VMware vSphere vCenter Server 4.0 - Manual (Page 10)

Resource Consumers
Virtual machines are resource consumers.
The default resource settings assigned during creation work well for most machines. You can later edit the
virtual machine settings to allocate a share-based percentage of the total CPU and memory of the resource
provider or a guaranteed reservation of CPU and memory. When you power on that virtual machine, the server
checks whether enough unreserved resources are available and allows power on only if there are enough
resources. This process is called admission control.
A resource pool is a logical abstraction for flexible management of resources. Resource pools can be grouped
into hierarchies and used to hierarchically partition available CPU and memory resources. Accordingly,
resource pools can be considered both resource providers and consumers. They provide resources to child
resource pools and virtual machines, but are also resource consumers because they consume their parents’
resources. See Chapter 4, “Managing Resource Pools,” on page 37.
An ESX/ESXi host allocates each virtual machine a portion of the underlying hardware resources based on a
number of factors:
n Total available resources for the ESX/ESXi host (or the cluster).
n Number of virtual machines powered on and resource usage by those virtual machines.
n Overhead required to manage the virtualization.
n Resource limits defined by the user.
Goals of Resource Management
When managing your resources, you should be aware of what your goals are.
In addition to resolving resource overcommitment, resource management can help you accomplish the
following:
n Performance Isolation—prevent virtual machines from monopolizing resources and guarantee
predictable service rates.
n Efficient Utilization—exploit undercommitted resources and overcommit with graceful degradation.
n Easy Administration—control the relative importance of virtual machines, provide flexible dynamic
partitioning, and meet absolute service-level agreements.
Configuring Resource Allocation Settings
When available resource capacity does not meet the demands of the resource consumers (and virtualization
overhead), administrators might need to customize the amount of resources that are allocated to virtual
machines or to the resource pools in which they reside.
Use the resource allocation settings (shares, reservation, and limit) to determine the amount of CPU and
memory resources provided for a virtual machine. In particular, administrators have several options for
allocating resources.
n Reserve the physical resources of the host or cluster.
n Ensure that a certain amount of memory for a virtual machine is provided by the physical memory of the
ESX/ESXi machine.
n Guarantee that a particular virtual machine is always allocated a higher percentage of the physical
resources than other virtual machines.
n Set an upper bound on the resources that can be allocated to a virtual machine.
vSphere Resource Management Guide
10 VMware, Inc.