Magento, in earlier versions, does not seem to handle log cleaning very well. A lot of configuration options are vailable under the configuration / Advanced / System menu, that seem to have an impact on the cron.php process. But trying to remove logs this way is a complicated task that involves full log counting (select count on mage_log_visitor) and the result is not always there, don't know why. A full truncate on log tables seems more appropriate, as described on the knowledge base, to use with caution obviously : Magento Maintenance Script was posted a few days ago but I tried it and it's pretty efficient. Let us know how it performed on your install, on mine it works well. Go ahead after you've performed the appropriate backups !