VMware ESX 4.0 Management Manual Download

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For example, consider a virtual machine with a configured size of 1GB. When the guest operating system boots,

it detects that it is running on a dedicated machine with 1GB of physical memory. The actual amount of physical

host memory allocated to the virtual machine depends on its memory resource settings and memory contention

on the ESX/ESXi host. In some cases, the virtual machine might be allocated the full 1GB. In other cases, it

might receive a smaller allocation. Regardless of the actual allocation, the guest operating system continues to

behave as though it is running on a dedicated machine with 1GB of physical memory.

Shares

Specify the relative priority for a virtual machine if more than the reservation

is available.

Reservation

Is a guaranteed lower bound on the amount of physical memory that the host

reserves for the virtual machine, even when memory is overcommitted. Set the

reservation to a level that ensures the virtual machine has sufficient memory

to run efficiently, without excessive paging.
After a virtual machine has accessed its full reservation, it is allowed to retain

that amount of memory and this memory is not reclaimed, even if the virtual

machine becomes idle. For example, some guest operating systems (for

example, Linux) might not access all of the configured memory immediately

after booting. Until the virtual machines accesses its full reservation, VMkernel

can allocate any unused portion of its reservation to other virtual machines.

However, after the guest’s workload increases and it consumes its full

reservation, it is allowed to keep this memory.

Limit

Is an upper bound on the amount of physical memory that the host can allocate

to the virtual machine. The virtual machine’s memory allocation is also

implicitly limited by its configured size.
Overhead memory includes space reserved for the virtual machine frame

buffer and various virtualization data structures.

Memory Overcommitment

For each running virtual machine, the system reserves physical memory for the virtual machine’s reservation

(if any) and for its virtualization overhead.
Because of the memory management techniques the ESX/ESXi host uses, your virtual machines can use more

memory than the physical machine (the host) has available. For example, you can have a host with 2GB memory

and run four virtual machines with 1GB memory each. In that case, the memory is overcommitted.
Overcommitment makes sense because, typically, some virtual machines are lightly loaded while others are

more heavily loaded, and relative activity levels vary over time.
To improve memory utilization, the ESX/ESXi host transfers memory from idle virtual machines to virtual

machines that need more memory. Use the Reservation or Shares parameter to preferentially allocate memory

to important virtual machines. This memory remains available to other virtual machines if it is not in use.

Memory Sharing

Many workloads present opportunities for sharing memory across virtual machines.
For example, several virtual machines might be running instances of the same guest operating system, have

the same applications or components loaded, or contain common data. ESX/ESXi systems use a proprietary

page-sharing technique to securely eliminate redundant copies of memory pages.
With memory sharing, a workload consisting of multiple virtual machines often consumes less memory than

it would when running on physical machines. As a result, the system can efficiently support higher levels of

overcommitment.

vSphere Resource Management Guide

VMware, Inc.