VMware vSphere vCenter Server 4.0 - Manual (Page 12)

For example, assume you have 2GHz available and specify a reservation of 1GHz for VM1 and 1GHz for VM2.
Now each virtual machine is guaranteed to get 1GHz if it needs it. However, if VM1 is using only 500MHz,
VM2 can use 1.5GHz.
Reservation defaults to 0. You can specify a reservation if you need to guarantee that the minimum required
amounts of CPU or memory are always available for the virtual machine.
Resource Allocation Limit
Limit specifies an upper bound for CPU or memory resources that can be allocated to a virtual machine.
A server can allocate more than the reservation to a virtual machine, but never allocates more than the limit,
even if there is unutilized CPU or memory on the system. The limit is expressed in concrete units (megahertz
or megabytes).
CPU and memory limit default to unlimited. When the memory limit is unlimited, the amount of memory
configured for the virtual machine when it was created becomes its effective limit in most cases.
In most cases, it is not necessary to specify a limit. There are benefits and drawbacks:
n Benefits — Assigning a limit is useful if you start with a small number of virtual machines and want to
manage user expectations. Performance deteriorates as you add more virtual machines. You can simulate
having fewer resources available by specifying a limit.
n Drawbacks — You might waste idle resources if you specify a limit. The system does not allow virtual
machines to use more resources than the limit, even when the system is underutilized and idle resources
are available. Specify the limit only if you have good reasons for doing so.
Resource Allocation Settings Suggestions
Select resource allocation settings (shares, reservation, and limit) that are appropriate for your ESX/ESXi
environment.
The following guidelines can help you achieve better performance for your virtual machines.
n If you expect frequent changes to the total available resources, use Shares to allocate resources fairly across
virtual machines. If you use Shares, and you upgrade the host, for example, each virtual machine stays
at the same priority (keeps the same number of shares) even though each share represents a larger amount
of memory or CPU.
n Use Reservation to specify the minimum acceptable amount of CPU or memory, not the amount you want
to have available. The host assigns additional resources as available based on the number of shares,
estimated demand, and the limit for your virtual machine. The amount of concrete resources represented
by a reservation does not change when you change the environment, such as by adding or removing
virtual machines.
n When specifying the reservations for virtual machines, do not commit all resources (plan to leave at least
10% unreserved.) As you move closer to fully reserving all capacity in the system, it becomes increasingly
difficult to make changes to reservations and to the resource pool hierarchy without violating admission
control. In a DRS-enabled cluster, reservations that fully commit the capacity of the cluster or of individual
hosts in the cluster can prevent DRS from migrating virtual machines between hosts.
Changing Resource Allocation Settings—Example
The following example illustrates how you can change resource allocation settings to improve virtual machine
performance.
Assume that on an ESX/ESXi host, you have created two new virtual machines—one each for your QA (VM-
QA) and Marketing (VM-Marketing) departments.
vSphere Resource Management Guide
12 VMware, Inc.