Information Technology Reference
In-Depth Information
used for spare capacity in the event of a failure. You can specify different percentages for
CPU and memory. The availability constraints are established by simply calculating the
specii ed percentage of the cluster's total available resources.
◆
The third option, Use Dedicated Failover Hosts, allows you to specify one or more ESXi
hosts as failover hosts. These hosts are used as spare capacity, and in the event of a failure,
vSphere HA will use these hosts to restart VMs.
Figure 7.18
h e Admission
Control Policy
settings will deter-
mine how a vSphere
HA-enabled cluster
determines avail-
ability constraints.
Be Careful about Using Failover Hosts
When you select an ESXi host as a vSphere HA failover host, it's almost like putting that host into
Maintenance mode. vSphere DRS, which we'll describe in more detail in Chapter 12, “Balancing
Resource Utilization,” won't place VMs here at startup and won't consider these hosts in its load-
balancing calculations. You can't manually power on VMs on the failover host(s) either. h ese hosts
are truly set aside as spare capacity.
For the most part, the Admission Control Policy settings are pretty easy to understand. One
area that can be confusing, however, involve slots and slot sizes, which are used by vSphere HA
when Admission Control Policy is set to failover capacity by a static number of hosts.
Why slots and slot sizes? vSphere HA uses slots and slot sizes because the ESXi hosts in the
cluster might have different coni gurations: One host might have 8 CPU cores and 24 GB of
RAM, while another host might have 12 CPU cores and 48 GB of RAM. Similarly, the VMs in the
cluster are likely to have different resource coni gurations. One VM might need 4 GB of RAM,
but another VM might require 8 GB of RAM. Some VMs will have 1 vCPU and other VMs will
