Information Technology Reference
In-Depth Information
percent of the resources in the cluster. This cluster is coni gured for a single ESXi host failure
(we'll go into more detail on these settings in a bit). Further, let's say now you want to power
on one more VM, and the resource consumption by that VM will push you past the 75 percent
resource usage mark. It is at this point that the Admission Control settings will come into play.
If Admission Control is set to Enabled, then vSphere HA would block the power-on operation
of this additional VM. Why? Because the cluster is already at the limit of the capacity it could
support if one of the ESXi hosts in the cluster failed (one host out of our four identical hosts is
equal to 25 percent of the cluster's capacity). Because you've told vSphere HA to prevent power-
on operations that violate availability constraints, vSphere HA will prevent you from starting
more VMs than it has resources to protect. In effect, vSphere HA is guaranteeing you that you'll
always have enough resources to restart all the protected VMs in the event of a failure.
If, on the other hand, Admission Control is set to Disabled, then vSphere HA will let you
power on VMs until all of the cluster's resources are allocated. If there is an ESXi host failure at
that point, it's possible that some of the VMs would not be able to be restarted because there are
not sufi cient resources to power on all the VMs. vSphere HA allowed you to exceed the avail-
ability constraints of the cluster.
Overcommitment in a vSphere HA-Enabled Cluster
When the Admission Control setting is set to allow VMs to be powered on even if they violate
availability constraints, you could fi nd yourself in a position where there is more physical memory
allocated to VMs than actually exists.
h is situation, called
overcommitment
, can lead to poor performance on VMs that become forced to
page information from fast RAM out to the slower disk-based swap fi le. Yes, your VMs will start,
but after the host gets maxed out, the whole system and all VMs will slow down dramatically.
h is will increase the amount of time that HA will need to recover the VMs. What should have
been a 20- to 30-minute recovery could end up being an hour or even more. Refer to Chapter 11,
“Managing Resource Allocation,” for more details on resource allocation and how vSphere handles
memory overcommitment.
You should be able to see now how integral the Admission Control Policy settings are to the
behavior of Admission Control. When Admission Control is enabled, the Admission Control
Policy settings control its behavior by determining how many resources need to be reserved and
the limit that the cluster can handle and still be able to tolerate failure.
The Admission Control Policy settings are illustrated in Figure 7.18.
There are three options for the Admission Control Policy:
The i rst option, Dei ne Failover Capacity By Static Number Of Hosts, allows you to specify
how many host failures the cluster should be coni gured to withstand. Because the ESXi
hosts may have different amounts of RAM and/or CPU capacity, and because the VMs in
the cluster may have different levels of resource allocation, vSphere HA uses the idea of a
slot to calculate the capacity of the cluster. This option also gives you the l exibility to spec-
ify the slot size of the cluster. We'll discuss slots in more detail in just a moment.
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The second option, Dei ne Failover Capacity By Reserving A Percentage Of The Cluster
Resources, allows you to specify a percentage of the cluster's total resources that should be
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