Most ESXi installations today come complete with a corresponding license for the standard edition of vCenter Operations Management for vSphere. If you have vSphere, are under SnS (software maintenance), or recently just bought it--install vCOps today!
Bound by Disk I/O
Wow! That sure was easy. Now, that doesn't mean disk I/O is actually a problem, it just means it is the most constrained resource. So if I have a server or VM that is giving me performance problems, a quick glance here can provide you a great place to begin your troubleshooting.