ubuntu服务器的正常磁盘读/写值

How would your determine a max value to set an alarm on the I/O activity of a ubuntu/linux server hosting upto 4 sites running apache, mysql and upto 4 tomcats? Or in general (if there is one such) what is a suitable operational range for I/O read/write per second in a production server with a 7.2K SATA HDD?How would your determine a max value to set an