Information Technology Reference
In-Depth Information
satisfy a reservation in order to power on a VM. Therefore, you cannot reserve more CPU
cycles than the host is actually capable of delivering.
Limits on CPU usage simply prevent a VM from gaining access to additional CPU cycles
even if CPU cycles are available to use. Even if the host has plenty of CPU processing power
available to use, a VM with a CPU limit will not be permitted to use more CPU cycles than
specii ed in the limit. Depending on the guest OS and the applications, this might or might
not have an adverse effect on performance.
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Shares are used to determine CPU allocation when the ESXi host is experiencing CPU
contention. Like memory, shares grant CPU access on a percentage basis calculated on the
number of shares granted out of the total number of shares assigned. This means that the
percentage of CPU cycles granted to a VM based on its Shares value is always relative to
the number of other VMs and the total number of shares granted, and it is not an absolute
value.
As you can see, there are some key differences as well as a number of similarities between
how these mechanisms work for memory when compared to how they work for CPU.
So far we've discussed two of the four major resource types (memory and CPU). Before
we can move on to the third resource type—networking—we need to discuss the concept of
resource pools.
Using Resource Pools
The settings for VM resource allocation (memory and CPU reservations, limits, and shares) are
methods used to modify or control the allocation of resources to individual VMs or to modify
the priority of VMs seeking access to resources. In much the same way as you assign users to
groups and then assign permissions to the groups, you can leverage resource pools to make
allocating resources to collections of VMs a less tedious and more effective process. In other
words, instead of coni guring reservations, limits, or shares on a per-VM basis, you can use a
resource pool to set those values on a group of VMs all at once.
A
resource
pool
is a special type of container object, much like a folder, in the Hosts And
Clusters view. You can create a resource pool on a stand-alone host or as a management object in
a DRS-enabled cluster. Figure 11.9 shows the creation of a resource pool.
If you examine the properties of the resource pool, you'll see two sections: one for CPU set-
tings (Reservation, Limit, and Shares) and another section with similar settings for memory.
When you apply resource settings to a resource pool, those settings affect all the VMs found
within that resource pool. This provides a scalable way to adjust the resource settings for groups
of VMs. Setting CPU and memory shares, reservations, and limits on a resource pool is very
much like setting these values on individual VMs. The behavior of these values, however, can be
quite different on a resource pool than on an individual VM.
To illustrate how to set shares, reservations, and limits on a resource pool, as well as to
explain how these values work when applied to a resource pool, we'll use an example of
an ESXi host with two resource pools. The resource pools are named ProductionVMs and
