From the Resource Management Guide:
"HA plans for a worst‐case failure scenario. When computing required failover capacity, HA calculates the maximum memory and CPU reservations needed for any currently powered on virtual machine and calls this a slot.... HA determines how many slots can “fit” into each host based on the host’s CPU and memory capacity. HA then determines how many hosts could fail with the cluster still having at least as many slots as powered on virtual machines. This number is the current failover level."
What this means is, if I have one really powerful VM on my cluster (say 4vCPU and 4GB RAM reservation), VC will calculate ALL of my VMs as being this powerful when determining failover capacity, even if all of the other VMs are 1vCPU and have a low memory reservation. Since the calculated failover capacity will obviously be much lower than what I can actually run on, I opted for a workaround.
1) Set Admission Control to allow VMs to power on even if they violate availability constraints.
2) Create a non-expandable resource pool with unlimited access to resources. Set the reservations for CPU and memory to be the sum of all of your remaining hosts should your highest resource machine die. Take the following example:
ESX1 - 2 x 2.4GHz, 24GB RAM
ESX2 - 2 x 2.4GHz, 32GB RAM
ESX3 - 4 x 2.4GHz, 24GB RAM
My resource pool reservation would be set to 4 x 2.4GHz (my two lowest CPU producers) and 48GB RAM (my two lowest memory producers). Make sure to adjust these numbers down to allow overhead for the host.
3) Make sure to add all VM's to this resource pool and keep your reservations up to date.
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