Does Admission Control have use cases?

[Nick_Andreev]

Hi guys, I've been reading about Admission Control for a bit and here is what I understand so far:

1. For "Host failures the cluster tolerates" and "Specify failover hosts" Admission Control uses slot based calculations. Which basically means that the moment you introduce a one VM with reservations, the number of slots in a cluster is significantly reduced, which causes a lot of problems to clients who all of a sudden cannot start VMs and do not know why.

2. For "Percentage of cluster resources reserved as failover spare capacity" Admission Control calculates amount of available CPU and Memory Failover Capacity based on memory overhead and reservations. If you don't have any VMs with reservations then for each running VM with an average config of 2 vCPUs and 4GB of RAM it will subtract roughly 50MB of RAM.

Lets take as an example a cluster of 5 hosts with 200GB of memory each and 20% of memory and CPU reserved for failover. In total we have 1000GB of RAM and say 200 running VMs with the above specs. Then we'll end up having roughly 200VMs x 50MB = 10GB of memory overhead, which translates into (1000GB-10GB)/1000GB * 100 = 99% of resources available for Admission Control. Whereas in fact we have only (200VMs x 4GB)/1000GB*100 = 20% available (lets not take into account granted/consumed amount of memory for the sake of this example).

To actually hit the point where Admission Control would kick in we need 800GB / 50MB = 16384 VMs. Which in it's turn is 16384 VMs x 4GB = 65536GB of actual memory granted to VMs on a 1000GB memory capacity cluster.

Am I missing something here? Because I'm really struggling to make any sense out of it.