Even if no reservations are used, admission control helps you to survive in disaster by reserving enough resources at least for overhead that each vm produces on host.
Let's take an extreme example scenario:
You have a cluster of 2 hosts, both have 70% avg active memory load (you may not have such a load now, but it tends to grow and you are going to reach it sooner or later).
One host fails and HA with disabled admission control tries to restart VMs on survivers island thus producing 140% ACTIVE (and most probably around 280% of total mem overcommit) memory load on remaining host. What will happen most probably is you are going to loose all your VMs (including ones on survived host) because of performance degradation.
Admisison control helps you to minimize these risks by sacrificing (reserving) resources and preparing for worst case scenario. You always have a choice to disable it, thus transfering the risks to your shoulders or other body parts :smileygrin:
It's a trade off you have to make - either having underutilized hosts with reserved capacity for failover, or degraded performance in event of failover, or only some VMs restarting, or disabling HA at all.
WBR
Imants
