Admission Control and Initial Placement
When you attempt to power on a single virtual machine or a group of virtual machines in a DRS-enabled
cluster, vCenter Server performs admission control. It checks that there are enough resources in the cluster to
support the virtual machine(s).
If the cluster does not have sufficient resources to power on a single virtual machine, or any of the virtual
machines in a group power-on attempt, a message appears. Otherwise, for each virtual machine, DRS generates
a recommendation of a host on which to run the virtual machine and takes one of the following actions
n
Automatically executes the placement recommendation.
n
Displays the placement recommendation, which the user can then choose to accept or override.
NOTE No initial placement recommendations are given for virtual machines on standalone hosts or in
non-DRS clusters. When powered on, they are placed on the host where they currently reside.
For more information about DRS recommendations and applying them, see “DRS Recommendations Page,”
on page 67.
Single Virtual Machine Power On
In a DRS cluster, you can power on a single virtual machine and receive initial placement recommendations.
When you power on a single virtual machine, you have two types of initial placement recommendations:
n
A single virtual machine is being powered on and no prerequisite steps are needed.
The user is presented with a list of mutually exclusive initial placement recommendations for the virtual
machine. You can select only one.
n
A single virtual machine is being powered on, but prerequisite actions are required.
These actions include powering on a host in standby mode or the migration of other virtual machines from
one host to another. In this case, the recommendations provided have multiple lines, showing each of the
prerequisite actions. The user can either accept this entire recommendation or cancel powering on the
virtual machine.
Group Power On
You can attempt to power on multiple virtual machines at the same time (group power on).
Virtual machines selected for a group power-on attempt do not have to be in the same DRS cluster. They can
be selected across clusters but must be within the same datacenter. It is also possible to include virtual machines
located in non-DRS clusters or on standalone hosts. These are powered on automatically and not included in
any initial placement recommendation.
The initial placement recommendations for group power-on attempts are provided on a per-cluster basis. If
all of the placement-related actions for a group power-on attempt are in automatic mode, the virtual machines
are powered on with no initial placement recommendation given. If placement-related actions for any of the
virtual machines are in manual mode, the powering on of all of the virtual machines (including those that are
in automatic mode) is manual and is included in an initial placement recommendation.
For each DRS cluster that the virtual machines being powered on belong to, there is a single recommendation,
which contains all of the prerequisites (or no recommendation). All such cluster-specific recommendations are
presented together under the Power On Recommendations tab.
When a nonautomatic group power-on attempt is made, and virtual machines not subject to an initial
placement recommendation (that is, those on standalone hosts or in non-DRS clusters) are included, vCenter
Server attempts to power them on automatically. If these power ons are successful, they are listed under the
Started Power-Ons tab. Any virtual machines that fail to power on are listed under the Failed Power-Ons tab.
vSphere Resource Management Guide
44 VMware, Inc.
