Vmware DRS - over exaggerated claims

Yesterday VMware issued a press release called "VMware vSphere^TM Provides Nearly 50 Percent Application Performance Improvement With VMware Distributed Resource Scheduler (DRS)

VMware DRS Enables Optimal Performance of Virtual Environments, Enabling Higher Consolidation Ratios and Lower Cost per Application

While I agree that load balancing in a virtual datacenter is a must have, I do find some of the claims in this press release a bit over exaggerated. The press release states that "These tests demonstrate how VMware DRS optimizes efficiency while providing guaranteed levels of performance. ". Guatanteed levels of performance? Vmware ESX platform does not provide a guaranteed level of performance. DRS will do its best to balance the worloads by moving VMs from one resource constrained host to a less constarained ESX host if one exists, but if there are no resources available, performance will quickly degrade.

Futhermore DRS only considers memory and cpu resources. In a typical Vmware ESX environmrent the most common resource constraint occures not in memory nor CPU. The most common bottlenecks occures in Storage I/O. As more VMs are added to ESX data center, I/O performed by additional VMs causes problems because all VMs roughly speaking read and write to the same disk. You may say that why would people put all or most VMs on the same disk or more specifcally common storage. The reason is VMotion. In order to take advanatge of vmotion, VMs have to be on common storage.

The press releases then goes on to say "DRS has eliminated the need for administrators to monitor CPU and memory for bottlenecks". Wow. I wish this was the case, but the reality is that DRS is not a substiture for capacity monitoring and capacity planning. Capacity bottlenecks can occur in memory and cpu at level ie. in VM in a host and in cluster or resource pool. what happens when you max out memory or cpu resources in a cluster created by multipe hosts. DRS moving VMs from one host to another will not ease the capacity over load if memory resources are maxed out.

We all love VMware ESX, but I think it is important for our customers to know the truth