1、背景
公司内部看到一则问题
1、kill -9 mysqld_safe 进程 2、systemd 检测到 mysqld_safe 进程不存在后，重新拉起 mysqld_safe 进程 3、mysqld_safe 进程启动后，发现 mysqld 进程也被重启
期望：启、停 mysqld_safe 进程，不会影响 mysqld 进程
2、systemd 服务启动
2.1、复现问题
1)、查看数据库服务状态
[greatsql@greatsql-1 ~]$ sudo systemctl status db-4306● db-4306.service - db-4306 Server Loaded: loaded (/usr/lib/systemd/system/db-4306.service; disabled; vendor preset: disabled) Active: active (running) since Wed 2023-07-19 11:15:18 CST; 6h ago Main PID: 14917 (mysqld_safe) CGroup: /system.slice/db-4306.service ├─14917 /bin/sh /greatsql/svr/greatsql/bin/mysqld_safe --defaults-file=/greatsql/conf/greatsql4306.cnf └─16340 /greatsql/svr/greatsql/bin/mysqld --defaults-file=/greatsql/conf/greatsql4306.cnf --basedir=/greatsql/svr/greatsql --datadir=/greatsql/dbdata/data4306/data -...Jul 19 11:15:18 greatsql-1 systemd[1]: Started db-4306 Server.Jul 19 11:15:19 greatsql-1 mysqld_safe[14917]: mysqld_safe Adding '/greatsql/svr/GreatSQL-8.0.32-24-Linux-glibc2.17-x86_64/lib/mysql/libjemalloc.so.1' t...or mysqldJul 19 11:15:19 greatsql-1 mysqld_safe[14917]: 2023-07-19T03:15:19.907338Z mysqld_safe Logging to '/greatsql/logs/error4306.log'.Jul 19 11:15:19 greatsql-1 mysqld_safe[14917]: 2023-07-19T03:15:19.953728Z mysqld_safe Starting mysqld daemon with databases from /greatsql/dbdata/data4306/dataHint: Some lines were ellipsized, use -l to show in full.
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2)、kill -9 mysqld_safe 进程，并再次查看数据库服务状态
[greatsql@greatsql-1 ~]$ kill -9 14917[greatsql@greatsql-1 ~]$ sudo systemctl status db-4306● db-4306.service - db-4306 Server Loaded: loaded (/usr/lib/systemd/system/db-4306.service; disabled; vendor preset: disabled) Active: active (running) since Wed 2023-07-19 18:00:33 CST; 43s ago Main PID: 15195 (mysqld_safe) Tasks: 50 CGroup: /system.slice/db-4306.service ├─15195 /bin/sh /greatsql/svr/greatsql/bin/mysqld_safe --defaults-file=/greatsql/conf/greatsql4306.cnf └─16613 /greatsql/svr/greatsql/bin/mysqld --defaults-file=/greatsql/conf/greatsql4306.cnf --basedir=/greatsql/svr/greatsql --datadir=/greatsql/dbdata/data4306/data -...Jul 19 18:00:33 greatsql-1 systemd[1]: Started db-4306 Server.Jul 19 18:00:34 greatsql-1 mysqld_safe[15195]: mysqld_safe Adding '/greatsql/svr/GreatSQL-8.0.32-24-Linux-glibc2.17-x86_64/lib/mysql/libjemalloc.so.1' t...or mysqldJul 19 18:00:34 greatsql-1 mysqld_safe[15195]: 2023-07-19T10:00:34.640240Z mysqld_safe Logging to '/greatsql/logs/error4306.log'.Jul 19 18:00:34 greatsql-1 mysqld_safe[15195]: 2023-07-19T10:00:34.679333Z mysqld_safe Starting mysqld daemon with databases from /greatsql/dbdata/data4306/dataHint: Some lines were ellipsized, use -l to show in full.
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确实在 kill -9 mysqld_safe 后，重新拉起了 mysqld_safe 和 mysqld 进程(它们的 PID 和之前不一样)
3)、查看数据库错误日志
2023-07-19T18:00:31.933020+08:00 0 [System] [MY-013172] [Server] Received SHUTDOWN from user <via user signal>. Shutting down mysqld (Version: 8.0.32-24).
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4)、查看 service 文件
[greatsql@greatsql-1 ~]$ cat /usr/lib/systemd/system/db-4306.service[Unit]Description=db-4306 ServerAfter=network.target
[Install]WantedBy=multi-user.target
[Service]User=greatsqlGroup=greatsql
Type=simpleExecStart=/greatsql/svr/greatsql/bin/mysqld_safe --defaults-file=/greatsql/conf/greatsql4306.cnfRestart=on-failureLimitNOFILE=1024000LimitNPROC=1024000TimeoutStopSec=15PrivateTmp=false
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2.2、分析原因
1)、查看进程信息
[greatsql@greatsql-1 ~]$ ps axj |head -1;ps axj |grep 4306 |grep -v grep PPID PID PGID SID TTY TPGID STAT UID TIME COMMAND 1 15195 15195 15195 ? -1 Ss 986 0:00 /bin/sh /greatsql/svr/greatsql/bin/mysqld_safe --defaults-file=/greatsql/conf/greatsql4306.cnf15195 16613 15195 15195 ? -1 Sl 986 0:06 /greatsql/svr/greatsql/bin/mysqld --defaults-file=/greatsql/conf/greatsql4306.cnf --basedir=/greatsql/svr/greatsql --datadir=/greatsql/dbdata/data4306/data --plugin-dir=/greatsql/svr/greatsql/lib/plugin --log-error=/greatsql/logs/error4306.log --open-files-limit=65535 --pid-file=/greatsql/dbdata/data4306/data/mysql.pid --socket=/greatsql/dbdata/data4306/data/mysql.sock --port=4306
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mysqld_safe：PID(进程 ID)=PGID(进程组 ID)=SID(会话 ID)，说明它是会话首进程，也是该进程组的组长
mysqld_safe 的 PID(进程 ID)=mysqld 的 PPID(父进程 ID)，说明 mysqld_safe 是 mysqld 的父进程
kill -9 mysqld_safe (会话首进程)，会向该进程组的每一个进程发送 SIGKILL，导致组中的进程被中止
2)、整体流程
3、mysqld_safe 命令启动
1)、使用 mysqld_safe 启动数据库
[greatsql@greatsql-1 ~]$ /greatsql/svr/greatsql/bin/mysqld_safe --defaults-file=/greatsql/conf/greatsql4306.cnf &[1] 18229[greatsql@greatsql-1 ~]$ mysqld_safe Adding '/greatsql/svr/GreatSQL-8.0.32-24-Linux-glibc2.17-x86_64/lib/mysql/libjemalloc.so.1' to LD_PRELOAD for mysqld2023-07-19T14:20:19.135297Z mysqld_safe Logging to '/greatsql/logs/error4306.log'.2023-07-19T14:20:19.173594Z mysqld_safe Starting mysqld daemon with databases from /greatsql/dbdata/data4306/data
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2)、查看进程信息
[greatsql@greatsql-1 ~]$ ps axj |head -1;ps axj |grep 4306 |grep -v grep PPID PID PGID SID TTY TPGID STAT UID TIME COMMAND17360 18229 18229 17206 pts/7 17360 S 986 0:00 /bin/sh /greatsql/svr/greatsql/bin/mysqld_safe --defaults-file=/greatsql/conf/greatsql4306.cnf18229 19658 18229 17206 pts/7 17360 Sl 986 0:02 /greatsql/svr/greatsql/bin/mysqld --defaults-file=/greatsql/conf/greatsql4306.cnf --basedir=/greatsql/svr/greatsql --datadir=/greatsql/dbdata/data4306/data --plugin-dir=/greatsql/svr/greatsql/lib/plugin --log-error=/greatsql/logs/error4306.log --open-files-limit=65535 --pid-file=/greatsql/dbdata/data4306/data/mysql.pid --socket=/greatsql/dbdata/data4306/data/mysql.sock --port=4306
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PID≠SID，不是会话首进程 PGID≠TPGID，是后台进程组 mysqld_safe 的 PID=mysqld 的 PPID，说明 mysqld_safe 是 mysqld 的父进程
3)、kill -9 mysqld_safe 进程，并再次查看进程信息
[greatsql@greatsql-1 ~]$ kill -9 18229[greatsql@greatsql-1 ~]$ ps axj |head -1;ps axj |grep 4306 |grep -v grep PPID PID PGID SID TTY TPGID STAT UID TIME COMMAND 1 19658 18229 17206 pts/7 17360 Sl 986 0:07 /greatsql/svr/greatsql/bin/mysqld --defaults-file=/greatsql/conf/greatsql4306.cnf --basedir=/greatsql/svr/greatsql --datadir=/greatsql/dbdata/data4306/data --plugin-dir=/greatsql/svr/greatsql/lib/plugin --log-error=/greatsql/logs/error4306.log --open-files-limit=65535 --pid-file=/greatsql/dbdata/data4306/data/mysql.pid --socket=/greatsql/dbdata/data4306/data/mysql.sock --port=4306
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kill -9 mysqld_safe (非会话首进程)，不影响同组的进程(mysqld)，此时 init 进程会自动领养 mysqld 进程
4)、重新启动 mysqld_safe 进程，并再次查看进程信息
[greatsql@greatsql-1 ~]$ /greatsql/svr/greatsql/bin/mysqld_safe --defaults-file=/greatsql/conf/greatsql4306.cnf &[1] 31401[greatsql@greatsql-1 ~]$ mysqld_safe Adding '/greatsql/svr/GreatSQL-8.0.32-24-Linux-glibc2.17-x86_64/lib/mysql/libjemalloc.so.1' to LD_PRELOAD for mysqld2023-07-19T14:38:42.429733Z mysqld_safe Logging to '/greatsql/logs/error4306.log'.2023-07-19T14:38:42.493870Z mysqld_safe A mysqld process already exists
[1]+ Exit 1 /greatsql/svr/greatsql/bin/mysqld_safe --defaults-file=/greatsql/conf/greatsql4306.cnf [greatsql@greatsql-1 ~]$ ps axj |head -1;ps axj |grep 4306 |grep -v grep PPID PID PGID SID TTY TPGID STAT UID TIME COMMAND 1 19658 18229 17206 pts/7 17360 Sl 986 0:09 /greatsql/svr/greatsql/bin/mysqld --defaults-file=/greatsql/conf/greatsql4306.cnf --basedir=/greatsql/svr/greatsql --datadir=/greatsql/dbdata/data4306/data --plugin-dir=/greatsql/svr/greatsql/lib/plugin --log-error=/greatsql/logs/error4306.log --open-files-limit=65535 --pid-file=/greatsql/dbdata/data4306/data/mysql.pid --socket=/greatsql/dbdata/data4306/data/mysql.sock --port=4306
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mysqld_safe 退出，原因是已存在 mysqld 进程
4、总结
mysqld_safe 进程和 mysqld 进程是父子进程关系
systemd 服务启动，kill 父进程(会话首进程)，会导致子进程也退出
mysqld_safe 命令启动，kill 父进程(非会话首进程)，不会导致子进程退出；重新启动父进程，报错子进程已存在
可以使用 mysqld_safe 命令启动 + 改造 mysqld_safe 脚本，实现【启、停 mysqld_safe 进程，不会影响 mysqld 进程】。此时不要混合使用 systemd 启动数据库，需要维护这个特殊的 mysqld_safe
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