(Carol Sliwa, Features Writer, SearchStorage) Storage performance measurement (SPM) has always been a difficult proposition for IT organizations, and the job has become even more challenging now that virtual servers are so popular.

Take RiskMetrics Group Inc., for example. The New York City-based financial services firm has 30 VMware Inc. ESX Servers spread across six locations, including data centers in the U.S. and Switzerland. Each ESX Server typically runs 10 to 15 virtual machines (VMs).

Many of those virtual machines were formerly physical machines that ran on local disk, so they were of no concern to the storage team. Now the staff needs to not only ensure the VMs perform in the same way they would if they were physical servers, but plan for their potential exponential growth.

"The physical host is easy to analyze, and when a problem surfaces in a physical server environment, it's usually on the host or the storage," said Ed Delgado, storage architect at RiskMetrics. But virtual server environments mean the storage team can't depend only on the performance numbers on the host or the storage. That's because of the amount of other virtual machines on the same datastore.

"Has one VM gone haywire with writes and is it now throttling the other 14 VMs on that datastore? How do you know the other 14 VMs aren't doing the same thing?" wrote Delgado in an email. "On a physical host you can check the read and write MB/sec of a host and trust that number, but on VMware you basically have to add the numbers from the 15 VMs to see how you're actually performing."