MySQL cannot start error! MySQL is running but PID file could not being found

After a spring festival, the server has been running, today check found that Tomcat posted on the site static page can be accessed, but need to access the database related pages are inaccessible;
I thought Tomcat was out of the question, so I restarted the Tomcat and the result was the same after the reboot. Through the vi to view the log file prompts error write errors, technology can not also confuse the situation, simply restart the server it!
Results The server has been stopped in the position of progress bar 100%, not into the login interface, it feels strange, after pressing the ESC key to see the reason, starting Mysql ..... ....... ..... Always this way, so through Xshell remote connection, log in and check the status of MySQL:
Execute command: Service MySQL status
Tip: error! MySQL is running but PID file could not being found
Search, some people say that the hard disk is full, clean up the junk file.
This came to understand, just use VI to view the log file to indicate the cause of the error.
Execute command at root/bottom: Df-h
The output is as follows:

[root @localhost /] # df-h  Filesystem Size used Avail use% mounted on  **/dev/mapper/volgroup-lv_root 50G 50G 0G 100%/ **tmpfs 1. 9  g 0  1.9  G 0 %/dev/shm/dev/sda1 485  M 39  M 421  M 9 %/boot/dev/mapper/volgroup-lv_home 534  G Span class= "Hljs-number" >824  M 506  G 1 %/ Home[root @localhost /]#  

/dev/mapper/volgroup-lv_root use has been 100%, a little space has not.
Then use the du-m–max-depth=1 command to see the space usage of each directory. In fact, some of the logs produced by Tomcat take up a lot of disk space and delete these log files (back up the log if necessary). After the disk space is released, check the status of MySQL to start successfully. Restart Tomcat and the relevant sites will be accessible.

MySQL cannot start error! MySQL is running but PID file could not being found