mysqld_safe之三言两语

    today,one buddy in IMG wechat group 2 asked "why i've installed the MySQL 5.7 on linux server,but there's no mysqld_safe command at all?"so,here i'd like to post this article to say something about it.first of all,let's see the command parameter and usage:

 #mysqld_safe --help
 Usage: /usr/local/mysql/bin/mysqld_safe [OPTIONS]
  The following options may be given as the first argument:
   --no-defaults              Don't read the system defaults file
   --defaults-file=FILE       Use the specified defaults file
   --defaults-extra-file=FILE Also use defaults from the specified file

  Other options:
   --ledir=DIRECTORY          Look for mysqld in the specified directory
   --open-files-limit=LIMIT   Limit the number of open files
   --core-file-size=LIMIT     Limit core files to the specified size
   --timezone=TZ              Set the system timezone
   --malloc-lib=LIB           Preload shared library LIB if available
   --mysqld=FILE              Use the specified file as mysqld
   --mysqld-version=VERSION   Use "mysqld-VERSION" as mysqld
   --nice=NICE                Set the scheduling priority of mysqld
   --plugin-dir=DIR           Plugins are under DIR or DIR/VERSION, if
                              VERSION is given
   --skip-kill-mysqld         Don't try to kill stray mysqld processes
   --syslog                   Log messages to syslog with 'logger'
   --skip-syslog              Log messages to error log (default)
   --syslog-tag=TAG           Pass -t "mysqld-TAG" to 'logger'
   --mysqld-safe-log-         TYPE must be one of UTC (ISO  UTC),
     timestamps=TYPE          system (ISO  local time), hyphen
                              (hyphenated date a la mysqld 5.6), legacy
                              (legacy non-ISO  mysqld_safe timestamps)

 All other options are passed to the mysqld program.

 [root@zlm3 :: /data/mysql/mysql3306]
 #

    the most simplest usage of mysqld_safe way is to just use '--defaults-file' to specify which "my.cnf" you want to use,just like:

 [root@zlm3 :: /usr/local/mysql/bin]
 #pkill mysqld

 [root@zlm3 :: /usr/local/mysql/bin]
 #ps aux|grep mysqld
 root        0.0  0.0     pts/    R+   :   : grep --color=auto mysqld

 [root@zlm3 :: /usr/local/mysql/bin]
 #mysqld_safe --defaults-file=/data/mysql/mysql3306/my.cnf &
 [] 

 [root@zlm3 :: /usr/local/mysql/bin]
 #--04T05::.758814Z mysqld_safe Logging to '/data/mysql/mysql3306/data/error.log'.
 --04T05::.786306Z mysqld_safe Starting mysqld daemon with databases from /data/mysql/mysql3306/data
 ^C

 [root@zlm3 :: /usr/local/mysql/bin]
 #ps aux|grep mysqld
 root        0.1  0.1    pts/    S    :   : /bin/sh /usr/local/mysql/bin/mysqld_safe --defaults-file=/data/mysql/mysql3306/my.cnf
 mysql       1.0 17.4   pts/  Sl   :   : /usr/local/mysql/bin/mysqld --defaults-file=/data/mysql/mysql3306/my.cnf --basedir=/usr/local/mysql --datadir=/data/mysql/mysql3306/data --plugin-dir=/usr/local/mysql/lib/plugin --user=mysql --log-error=error.log --open-files-limit= --pid-file=mysql.pid --socket=/tmp/mysql3306.sock --port=
 root        0.0  0.0     pts/    R+   :   : grep --color=auto mysqld

 [root@zlm3 :: /usr/local/mysql/bin]
 #

    here we can see,there're two processes running,one is mysqld_safe,another one is the mysqld.even if you use "kill -9 7328" to stop the mysqld process,but subsequently you'll find that the mysqld will startup again soon,unless you kill mysqld process by using "pkill mysqld" as below:

 [root@zlm3 :: /usr/local/mysql/bin]
 #kill - 

 [root@zlm3 :: /usr/local/mysql/bin]
 #/usr/local/mysql/bin/mysqld_safe: line :   Killed                  nohup /usr/local/mysql/bin/mysqld --defaults-file=/data/mysql/mysql3306/my.cnf --basedir=/usr/local/mysql --datadir=/data/mysql/mysql3306/data --plugin-dir=/usr/local/mysql/lib/plugin --user=mysql --log-error=error.log --open-files-limit= --pid-file=mysql.pid --socket=/tmp/mysql3306.sock --port= < /dev/null > /dev/null >&
 --04T05::.076914Z mysqld_safe Number of processes running now:
 --04T05::.083092Z mysqld_safe mysqld restarted
 ^C

 [root@zlm3 :: /usr/local/mysql/bin]
 #ps aux|grep mysqld
 root        0.0  0.1    pts/    S    :   : /bin/sh /usr/local/mysql/bin/mysqld_safe --defaults-file=/data/mysql/mysql3306/my.cnf
 mysql       2.4 17.7   pts/  Sl   :   : /usr/local/mysql/bin/mysqld --defaults-file=/data/mysql/mysql3306/my.cnf --basedir=/usr/local/mysql --datadir=/data/mysql/mysql3306/data --plugin-dir=/usr/local/mysql/lib/plugin --user=mysql --log-error=error.log --open-files-limit= --pid-file=mysql.pid --socket=/tmp/mysql3306.sock --port=
 root        0.0  0.0     pts/    R+   :   : grep --color=auto mysqld

 [root@zlm3 :: /usr/local/mysql/bin]
 #pkill mysqld

 [root@zlm3 :: /usr/local/mysql/bin]
 #--04T05::.957789Z mysqld_safe mysqld from pid file /data/mysql/mysql3306/data/mysql.pid ended
 ^C
 []+  Done                    mysqld_safe --defaults-file=/data/mysql/mysql3306/my.cnf

 [root@zlm3 :: /usr/local/mysql/bin]
 #ps aux|grep mysqld
 root        0.0  0.0     pts/    R+   :   : grep --color=auto mysqld

 [root@zlm3 :: /usr/local/mysql/bin]
 #

    why will that happen?the consequence is trigged by mysqld_safe which can protect mysqld from killing by some command like "kill -9" accidentally.or in the other case,the mysqld process shuted down by the OS because of some unknowing issue or bug.that will help you to prevent the application from disconnecting the MySQL server when mysqld process down.but why the buddy's MySQL server hasnot the mysqld_safe command?here's the explaination:

 

---

Note

For some Linux platforms, MySQL installation from RPM or Debian packages includes systemd support for managing MySQL server startup and shutdown. On these platforms, mysqld_safe is not installed because it is unnecessary. For more information, see Section 2.5.9, “Managing MySQL Server with systemd”.

来源： https://dev.mysql.com/doc/refman/8.0/en/mysqld-safe.html 

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    therefore,if you want the mysqld_safe feature,i rather recommend you to install MySQL server with binary distribution instead of rpm distribution.

someone said that mysqld_safe will not be supported in the future release,but what i've seen is the version 8.0 official document is that it still be recommended:

 

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 MySQL 8.0 Reference Manual  /  ...  /  mysqld_safe — MySQL Server Startup Script

4.3.2 mysqld_safe — MySQL Server Startup Script

mysqld_safe is the recommended way to start a mysqld server on Unix. mysqld_safe adds some safety features such as restarting the server when an error occurs and logging runtime information to an error log. A description of error logging is given later in this section.

来源： https://dev.mysql.com/doc/refman/8.0/en/mysqld-safe.html

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summary:

    another buddy in zst techique wechat group said that it will be messy in troubleshooting while using mysqld_safe to startup mysqld,'cause in some case,the mysqld_safe may lead to the ceaseless restarting of mysqld.furthermore,it may destroy the evidences and logs which can be diagnosted by DBAs.anyhow,in my opinion it depends:

• if the bussines continuity is the first thing you need to consider,i recommend to use mysqld_safe method.
• if your monitor system is strong enough or the application on the MySQL server is not so important such as enterprise management system,BBS system,i recommend to use mysqld method.