You can never have enough memory, says virtualization report

Forget about CPU power; memory is the biggest hold-up for those implementing virtual machines.

A new survey indicates that the biggest issue for virtualization deployment is insufficient memory installed in servers and not, as is commonly believed, CPU performance.

The survey found that, on average, administrators allocate 70 percent of a server's RAM to virtual machines (VMs), leaving 30 percent in reserve. But once memory is exhausted, no more VMs can be installed on that server, regardless of remaining CPU or storage capacity, and the survey suggests this leads to servers being inefficiently utilized.

The lack of memory boils down to cost: Higher density modules are required to boost a machine's capacity to the levels required for virtualization (into the double-digit GB range), due to a limited number of memory sockets. But these are usually very expensive. Investing in faster or multiple processing cores, which are comparatively cheaper, is often mistakenly seen as the best way of adding capacity.

The survey was conducted by systems management vendor VKernel and was drawn from 2500 virtualized environments representing over 550,000 virtualized machines. The survey was limited to environments containing more than 50 virtual machines, so modest VM setups of just a handful of physical servers are not represented. However, the messages drawn from the data still apply.

The survey also found that, on average, each physical machine hosts 15.6 VMs, is dual-socket with quad-core processors, and contains 50GB of memory alongside 2TB of storage.

The chief benefit of virtualization is one of cost. Whereas previously each operating system required its own physical computer, virtualization allows many instances of operating systems to be installed on one physical computer. All can run simultaneously.

For example, one VM could run a company's mail server, while another could run a Web server. Yet another could provide a VPN gateway. Administrating multiple VMs is also easier than a similar setup involving multiple physical machines.

Interestingly, larger outfits with more physical machines tend to be more relaxed about capacity. Sites with 24 or more physical hosts have on average 11 VMs per physical machine, while sites with 10 or fewer machines almost double this to 20 VMs. In other words, businesses with fewer servers tend to wring more value out of them.

Some of the report's findings might be moot because the physical memory (DRAM) market has seen price falls lately that favor high-powered virtualization environments. In November, memory market researchers DRAMeXchange reported that improvements in yields via new technology combined with falling consumer demand meant that DRAM prices have dropped significantly. It quoted the example of a 2GB DDR3 module that started this year priced at $46.50, but which can be had for just $20 now.

Additionally, an industry report by Deutsche Bank suggests that memory prices are unlikely to rise again until the second half of 2011. In fact, the report says, they may fall even lower in the short term.

In other words, there's never been a better time to invest in efficient virtualization computing, but beware that the DRAM market tends to go up and down like a yo-yo, so this time next year it might be a difficult story.

Keir Thomas has been writing about computing since the last century, and more recently has written several best-selling books. You can learn more about him at and his Twitter feed is @keirthomas.

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Keir Thomas

PC World (US online)
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