Admission control can be set to tolerate a maximum of N-1 number of host failures in a cluster.
With the Cluster resource percentage option, a computed or user defined failover capacity is reserved in the cluster. Any operation violating this constraint is disallowed. This is enforced as follows:
- Computes the total resource requirements for all powered-on virtual machines in the cluster.
- In parallel, it calculates the total host resources available for virtual machines.
- Calculates the Current CPU Failover Capacity and Current Memory Failover Capacity for the cluster.
If either the Current CPU Failover Capacity or Current Memory Failover Capacity is less than the corresponding configured failover capacity then the operation is disallowed.
vSphere HA by default uses the actual reservations of the virtual machines. If a virtual machine does not have reservations, a default of 0 MB memory and 32 MHz CPU is applied.
With the Slot capacity option, a placeholder slot is derived at. Each slot acts as a placeholder for a virtual machine, that is, a slot can satisfy resource requirements of any given powered-on virtual machine in the cluster:
- Compute Slot capacity.
- Determines how many slots each host in the cluster can hold.
- Determines the current failover capacity of the cluster.
- This is the number of hosts that can fail and still leave enough slots to satisfy all of the powered-on virtual machines.
- Determines whether the current failover capacity is less than the configured failover capacity (user-defined). If it is, Admission Control disallows the operation.
With the Dedicated failover hosts option, specific targeted hosts are intended for restarting the virtual machines. These hosts play the role of spares. You can neither vMotion nor power-on virtual machines to the dedicated Failover Hosts:
