Virtual machines per server: A viable metric for hardware selection?

When server vendors introduce new blade servers these days, they often mention virtualization in the same breath, often touting the number of virtual machines (VMs) their hardware can support. But those numbers are hardly the result of scientific method.

For instance, San Diego, Calif.-based Verari Systems recently announced that its VMware ESX 3.5-certified VB1257 for BladeRack 2 XL supports up to twice as many VMs as competitive offerings (16). After speaking with Verari, I asked the competition — Sun Microsystems, Hewlett-Packard and Dell — how many VMs their blades can hypothetically support, and was given some big numbers.

But are these server vendors asking the right question? According to Anne Skamarock, a research director at Focus Consulting, the answer is no. Although vendors boast about the number of VMs their hardware supports, “it really is a silly way to look at it,” she said.

“The number of VMs supported depends on the workload. For CPU-intensive workloads, memory will also be a significant factor in performance,” Skamarock said. ”I have spoken with customers who are running 30 VMs per 8-core system and expect to increase that to 50 VMs per system.”

Skamarock said Virtual Desktop Infrastructure adds another twist. “The rule of thumb is six to eight virtual desktops per core, but again, memory will be a big issue here depending on the OS.”

According to preliminary data from SearchDataCenter.com’s 2008 Purchasing Intentions Survey, 61% of the respondents run less than 10 VMs per server, though 33% run 10 to 25, and a mere 5% run more than 25 VMs on a server.

Vendors make big VM support claims

According to VMware Inc.’s website, server consolidation ratios commonly exceed 10 virtual machines per physical processor; so presumably, a blade server with two CPUs, like Verari’s VB1257, should be able to support at least 20 VMs.

Within HP’s ProLiant blade server line, the ProLiant BL460c/465c and BL680c/BL685c would be a good choice for a virtual server platform, primarily because they offer a large memory footprint, which means more than 16 VMs per blade in both cases, plus more network expansion and storage performance, HP spokesman Eric Krueger said.

“Keep in mind of course the number of VMs always vary – the number could be higher or lower depending on the needs the application/VM — but based on the rule of thumb … the BL460c can support up to 16 VMs and the BL680c up to 32,” Krueger said.

Sun Microsystems Inc. claims its Sun Blade servers pack two and three times that many VMs. The Sun Blade X6250, which has up to eight cores with Intel Xeon processors, 64 GB RAM, 110 Gbps I/O and 800GB of internal storage, supports 36 VMs; the Sun Blade X6450, with two or four dual-core or quad-core Intel Xeon processors and up to 96 GB of memory, can support up to 42 VMs and the Sun Blade X8450 with 16 cores per module and 128 GB Memory, supports up to 48 VMs, according to Sun.

Dell was hesitant to name a number of VMs that its PowerEdge blade servers can support, because the number is dependent on a number of factors, like workload, memory, I/O. A spokesperson did say that “Dell has blades that support up to 66 loaded VMs. This is based on VMware’s VMmark benchmark test,” a spokesperson said. “This is an area where we are doing quite a bit of work, so stay tuned.”

So I’m wondering: Are VM support numbers a consideration when buying server hardware, or is it too subjective? Let us know what you think.