6.7

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, memory, or storage I/O 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 are unused resources on the system. The limit is expressed in concrete units
(megahertz, megabytes, or I/O operations per second).
CPU, memory, and storage I/O resource limits 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, it is not necessary to specify a limit. There are benefits and drawbacks:
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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.
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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 (reservation, limit and shares) that are appropriate for your ESXi
environment.
The following guidelines can help you achieve better performance for your virtual machines.
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Use Reservation to specify the minimum acceptable amount of CPU or memory, not the amount you
want to have available. 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. The host
assigns additional resources as available based on the limit for your virtual machine, the number of
shares and estimated demand.
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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.
vSphere Resource Management
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