The MySQL Server can create a number of different log files, which make it easy to see what is going on. See Log Files. However, you must clean up these files regularly, to ensure that the logs don't take up too much disk space.
When using MySQL with log files, you will want to remove/backup old log files from time to time and tell MySQL to start logging to new files. See Backup.
On a Linux (Red Hat) installation, you can use the mysql-log-rotate script for this. If you installed MySQL from an RPM distribution, the script should have been installed automatically. Note that you should be careful with this script if you are using the binary log for replication!
On other systems you must install a short script yourself that you start from cron to handle log files.
You can force MySQL to start using new log files by using mysqladmin flush-logs or by using the SQL command FLUSH LOGS. If you are using MySQL Version 3.21, you must use mysqladmin refresh.
The above command does the following:
If standard logging (--log) or slow query logging (--log-slow-queries) is used, closes and reopens the log file (mysql.log and `hostname`-slow.log as default).
If update logging (--log-update) is used, closes the update log and opens a new log file with a higher sequence number.
If you are using only an update log, you only have to flush the logs and then move away the old update log files to a backup. If you are using the normal logging, you can do something like:
shell> cd mysql-data-directory shell> mv mysql.log mysql.old shell> mysqladmin flush-logs
and then take a backup and remove mysql.old.