VMware vSphere vCenter Server 4.0 - Manual (Page 47)

Group Power-On Example
The user selects three virtual machines in the same datacenter for a group power-on attempt. The first two
virtual machines (VM1 and VM2) are in the same DRS cluster (Cluster1), while the third virtual machine (VM3)
is on a standalone host. VM1 is in automatic mode and VM2 is in manual mode. For this scenario, the user is
presented with an initial placement recommendation for Cluster1 (under the Power On Recommendations
tab) which consists of actions for powering on VM1 and VM2. An attempt is made to power on VM3
automatically and, if successful, it is listed under the Started Power-Ons tab. If this attempt fails, it is listed
under the Failed Power-Ons tab.
Virtual Machine Migration
Although DRS performs initial placements so that load is balanced across the cluster, changes in virtual
machine load and resource availability can cause the cluster to become unbalanced. To correct such imbalances,
DRS generates migration recommendations.
If DRS is enabled on the cluster, load can be distributed more uniformly to reduce the degree of this imbalance.
For example, see Figure 5-1 . The three hosts on the left side of this figure are unbalanced. Assume that Host 1,
Host 2, and Host 3 have identical capacity, and all virtual machines have the same configuration and load
(which includes reservation, if set). However, because Host 1 has six virtual machines, its resources might be
overused while ample resources are available on Host 2 and Host 3. DRS migrates (or recommends the
migration of) virtual machines from Host 1 to Host 2 and Host 3. On the right side of the diagram, the properly
load balanced configuration of the hosts that results is displayed.
Figure 5-1. Load Balancing
Host 1
VM1
VM4
VM2 VM3
VM5 VM6
Host 2
VM7
Host 3
VM8 VM9
Host 1
VM1 VM2 VM3
Host 2
VM7 VM4 VM5
Host 3
VM8 VM9 VM6
When a cluster becomes unbalanced, DRS makes recommendations or migrates virtual machines, depending
on the default automation level:
n If the cluster or any of the virtual machines involved are manual or partially automated, vCenter Server
does not take automatic actions to balance resources. Instead, the Summary page indicates that migration
recommendations are available and the DRS Recommendations page displays recommendations for
changes that make the most efficient use of resources across the cluster.
Chapter 5 Creating a DRS Cluster
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