We have a live webserver which doesn't seem to be performing all that well. It's a Dell PowerEdge machine, a few years old (dual core, 4GB) which is hosting about 20 low-traffic websites. However it doesn't seem to be as fast as it used to be. How can we determine the cause of this? If it's website traffic, I would be expecting high CPU but CPU usage is quite low and hovers around the 15-30% mark except for very brief periods.

I'm wondering perhaps, if rather than CPU performance being a problem, perhaps it's disk thrashing due to the constant read/writes of all the small web files and database queries. It has 4x 7200 RPM SATA drives in RAID 5.

So is there a way to check that it's not disk thrashing?