Installation guide

C H A P T E R 1 2 VMware ESX Server Resource Management
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times that of low. For example, a virtual machine with high shares has twice as many
shares as a virtual machine with normal shares, or four times as many shares as a
virtual machine with low shares. If you want to change these defaults, see Service
Console Commands on page 412.
ESX Server interprets this allocation so that virtual machine A will never have less than
192MB memory, while virtual machine B will never have less than 256MB memory, in
any situation.
However, if one or more virtual machines are not actively using their allocated
memory (for example, the virtual machines are idling), then ESX Server may
redistribute a portion of this unused memory proportionally, based on the virtual
machines’ memory shares. Active virtual machines benefit when extra resources are
available. In this example, because virtual machine A has high shares, it can get twice
as much memory as virtual machine B (low shares), subject to the specified memory
minimum or maximum.
For detailed information on how ESX Server dynamically redistributes memory, see
Allocating Memory Dynamically on page 406.
Admission Control Policy
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 54MB for a
single virtual CPU virtual machine, and 64MB for a dual-virtual CPU SMP virtual
machine. Additional overhead memory is reserved for virtual machines larger than
512MB.
Note: To create SMP virtual machines with ESX Server, you must also have
purchased the VMware Virtual SMP for ESX Server product. For more information on
the VMware Virtual SMP for ESX Server product, contact VMware, Inc. or your
authorized sales representative.
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.