Red Hat NETSCAPE MANAGEMENT SYSTEM 4.5 Instrukcja Użytkownika Pobierz pdf (Strona 240)

Resource Management

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and maximum constraints, provided they are both actively using the memory they

have been allocated.

Admission Control

VMware ESX Server uses an admission control policy to ensure that sufficient

unreserved memory and swap space are available before powering on a virtual

machine. Memory must be reserved for the virtual machine’s guaranteed minimum

size; additional overhead memory is required for virtualization. Thus the total required

for each virtual machine is the specified minimum plus overhead.

The overhead memory size is determined automatically; it is typically 32MB per virtual

machine. Additional overhead memory is reserved for virtual machines larger than

1GB.

Swap space must be reserved on disk for the remaining virtual machine memory —

that is the difference between the maximum and minimum settings. This swap

reservation is required to ensure the system is able to preserve virtual machine

memory under any circumstances. In practice, only a small fraction of the swap space

may actually be used.

Similarly, while memory reservations are used for admission control, actual memory

allocations vary dynamically, and unused reservations are not wasted.

The amount of swap space configured for the system limits the maximum level of

overcommitment. A default swap file size equal to the physical memory size of the

computer is recommended in order to support a reasonable 2x level of memory

overcommitment. You may configure larger or smaller swap files. If you do not

configure a swap file, memory may not be overcommitted. The swap file may be

configured using the management interface or from the console operating system

using the vmkfstools program. For details on vmkfstools see Using vmkfstools

on page 199.

Dynamic Allocation

Virtual machines are allocated their maximum memory size unless memory is

overcommitted. When memory is overcommitted, each virtual machine is allocated

an amount of memory somewhere between its minimum and maximum sizes. The

amount of memory granted to a virtual machine above its minimum size may vary

with the current memory load. The system automatically determines allocations for

each virtual machine based on two factors: the number of shares it has been given

and an estimate of its recent working set size.

VMware ESX Server uses a modified proportional-share memory allocation policy.

Memory shares entitle a virtual machine to a fraction of physical memory. For

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Red Hat NETSCAPE MANAGEMENT SYSTEM 4.5 Instrukcja Użytkownika Strona 240