Datasheet

CHAPTER 1 VMware VSphere 4 OVerView

Advanced Memory Management

VMware vSphere uses several advanced memory management features to efﬁciently use the

physical memory available. These features make sure that in a highly consolidated environment

virtual machines are allocated the required memory as needed without impacting the perfor-

mance of other virtual machines. These advanced features include the following:

Memory over-commitment Similar to CPU over-commitment, memory over-commitment

improves memory utilization by enabling you to conﬁgure virtual machine memory that

exceeds the physical server memory. For example, the total amount of memory allocated for

all virtual machines running on a vSphere host can be more than the total physical memory

available on the host.

Transparent page sharing Transparent page sharing uses available physical memory more

efﬁciently by sharing identical memory pages across multiple virtual machines on a vSphere

host. For example, multiple virtual machines running Windows Server 2008 will have many

identical memory pages. ESX will store a single copy of these identical memory pages in

memory and create additional copies only if a memory page changes.

Memory ballooning Memory ballooning dynamically transfers memory from idle virtual

machines to active ones. It puts artiﬁcial memory pressure on idle virtual machines, forcing

them to use their own paging areas and release memory. This allows active virtual machines

in need of memory to use this memory. Keep in mind that ESX will ensure that a virtual

machine memory usage cannot exceed its conﬁgured memory.

Large memory pages Newer x86 processors support the use of large 2 MB memory pages

in addition to the small 4 KB pages. Operating systems rely on the translation lookaside buf-

fers inside the processor to translate virtual to physical memory addresses. Larger page sizes

mean that a TLB cache of the same size can keep track of larger amounts of memory, thus

avoiding the costly TLB misses. Enterprise applications such as database servers and Java

virtual machines commonly use large memory pages to increase TLB access efﬁciency and

improve performance. ESX supports the use of large memory pages in virtual machines and

backs up with its own large memory pages to maintain efﬁcient memory access.

Resource Management

VMware vSphere allows you to establish minimum, maximum, and proportional resource

shares for CPU, memory, disk, and network bandwidth for virtual machines. The minimum

resource setting or reservation guarantees the amount of CPU and memory resources for a

virtual machine, while the maximum resource setting or limit caps the amount of CPU and

memory resources a virtual machine can use. The proportional resource allocation mechanism

provides three levels—normal, low, and high—out of the box. These settings help conﬁgure vir-

tual machine priority for CPU and memory resources relative to each other. These can be set at

the resource pool level and are inherited or overridden at the individual virtual machine level.

You can leverage these resource allocation policies to improve service levels for your software

applications. The key advantage of these settings is that you can change resource allocations

while virtual machines are running, and the changes will take place immediately without any

need to reboot.

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