MySQL Troubleshooting

Errors and Logs

Another important troubleshooting technique sounds simple: read and analyze information from the server. This is a very important step. In Chapter 1, I discussed tools that can help you get and analyze information, along with examples. Here I want to add details I skipped before.

Error Information, Again

Error messages are key and should never be ignored. You can find information about errors in the MySQL Reference Manual at http://dev.mysql.com/doc/refman/5.5/en/error-handling.html. This page lists client and server error messages, but omits messages specific to a storage engine. Nor does it explain errors that come from the operating system. Strings of information describing operating system errors can be derived through the perror utility (see Retrieving Error Strings Through perror).

Another very important tool is the mysqld error logfile, which contains information about table corruption, server crashes, replication errors, and much more. Always have it turned on, and analyze it when you encounter a problem. A log from an application cannot always replace the MySQL server error log, because the latter can contain problems and details not visible to the application.

Crashes

I discussed crashes and a general troubleshooting sequence applicable to them in When the Server Does Not Answer. Start by using the techniques described in the previous section: look in the error logfile, and analyze its content. This works in most cases, but this section discusses what to do ...