Troubleshooting MySQL job failed to start

MySQL start fails

$ sudo service mysql start
start: Job failed to start

Troubleshoot

1. Try to find out what is wrong.
    
   • Start mysqld manually in verbose mode
     
     $ sudo mysqld --verbose --user=root
     
     
   • Check dmesg
     
     $ dmesg
   
   
   
2. Mysqld start manually (Step1) without any error, however,  `sudo service mysql start` still fails. Check following settings are proper  in your /etc/mysql/my.cnf; 
   
   
   # The MySQL server
   [mysqld]
   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
   

   
   

   
   
   

MySQL crashes under heavy load on Webserver.

 Simulate heavy traffic

 $ ab -n 1000 -c 100 http://yourdomainname/

         ab : Apache HTTP server benchmarking tool
         -n number of requests
         -c concurrent requests

 General cause

1. Generally mysql crashes because it runs out of memory.
   
   
   • Check innodb settings if your using it. Default innodb_buffer_pool_size is 128M which is very high for low footprint server. Change it to 16M.
     
     
   • Use mysql tunner script.
     
      Download: https://github.com/rackerhacker/MySQLTuner-perl
   
   
2. There could be other reasons also. Execute dmesg or look at mysql logs. Path for MySQL logs are specified in mysql.cnf.