We are looking at hardware for our new ESX farm and are wondering if we should go dual-core quad-processor systems, or quad-core dual-processor systems. Essentially we will have the same number of cores in either case, but the dual processor servers are much cheaper. Thoughts?
Ahh, multiple cores, a great thing to happen in virtualization. The reality with the multi-core systems (for sure in the Intel space) is that you are right now trading clock speed for cores. Most quad core systems with a reasonable price are 1.6 or 1.8 GHz, while you can get dual core systems at 3.0 GHz all day long for cheap. Your question comes down to two answers. 1- What is best for the virtualization systems, speed or cores? And what hits the pocket book the least (i.e. gives you a better return on investment (ROI)/lowest cost per VM)?
What is better for the virtualization system (in my opinion) is cores vs. speed. Comparing quad processors to dual processors is another argument. But we'll get there in a minute. The scheduling mechanism in use today allows for only a single VM (virtual machine) to access a core at any given time. Essentially VMs are treated like processes and the host schedules them according to priority. This means that the more cores you have the more VMs you can have simultaneously executing on a processor/core. Trading clock speed on the processor for more cores is OK in my book. The reason being is that most (90%+) of the x86 servers out there do nothing with the processor. Sure there are spikes, but sustained utilization above 1 or 2 GHz is very rare as a percentage of the environment. Get more cores when possible.
On the money/ROI side, multi-core systems are the best thing to happen to virtualization hosts since… well, since VMware hit the market hard. Essentially you can now get four and eight cores in a dual processor chassis. If you use standard industry consolidation ratios on multiple core systems and get three VMs per core, you have a 12:1 ratio on four cores and a 24:1 ratio for eight. The question then becomes about cost of memory and memory expandability. Can the DP system handle enough memory to support all your VMs, or do you wind up with an eight core system that due to memory limits only handles 12 VMs and the processor remains unused? Best bet: Compare like to like. If comparing the quad to dual, assume the quad will host MORE VMs and therefore you need to add (really double) the memory you need. Run the costs down, and use a standard like 3:1 per core to figure out your cost per VM in different configurations. The cheapest one that provides the same levels of hardware redundancy should win.
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