Back when I had ESX servers that were memory constrained there where times when I could not start a VM because there was not enough free memory. From what I understood this was due to the fact that ESX had to reserve the full memory size of the VM on boot until the OS was loaded. I was doing some testing with the VirtualCenter appliance on my ESXi 5.1 server that has 4G of RAM. The appliance needed 8G of RAM to work so I set it for 8 assuming it would not start. I was very surprised when it successfully started and even more surprised when it reported 7G of active RAM. Has something changed in the guest boot RAM allocation process that allows machines to start even when there is not enough available RAM?
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