I'm planning to have an offsite DR server and would like to host about 12 VMs on it. There will be 3 separate machines with SQL server instances that will, on average, be lightly loaded, in addition to an Exchange 2K3 environment with 120 mailboxes and AD servers, and a Windows 2K3 Terminal Services box for about 10 concurrent users doing office productivity (Office 2K3 or 2K7). I was wondering what the community thinks about running that environment on a Dell R905 with 4 x quad core 2GHz processors and about 500GB of RAID 5 storage with a hot spare. Any suggestions/advice would be appreciated. Thanks.
If they truly are lightly loaded I woul say that should be ok as long as you have enough memory - the one resource I would pay close attention to is disk bandwidth -
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That should be sufficient memory. It's better to over-allocate memory anyway so that no single VM gets short on memory and start slowing down too much. This forces all other VMs that interact with it to wait. Basically the slowest machine dictates the overall speed of your entire system. I set all my 32-bit machines to 4gb and 64-bit ones to 8-16gb, then let ESX pool and share memory as needed. I've got one host right now in similar configuration as you with 8gb without any problems.
Other areas such as disk-speed and network-bandwidth becomes a concern depending upon the usage-pattern. Exchange is very disk-intensive. But with 10-mailboxes, you shouldn't have to worry about that too much.