Red Hat NETSCAPE MANAGEMENT SYSTEM 4.5 User Manual download pdf (Page 251)

Resource Management

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Memory Utilization page (Overview > Configure System > Memory Utilization; you

must be logged in as root to see this page).

ESX Server memory sharing runs as a background activity that scans for sharing

opportunities over time. The amount of memory saved may vary over time; for a fairly

constant workload, the amount generally increases slowly until all sharing

opportunities are exploited.

Memory Overcommitment

In many consolidated workloads, it is rare for all virtual machines to be actively using

all of their memory simultaneously. Typically, some virtual machines are lightly loaded,

while others are more heavily loaded, and relative activity levels generally vary over

time. In such cases, it may be reasonable to overcommit memory to reduce hardware

memory requirements.

ESX Server automatically transfers memory from idle virtual machines to virtual

machines that actively need more memory in order to improve memory utilization.

You may also specify configuration parameters to preferentially devote space to

important virtual machines.

The minimum size for a virtual machine defines a guaranteed lower bound on the

amount of memory that it is allocated, even when memory is overcommitted. You

can also use memory shares to specify the relative importance of different virtual

machines. In any case, you should configure an appropriate minimum size for each

virtual machine to ensure that each virtual machine can function effectively (without

excessive paging), even when all virtual machines are active concurrently.

When memory is scarce, ESX Server dynamically reclaims space from some virtual

machines based on importance and current working sets. For optimal performance,

the server attempts to reclaim memory from a virtual machine via a VMware-supplied

vmmemctl module running in the guest. This allows the guest operating system to

invoke its own native memory management policies, causing it to swap to its own

virtual disk only when necessary.

ESX Server also has its own swap file and may also swap memory from a virtual

machine to the ESX Server swap file directly, without any involvement by the guest

operating system.

Example: Web Server Consolidation

Suppose that you are using ESX Server to consolidate eight nearly-identical Web

servers running IIS on Windows 2000. Each Windows 2000 machine is configured with

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Red Hat NETSCAPE MANAGEMENT SYSTEM 4.5 User Manual Page 251