装目录，其他 Linux 发布版可能不太一样，这个文件中 MySql 服务器的参数如下：
# * 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 logging goes to syslog due to /etc/mysql/conf.d/mysqld_safe_syslog.cnf.
#
# 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
MySql 安装默认是不启用所有日志文件的(Windows 平台下的 error 日志除外)。Debian 安装 MySql 默认是将 error 日志发送给 syslog。
error 日志
根据 /etc/mysql/conf.d/mysqld_safe_syslog.cnf 配置，error 日志推送给 syslog：[mysqld_safe]
syslog
这是推荐的做法。如果你由于某种原因，不想讲 error 日志推给 syslog，将 /etc/mysql/conf.d/mysqld_safe_syslog.cnf 文件中的上述行注掉，或者直接删除掉这个文件，然后在 /etc/mysql/my.cnf 中添加以下行：
[mysqld_safe]
log_error=/var/log/mysql/mysql_error.log
[mysqld]
log_error=/var/log/mysql/mysql_error.log
一般查询日志
要启用一般查询日志，将相关行取消注释(或者添加)即可：general_log_file = /var/log/mysql/mysql.log
general_log = 1
慢查询日志
要启用慢查询日志，将相关行取消注释(或者添加)即可：log_slow_queries = /var/log/mysql/mysql-slow.log
long_query_time = 2
log-queries-not-using-indexes
配置修改后重启 MySql 服务器
以上方法要求服务重启才能生效：service mysql restart
或者使用 systemd：
systemctl restart mysql.service
运行时启用日志
MySql 5.1 之后我们可以在运行时启用或者禁用日志。运行时启用日志，登录 MySql 客户端(mysql -u root -p)然后执行：
SET GLOBAL general_log = 'ON';
SET GLOBAL slow_query_log = 'ON';
运行时禁用日志，登录 Mysql 客户端(mysql -u root -p)后执行：
SET GLOBAL general_log = 'OFF';
SET GLOBAL slow_query_log = 'OFF';
这种方式适用于所有平台并且不需要重启服务。
显示日志结果
error 日志
按以上办法设置以后，你可以通过以下命令显示 error 日志：tail -f /var/log/syslog
备注：如果你没有配置 error 日志文件，MySql 将把 error 日志保存在数据目录(通常是 /var/lib/mysql)下的一个名为 {host_name}.err 的文件中。
普通查询日志
按以上办法设置以后，你可以通过使用以下命令来显示普通日志：tail -f /var/log/mysql/mysql.log
备注：如果你没有配置普通日志文件，MySql 将把普通日志保存在数据目录(通常是 /var/lib/mysql)下的一个名为 {host_name}.log 的文件中。
慢查询日志
按以上办法设置以后，你可以通过使用以下命令来显示慢查询日志：tail -f /var/log/mysql/mysql-slow.log
备注：如果你没有配置慢查询日志文件，MySql 将把普通日志保存在数据目录(通常是 /var/lib/mysql)下的一个名为 {host_name}-slow.log 的文件中。
循环日志
别忘了滚动日志，否则的话日志文件可能会变得很庞大。在 Debian(以及 Debian 派生系列诸如 Ubuntu 等)系统，MySql 初始安装之后，循环日志就已经使用了 logrotate：
nano /etc/logrotate.d/mysql-server
对于其他 Linux 发行版，可能需要做一些改动：
# - I put everything in one block and added sharedscripts, so that mysql gets# flush-logs'd only once.# Else the binary logs would automatically increase by n times every day.# - The error log is obsolete, messages go to syslog now./var/log/mysql.log /var/log/mysql/mysql.log /var/log/mysql/mysql-slow.log { daily rotate 7 missingok create 640 mysql adm compress sharedscripts postrotate test -x /usr/bin/mysqladmin || exit 0 # If this fails, check debian.conf! MYADMIN="/usr/bin/mysqladmin --defaults-file=/etc/mysql/debian.cnf" if [ -z "`$MYADMIN ping 2>/dev/null`" ]; then # Really no mysqld or rather a missing debian-sys-maint user? # If this occurs and is not a error please report a bug. #if ps cax | grep -q mysqld; then if killall -q -s0 -umysql mysqld; then exit 1 fi else $MYADMIN flush-logs fi endscript}检验服务器配置
使用 show variables like '%log%'; 来检查服务器和日志文件相关的变量：[email protected] ~ # mysql -uroot -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or \g.
Your MySQL connection id is 144332
Server version: 5.5.31-0+wheezy1 (Debian)
Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
mysql> show variables like '%log%';
+-----------------------------------------+--------------------------------+
| Variable_name | Value |
+-----------------------------------------+--------------------------------+
| back_log | 50 |
| binlog_cache_size | 32768 |
| binlog_direct_non_transactional_updates | OFF |
| binlog_format | STATEMENT |
| binlog_stmt_cache_size | 32768 |
| expire_logs_days | 10 |
| general_log | OFF |
| general_log_file | /var/lib/mysql/cosmos.log |
| innodb_flush_log_at_trx_commit | 1 |
| innodb_locks_unsafe_for_binlog | OFF |
| innodb_log_buffer_size | 8388608 |
| innodb_log_file_size | 5242880 |
| innodb_log_files_in_group | 2 |
| innodb_log_group_home_dir | ./ |
| innodb_mirrored_log_groups | 1 |
| log | OFF |
| log_bin | OFF |
| log_bin_trust_function_creators | OFF |
| log_error | |
| log_output | FILE |
| log_queries_not_using_indexes | OFF |
| log_slave_updates | OFF |
| log_slow_queries | OFF |
| log_warnings | 1 |
| max_binlog_cache_size | 18446744073709547520 |
| max_binlog_size | 104857600 |
| max_binlog_stmt_cache_size | 18446744073709547520 |
| max_relay_log_size | 0 |
| relay_log | |
| relay_log_index | |
| relay_log_info_file | relay-log.info |
| relay_log_purge | ON |
| relay_log_recovery | OFF |
| relay_log_space_limit | 0 |
| slow_query_log | OFF |
| slow_query_log_file | /var/lib/mysql/cosmos-slow.log |
| sql_log_bin | ON |
| sql_log_off | OFF |
| sync_binlog | 0 |
| sync_relay_log | 0 |
| sync_relay_log_info | 0 |
+-----------------------------------------+--------------------------------+
41 rows in set (0.00 sec)
服务器变量相关官方文档参考 http://dev.mysql.com/doc/refman/5.7/en/server-options.html。
何时启用日志
MySql 默认安装的话，所有的日志文件都不会被启用的(除了 Windows 平台上的 error 日志)。Debian 上安装默认将 error 日志发给 syslog。实际上，在很多情况下日志文件都可以提供关键问题的解决办法：
- 总是启用 error 日志
- 在这些情况下开启普通查询日志(最好在运行时)：检查你的应用是否正确处理了 MySql 数据库连接(一个常见的错误就是从一个单一脚本多次连接到 MySql)；监控来自你的应用的查询的执行情况；测试 memcached(或者类似的软件)，检查某查询是被 db 执行还是被 memcached 处理
- 当你的应用由于某些原因造成性能下降而你想找到这些慢查询时，启用慢查询日志(MySql 最好是在短期内这样配置，比如 2-3 天)
示例
以下是一个 MySql 普通日志的示例：131021 17:43:50 43 Connect [email protected] as anonymous on pnet_blog
43 Init DB pnet_blog
43 Query SELECT count(id) as total_posts FROM posts WHERE date_published is not null AND date_published <= '20131021144350'
43 Query SELECT * FROM posts WHERE date_published is not null AND date_published <= '20131021144350' ORDER BY date_published DESC LIMIT 0,10
44 Connect [email protected] as anonymous on pnet_blog
44 Query SELECT id, title, impressions FROM tips WHERE date_published IS NOT NULL AND date_published <= '20131021144350' ORDER BY date_published DESC LIMIT 0, 10
44 Quit
43 Quit
131021 17:44:28 45 Connect [email protected] as anonymous on pnet_blog
45 Init DB pnet_blog
45 Query SELECT * FROM posts WHERE url='how-and-when-to-enable-mysql-logs'
45 Query UPDATE posts SET impressions=impressions+1 WHERE id='41'
45 Query SELECT url, post_title FROM posts WHERE date_published IS NOT NULL AND date_published < '20131020150000' ORDER BY date_published DESC LIMIT 0,1
45 Query SELECT url, post_title FROM posts WHERE date_published IS NOT NULL AND date_published > '20131020150000' ORDER BY date_published ASC LIMIT 0,1
45 Query SELECT * FROM posts WHERE date_published is not null AND date_published <= '20131021144428' AND date_published >= '20130421144428' ORDER BY impressions DESC LIMIT 0,10
46 Connect [email protected] as anonymous on pnet_blog
46 Query SELECT id, title, impressions FROM tips WHERE date_published IS NOT NULL AND date_published <= '20131021144428' ORDER BY date_published DESC LIMIT 0, 10
46 Quit
45 Quit
原文链接：http://www.pontikis.net/blog/how-and-when-to-enable-mysql-logs。