Technical information

VMware, Inc. 39
Chapter 4 Virtual Infrastructure Management
300
</pollPeriodSec>
</drm>
</config>
The default frequency is 300 seconds, but it can be set to anything between 60 seconds and 3600 seconds.
Users are discouraged from changing the default value. This is recommended only in specific cases where
the user would like to invoke the algorithm less frequently at the cost of potential loss in throughput.
 DRS affinity rules can keep virtual machines on the same ESX host or make sure they are always on
different hosts. In most cases, the default affinity settings provide the best results. In rare cases, however,
specifying affinity rules can help improve performance. To change affinity settings from the vSphere
Client select the cluster, choose the Summary tab, click Edit Settings, choose Rules, click Add, create a
name for the new rule, choose one of the two settings, click Add, select the virtual machines to which this
rule should apply, click OK, then click OK again.
Besides the default setting, the two affinity settings are:
 Keep Virtual Machines Together can improve performance due to lower latencies of
communication between machines.
 Separate Virtual Machines maintains maximal availability of the virtual machines. For instance, if
they are both web server front ends to the same application, you might want to make sure that they
don't both go down at the same time. Also co-location of I/O intensive virtual machines could end up
saturating the host I/O capacity, leading to performance degradation. DRS currently does not make
virtual machine placement decisions based on their I/O resources usage.
 To allow DRS the maximum flexibility:
 Place virtual machines on shared datastores accessible from all hosts in the cluster.
 Make sure virtual machines are not connected to host devices that would prevent them from moving
off of that host.
 Carefully select the resource settings (that is, reservations, shares, and limits) for your virtual machines.
 Setting reservations too high can leave few unreserved resources in the cluster, thus limiting the
options DRS has to balance load.
 Setting limits too low could keep virtual machines from using extra resources available in the cluster
to improve their performance.
Use reservations to guarantee the minimum requirement a virtual machine needs, rather than what you
might like it to get. Note that shares take effect only when there is resource contention. Note also that
additional resources reserved by virtual machine memory overhead need to be accounted for when sizing
resources in the cluster.
 Resource pools help improve manageability and troubleshooting of performance problems. We
recommend, however, that resource pools and virtual machines not be made siblings in a hierarchy.
Instead, each level should contain only resource pools, or only virtual machines. This is because by default
resource pools are assigned share values that might not compare appropriately with those assigned to
virtual machines, potentially resulting unexpected performance.