6.0.1

Figure 2‑1. Admission Control Example with Host Failures Cluster Tolerates Policy
6 slots remaining
if H1 fails
slot size
2GHz, 2GB
2GHz
1GB
2GHz
1GB
1GHz
2GB
1GHz
1GB
1GHz
1GB
VM1
9GHz
9GB
4 slots
H1
9GHz
6GB
3 slots
H2
6GHz
6GB
3 slots
H3
VM2 VM3 VM4 VM5
1 Slot size is calculated by comparing both the CPU and memory requirements of the virtual machines
and selecting the largest.
The largest CPU requirement (shared by VM1 and VM2) is 2GHz, while the largest memory
requirement (for VM3) is 2GB. Based on this, the slot size is 2GHz CPU and 2GB memory.
2 Maximum number of slots that each host can support is determined.
H1 can support four slots. H2 can support three slots (which is the smaller of 9GHz/2GHz and
6GB/2GB) and H3 can also support three slots.
3 Current Failover Capacity is computed.
The largest host is H1 and if it fails, six slots remain in the cluster, which is sufficient for all five of the
powered-on virtual machines. If both H1 and H2 fail, only three slots remain, which is insufficient.
Therefore, the Current Failover Capacity is one.
The cluster has one available slot (the six slots on H2 and H3 minus the five used slots).
Percentage of Cluster Resources Reserved Admission Control Policy
You can configure vSphere HA to perform admission control by reserving a specific percentage of cluster
CPU and memory resources for recovery from host failures.
With the Percentage of Cluster Resources Reserved admission control policy, vSphere HA ensures that a
specified percentage of aggregate CPU and memory resources are reserved for failover.
With the Cluster Resources Reserved policy, vSphere HA enforces admission control as follows:
1 Calculates the total resource requirements for all powered-on virtual machines in the cluster.
2 Calculates the total host resources available for virtual machines.
3 Calculates the Current CPU Failover Capacity and Current Memory Failover Capacity for the cluster.
4 Determines if either the Current CPU Failover Capacity or Current Memory Failover Capacity is less
than the corresponding Configured Failover Capacity (provided by the user).
If so, admission control disallows the operation.
vSphere Availability
26 VMware, Inc.