It is important to have a basic understanding of how to check out your Linux server's performance to help diagnose / troubleshoot various issues. Below is some useful tools and reference materials I have found helpful.

iostat - disk utilization

This article is a good simple reference on using IOSTAT. http://www.cyberciti.biz/tips/linux-disk-performance-monitoring-howto.html

top - cpu utilization

"The top program provides a dynamic real-time view of a running system. It can display system summary information as well as a list of tasks currently being managed by the Linux kernel. http://www.cyberciti.biz/tips/how-do-i-find-out-linux-cpu-utilization.html

ifstat - network interface utilization

Extremely handy tool to see if you are saturating your network interfaces. eg, in an NAS environment this could be very useful. http://freecode.com/projects/ifstat

20 Linux System Monitoring Tools Every SysAdmin Should Know

http://www.cyberciti.biz/tips/top-linux-monitoring-tools.html