All of this is implemented through VMware's new SIOC (Storage I/O Control) feature set built into vSphere 4.1. The most important aspect of SIOC is that the hypervisor can now determine the available storage performance resources through the use of an administrator-set congestion threshold. Essentially, if the hypervisor detects high storage latency for a few seconds, it starts to throttle back the I/O the virtual machines are allowed to produce. Though high-load performance characteristics vary from one storage platform to another, once a certain latency threshold (usually 25ms to 35ms) has been crossed, overall array performance starts to decrease rapidly in most SAN environments. Restricting the I/O sent before that threshold prevents storage resources from becoming saturated -- which in itself is a great capability.
Across-the-board slowdowns, however, have obvious drawbacks, especially if you can't control which of the potentially thousands of virtual machines are allowed to use what resources. SIOC offers a solution by providing the ability to set I/O performance priorities down to the level of individual virtual disks. This means that when storage congestion occurs, lower-priority virtual machines can be given a smaller piece of available performance than higher-priority machines -- allowing mission-critical services to continue relatively unaffected while background processes feel the pinch. Coupled with SIOC are a new set of detailed storage performance graphs that make it easy to pinpoint high storage users and observe the effects of set storage rules.
From a storage administrator's perspective, the value of having this kind of capability is difficult to overstate. Being able to keep your virtualization environment from saturating your SAN while also controlling I/O on a per-machine or even per-disk basis is an amazing capability that's largely unparalleled even in the physical world. In short, VMware just transformed virtualization from being one of the most difficult storage performance consumers to manage to one of the easiest -- no small trick.
This article, "VMware vSphere raises the bar -- again," originally appeared at InfoWorld.com. Read more of Matt Prigge's Information Overload blog and follow the latest developments in data storage and information management at InfoWorld.com.