Hi,
ESX host will start ballooning activity if its left with 2% free memory. ESX will try to move Physical RAM pages to VM's virtual disk until it has recommended free memory (not sure about correct value). The above scenario is without Reservation and DRS/HA. If your environement has DRS/HA enabled, it will validate available resources every time when you power on a VM based on HA failover capacity configured in ESX Cluster.
