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### how to calculate physical servers needed for an openstack project

Hello I would like to try to calculate the number of physical servers needed to host 100 Virtual machines of 4GB RAM each a quad-core at 2.2Ghz and 40GB of HDD. Let's say I have a 2 types of servers I can have either

2x AMD Opteron 6272, 32 Cores (2x16)
64 GB DDR3 RAM ECC
2x 2,000 GB SATA II HDD


or

2x Intel Xeon E5-2420, 12 Cores (2x6)
32 GB DDR3 RAM ECC
2x 2,000 GB SATA II HDD


what is the formula to come up with the total physical servers needed for the project.

I have a little doubt in my understand of how the whole system would work out so in a system like mine where there are physical servers available to host all of the 100, how are backups of the instances catered for?

Thanks for helping