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Can you help me with my capacity planning?

My query is regarding how to specify a physical host, to serve virtual machines, and what 'rules of thumb' you would use regarding how many virtual instances could reside on a physical host. I appreciate that this can vary greatly depending upon the server instance, and the workload, and these are easy to understand in isolation, but this changes once you start putting several VMs on a box, they compete for resources, and you need to think about the overall performance of the server, rather than the individual VM.

I've read on another site as rough guide to have roughly 2-4GB RAM per core, 2 cores per VM. So if you had 8 Cores, 32GB RAM you'd be looking at running 4 WS2008R2 full installs comfortably. Disk IOPs important too, so should you be looking at a nice fast RAID array? Finally presumably you'd want two NICs minimum?

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marked as duplicate by TomTom, Sven, Khaled, sysadmin1138 Jan 17 '13 at 13:14

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My query is regarding how to specify a physical host,

?? WHat you mean. Ever seen a computer price list. Depends.

and what 'rules of thumb' you would use regarding how many virtual instances could reside on physical host.

None. How many people can you seat into a room?

Generally I put in a lot of memory unless the VM's will be processor intensive, and generalöly I let memory drive the amount of VM's. Assuming a heavy high performance disc subsystem or SAN behind it.

I've read on another site as rough guide to have roughly 2-4GB RAM per core, 2 cores per > VM

Bull, like all those numbes. I have VM's wih 16gb on 2 cores, 32-48gb using a whole computer processing power and 512mb 1 core small machines (routers, small domain controllers for mini domains).

Generally these days:

  • Start a VM with 1gb memory, scaling down to 512mb. 1GB Makes sure it starts fast.
  • Assign 2 cores. No sense to give it only 1, even if it does nothing - comes in handy during patching etc.

And thT IS IT.

So if you had 8 Cores, 32GB RAM you'd be looking at running 4 WS2008R2 full installs comfortably.

First, that is a crappy server. SHould be 64gb at least to use up the cores unless you run special load. And that really depends. THere really is no number. Not without at least having an idea what the servers do.

THAT SAID: If that is a separate host, lookat at least 6-8 discs to use up 8- virtual machines. At least during patch day you get IO hell. THis is why a central SAN really shines.

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