VMware vSphere vCenter Server 4.0 - Manual (Page 48)

n If the cluster and virtual machines involved are all fully automated, vCenter Server migrates running
virtual machines between hosts as needed to ensure efficient use of cluster resources.
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Even in an automatic migration setup, users can explicitly migrate individual virtual machines, but
vCenter Server might move those virtual machines to other hosts to optimize cluster resources.
By default, automation level is specified for the whole cluster. You can also specify a custom automation level
for individual virtual machines.
DRS Migration Threshold
The DRS migration threshold allows you to specify which recommendations are generated and then applied
(when the virtual machines involved in the recommendation are in fully automated mode) or shown (if in
manual mode). This threshold is also a measure of how much cluster imbalance across host (CPU and memory)
loads is acceptable.
You can move the threshold slider to use one of five settings, ranging from Conservative to Aggressive. The
five migration settings generate recommendations based on their assigned priority level. Each setting you
move the slider to the right allows the inclusion of one more lower level of priority. The Conservative setting
generates only priority-one recommendations (mandatory recommendations), the next level to the right
generates priority-two recommendations and higher, and so on, down to the Aggressive level which generates
priority-five recommendations and higher (that is, all recommendations.)
A priority level for each migration recommendation is computed using the load imbalance metric of the cluster.
This metric is displayed as Current host load standard deviation in the cluster's Summary tab in the vSphere
Client. A higher load imbalance leads to higher-priority migration recommendations. For more information
about this metric and how a recommendation priority level is calculated, see the VMware Knowledge Base
article "Calculating the priority level of a VMware DRS migration recommendation."
After a recommendation receives a priority level, this level is compared to the migration threshold you set. If
the priority level is less than or equal to the threshold setting, the recommendation is either applied (if the
relevant virtual machines are in fully automated mode) or displayed to the user for confirmation (if in manual
or partially automated mode.)
Migration Recommendations
If you create a cluster with a default manual or partially automated mode, vCenter Server displays migration
recommendations on the DRS Recommendations page.
The system supplies as many recommendations as necessary to enforce rules and balance the resources of the
cluster. Each recommendation includes the virtual machine to be moved, current (source) host and destination
host, and a reason for the recommendation. The reason can be one of the following:
n Balance average CPU loads or reservations.
n Balance average memory loads or reservations.
n Satisfy resource pool reservations.
n Satisfy DRS (affinity or anti-affinity) rule.
n Host is entering maintenance mode or standby mode.
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If you are using the VMware Distributed Power Management feature, in addition to migration
recommendations, DRS provides host power state recommendations.
vSphere Resource Management Guide
48 VMware, Inc.