Server Farming

Aug 20 2008   1:56PM GMT

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

Bridget Botelho Profile: Bridget Botelho

Active Directory

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’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. VMware Virtualization diagram

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.

3  Comments on this Post

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  • Iansane
    As owner of a small IT repair/network consulting business with no virtualization, and as someone new to the IT field all together, It makes good sense that this would be a strong consideration for large scale hosting and data storage at the enterprise level. It is good to see articles like this. For those of us just getting into the field virtualization is something new that they haven't started teaching in most universities even though it's actually been around for years so it is always good to see what enterprise admins have to say on the subject.
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  • MSubramanian
    I agree with Anne from Focus Consulting......having been involved in rolling out many virtual environments, I can confidently say it is dependent on the application needs not what the vendors claim it to be. The vendors claim is based on some theoretical set of parameters which do not work in the corporate environment. The main advantages of virtualization is the increase in utilization, reduction in space, energy, cooling costs, RAS features. Wish the vendors would highlight these and sell based on some other metric other than the number of VM's. False promises are made to IT management and the operations staff/users struggle with poorly designed VM's which address the promise made by marketing people rather than reality.
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  • VMwareTips
    It is really bad for hardware vendors to make these claims. The amount of virtual machines you're going to be able to populate on a machine is really dependent on the type of application your deploying and its utilization. I have one server with 8 cores @ 2.6GHz and 64GB of RAM and get a consolidation ratio of about 35:1 running small COTS applications and other tasks like IIS/.NET/Sharepoint --- but on the same type box running SAP I only get a consolidation ratio of about 10:1 so--it REALLY depends on what your deploying. For vendors to say YOU'LL GET 35:1 is really bad business...they really should be asking their customers what they're going to deploy, before making assumptions. [A href=""][/A]
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