The MyISAM log records all changes to MyISAM tables. It is not enabled by default. To enable it, start the server with the --log-isam option, for example:

--log-isam=myisam.log

The isam instead of myisam above is not a typo - it's a legacy from when the predecessor to the MyISAM format, called ISAM. The option can be used without specifying a filename, in which case the default, myisam.log is used.

To process the contents of the log file, use the myisamlog utility.