Information Technology Reference
In-Depth Information
vSphere DRS runs as a process within vCenter Server, which means that you must have
vCenter Server in order to use vSphere DRS. By default, DRS checks every 5 minutes (or 300
seconds) to see if the cluster's workload is balanced. DRS is also invoked by certain actions
within the cluster, such as adding or removing an ESXi host or changing the resource settings
of a VM. When DRS is invoked, it will calculate the imbalance of the cluster, apply any resource
controls (such as reservations, shares, and limits), and, if necessary, generate recommendations
for migrations of VMs within the cluster. Depending on the coni guration of vSphere DRS,
these recommendations could be applied automatically, meaning that VMs will automatically
be migrated between hosts by DRS in order to maintain cluster balance (or, put another way, to
minimize cluster imbalance).
vSphere Distributed Resource Scheduler Enables Resource Pools
DRS enables the use of resource pools when clustering ESXi hosts. If hosts are part of a HA enabled
cluster, DRS must also be enabled to allow Resource Pools to be created.
Fortunately, if you like to retain control, you can set how aggressively DRS will automatically
move VMs around the cluster.
If you start by looking at the DRS properties—you can view these properties by right-
clicking a DRS-enabled cluster, selecting Settings, clicking the vSphere DRS heading on the left,
and then clicking Edit—you will see that there are three selections regarding the automation
level of the DRS cluster: Manual, Partially Automated, and Fully Automated. If you click the
triangle next to DRS Automation, you will i nd the slider bar that affects the actions of the Fully
Automated setting on the cluster. These settings control the initial placement of a VM and the
automatic movement of VMs between hosts. We'll examine the behavior of these automation
levels in the next three sections.
Understanding Manual Automation Behavior
When a DRS cluster is set to Manual, every time you power on a VM, the cluster prompts you
to select the ESXi host where that VM should be hosted. The dialog box rates the available hosts
according to suitability at that moment: the lower the priority, the better the choice, as shown in
Figure 12.19.
Figure 12.19
A DRS cluster
set to Manual
requires you to
specify where the
VM should be pow-
ered on.
