VMware Distributed Resource Scheduler (DRS) is a key feature included with vSphere Enterprise Plus and vSphere with Operations Management Enterprise Plus. DRS balances computing capacity across a collection of VxRail server resources that have been aggregated into logical pools. It continuously balances and optimizes compute resource allocation among the VMs.
When a VM experiences an increased workload, DRS evaluates the VM priority against user-defined resource-allocation rules and policies. If justified, DRS allocates additional resources. It can also be configured to dedicate consistent resources to the VMs of particular business-unit applications to meet SLAs and business requirements.
DRS allocates resources to the VM either by migrating the VM to another server with more available resources or by making more “resources” for the VM on the same server by migrating other VMs off the server. In the VxRail system, all ESXi hosts are part of a vMotion network. The live migration of VMs to different node servers is completely transparent to end users through vMotion (see the figures below). DRS adds tremendous value to the VxRail cluster by automating VM placement and ensuring consistent and predictable application-workload performance.
Figure 35. DRS movement of VMs across node servers
Figure 36. VM migration across the vMotion network (requires external vCenter)
DRS offers a considerable advantage to VxRail users during maintenance situations, because it automates the tasks normally involved in manually moving live machines during upgrades or repairs. DRS facilitates maintenance automation, providing transparent, continuous operations by dynamically migrating all VMs to other physical servers. That way, servers can be attended to for maintenance, or new node servers can be added to a resource pool, all while DRS automatically redistributes the VMs among the available servers as the physical resources change.
In other words, DRS dynamically balances VMs as soon as additional resources become available when new server is added or when an existing server has finished its maintenance cycle. DRS allocates only CPU and memory resources for the VMs and uses vSAN for shared storage.
The following figure shows the settings for configuring DRS.
Figure 37. Configuring DRS settings
Some conditions and business operations warrant a more aggressive DRS migration strategy than others. Adjustable cluster parameters establish the thresholds that trigger DRS migrations (as shown in the screen shot above.) For example, a Level-2 threshold only applies specified migration recommendations to make a significant impact on the cluster’s load balance, whereas a Level-5 threshold applies all the recommendations to even slightly improve the cluster’s load balance.
DRS applies only to VxRail virtual machines. (vSAN uses a single datastore and handles placement and balancing internally. vSAN does not currently support Storage DRS or Storage I/O Control.)