What Is a VMware DRS Cluster?

A cluster is a group of hosts connected to each other with special software that makes them elements of a single system. At least two hosts (also called nodes) must be connected to create a cluster. When hosts are added to the cluster, their resources become the cluster’s resources and are managed by the cluster.

The most common types of VMware vSphere clusters are High Availability (HA) and Distributed Resource Scheduler (DRS) clusters. HA clusters are designed to provide high availability of virtual machines and services running on them; if a host fails, they immediately restart the virtual machines on another ESXi host. DRS clusters provide the load balancing among ESXi hosts, and in today’s blog post, we are going to explore the DRS cluster system in depth.

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How Does the DRS Cluster Work?

Distributed Resource scheduler (DRS) is a type of VMware vSphere cluster that provides load balancing by migrating VMs from a heavily loaded ESXi host to another host that has enough computing resources, all while the VMs are still running. This approach is used to prevent overloading of ESXi hosts. Virtual machines can have uneven workloads at different times, and if an ESXi host is overloaded, performance of all VMs running on that host is reduced. The VMware DRS cluster helps in this situation by providing automatic VM migration.

For this reason, DRS is usually used in addition to HA, combining failover with load balancing. In a case of failover, the virtual machines are restarted by the HA on other ESXi hosts and the DRS, being aware of the available computing resources, provides the recommendations for VM placement. vMotion technology is used for this live migration of virtual machines, which is transparent for users and applications.

Resource pools are used for flexible resource management of ESXi hosts in the DRS cluster. You can set processor and memory limits for each resource pool, then add virtual machines to them. For example, you could create one resource pool with high resource limits for developers’ virtual machines, a second pool with normal limits for testers’ virtual machines, and a third pool with low limits for other users. vSphere lets you create child and parent resource pools.

Resource pools of a DRS cluster

When Are DRS Clusters Used?

The DRS solution is usually used in large VMware virtual environments with uneven workloads of VMs in order to provide rational resource management. Using a combination of DRS and HA results in a high-availability cluster with load balancing. The DRS is also useful for automatic migration of VMs from an ESXi server put in maintenance mode by an administrator. This mode must be turned on for the ESXi server to perform maintenance operations such as firmware upgrades, installing security patches, ESXi updates etc. There cannot be any virtual machines running on an ESXi server entering maintenance mode.

DRS Clustering Features

Main DRS clustering features are Load Balancing, Distributed Power Management, and Affinity Rules.

Load Balancing is the feature that optimizes the utilization of computing resources (CPU and RAM). Utilization of processor and memory resources by each VM, as well as the load level of each ESXi host within the cluster, is continuously monitored. The DRS checks the resource demands of VMs and determines whether there is a better host for the VM to be placed on. If there is such host, the DRS makes a recommendation to migrate the VM in automatic or manual mode, depending on your settings. The DRS generates these recommendations every 5 minutes if they are necessary. The figure below illustrates the DRS performing VM migration for load balancing purposes.

Load balancing in a VMware DRS cluster

Distributed Power Management (DPM) is a power-saving feature that compares the capacity of cluster resources to the resources utilized by VMs within the cluster. If there are enough free resources in the cluster, then DPM recommends migrating the VMs from lightly loaded ESXi hosts and powering off those hosts. If the cluster needs more resources, wake-up packets are sent to power hosts back on. For this to function, the ESXi servers must support one of the following power management protocols: Wake-On-LAN (WOL), Hewlett-Packard Integrated Lights-Out (iLO), or Intelligent Platform Management Interface (IPMI). With the DRS cluster’s DPM, you can save up to 40% in electricity costs.

The Distributed Power Management feature of VMware Distributed Resource Scheduler cluster

Affinity Rules allow you some control over placement of VMs on hosts. There are two types of rules that allow keeping VMs together or separated:

  • affinity or anti-affinity rules between individual VMs.
  • affinity or anti-affinity rules between groups of VMs and groups of ESXi hosts.

Let’s explore how these rules work with examples.

1. Suppose you have a database server running on one VM, a web server running on a second VM, and an application server running on a third VM. Because these servers interact with each other, three VMs would ideally be kept together on one ESXi host to prevent overloading the network. In this case, we would select the “Keep Virtual Machines Together” (affinity) option.

2. If you have an application-level cluster deployed within VMs in a DRS cluster, you may want to ensure the appropriate level of redundancy for the application-level cluster (this provides additional availability). In this case, you could create an anti-affinity rule and select the “Separate Virtual Machines” option. Similarly, you can use this approach when one VM is a main domain controller and the second is a replica of that domain controller (Active Directory level replication is used for domain controllers). If the ESXi host with the main domain controller VM fails, users can connect to the replicated domain controller VM, as long as the latter is running on a separate ESXi host.

3. An affinity rule between a VM and an ESXi host might be set, in particular, for licensing reasons. As you know, in a VMware DRS cluster, the virtual machines can migrate between hosts. Many software licensing policies – such as database software, for example – require you to buy a license for all hosts on which the software runs, even if there is only one VM running the software within the cluster. Thus, you should prevent such VM from migrating to different hosts and costing you more licenses. You can accomplish this by applying an affinity rule: the VM with database software must run only on the selected host for which you have a license.  In this case, you should select the “Virtual Machines to Hosts” option. Choose “Must Run on Host” and then input the host with the license. (Alternatively, you could select “Must Not Run on Hosts in Group” and specify all unlicensed hosts.)

You can see how to set affinity rules in the setup section below.

Requirements for Setting Up a DRS Cluster

The following requirements must be met to set up a DRS cluster:

  • CPU compatibility. The maximum compatibility of processors between ESXi hosts is required. Processors must be produced by the same vendor and belong to the same family with equivalent instructions sets. Ideally, the same processor model should be used for all ESXi hosts.
  • Shared datastore. All ESXi hosts must be connected to shared storage such as SAN (Storage Area Network) or NAS (Network Attached Storage) that can access shared VMFS volumes.
  • Network connection. All ESXi hosts must be connected to each other. Ideally, you would have a separate vMotion network, with at least 1Gbit of bandwidth, for VM migration between hosts.
  • vCenter Server must be deployed to manage and configure the cluster.
  • At least 2 ESXi servers must be installed and configured (3 or more ESXi servers are recommended).

How to Set Up the DRS Cluster

First, you need to configure the ESXi hosts, network connection, shared storage, and vCenter server. After configuring those, you can set up your DRS cluster. Log in to vCenter server with the vSphere web client. Create a datacenter in which to place your ESXi hosts: vCenter -> Datacenters -> New Datacenter. Then, select your datacenter and click Actions -> Add Host to add the ESXi hosts you need, following the recommendations of the wizard. Now you are ready to create a cluster.

In order to create a cluster, do the following:

  • Go to vCenter -> Hosts and Clusters.
  • Right-click on your datacenter and select “New Cluster”.
  • Set the name of the cluster and check the box marked “Turn on DRS”. Click “OK” to finish.

If you have already created a cluster, follow these steps:

  • Go to vCenter -> Clusters -> Your cluster name.
  • Open Manage -> Settings tab.
  • Select “vSphere DRS” and click “Edit”.
  • Check the box marked “Turn ON vSphere DRS”. Click “OK” to finish.

Now that you have created the DRS cluster, you can configure DRS automation, DPM, affinity rules, and other options.

DRS automation. In order to set up the load balancing, you need the “DRS Automation” section. Here, you can select the Automation Level (Manual, Partially Automated, or Fully Automated), as well as the Migration Threshold (values from 1 to 5, with 1 being conservative and 5 being aggressive). If you want to set up individual virtual machine automation levels, then check the appropriate box.

DRS automation settings

Power Management. You can set up DPM by selecting one of the following values: Off, Manual, or Automatic. As with the load balancing feature described above, you can select the DPM threshold values from 1 (conservative) to 5 (aggressive).

Distributed Power Management settings for the DRS cluste

Advanced Options. You can manually set the advanced options for detailed tuning of your cluster.

For example, you can set “MinImbalance 40” for computing target imbalance. The default value is 50, while 0 is the most aggressive. You can read more about this and explore all the advanced options in the VMware documentation.

Affinity Rules. In order to set up affinity and anti-affinity rules, follow these steps:

1. Go to vCenter -> Clusters -> your cluster name
2. Go to Manage -> Settings tab
3. Select “DRS Rules” and click “Add”Set a name for the rule
4. Select the rule type:

  • Keep Virtual Machines Together (affinity)
  • Separate Virtual Machines (anti-affinity)
  • Virtual Machines to Hosts (affinity or anti-affinity)

5. Select VMs for the first two rule types, or VM groups, host groups and policy for the third rule type
6. Click “OK” to finish.

Affinity rules settings

Resource Pools. If you would like to create a resource pool for your VMs in a cluster, do the following:

  • Go to vCenter -> Clusters -> Your cluster name.
  • Click Actions -> New Resource Pool.
  • Give the pool a name, then define limits and reservations for CPU as well as memory. Click “OK” when done.

Creating a new resource pool for the DRS cluster

Now you can add your virtual machines to the resource pool. Here is how you can migrate an existing VM to the resource pool:

  • Go to vCenter -> Virtual Machines.
  • Select your virtual machine.
  • Click Actions -> Migrate. The wizard window appears.
  • Select “Change Host” in the “Migration Type” section and click “Next”.
  • Select your resource pool in the “Select Destination Resource” section and click “Next”.
  • In the “Review Selections” section, click “Finish”.

After configuration, you can check the state of your newly created DRS cluster. Just go to vCenter -> Clusters -> Your cluster name and click the “Summary” tab.

Viewing the summary of a VMware DRS cluster with vSphere Web Client

The Advantages of Using DRS

The main advantage of using a VMware DRS cluster is effective resource management with load balancing. This improves the quality of services provided, while also allowing you to save power (and, thus, money) with DPM. You can control the virtual machine placement manually or automatically, which makes maintenance and support more convenient.


The DRS cluster solution is a part of VMware’s vSphere virtualization software, and is particularly useful in large virtual environments. DRS features such as load balancing, power management, and affinity rules help you optimize resource use, as well as performance of the cluster. With Distributed Power Management, you can save on electricity costs. Using DRS in combination with HA gives you a balanced High Availability VMware vSphere cluster that is an effective, high-performance solution for any virtual infrastructure.

NAKIVO Backup & Replication is a product designed for protection of VMware virtual machines as well as clusters. When adding vCenter with a cluster to the product’s inventory, all the VMs of the cluster are automatically added, too. If a cluster is selected for backup or replication job, all the VMs of such cluster are selected automatically regardless of the ESXi host on which they are residing.

Try out the cluster-related and other features of NAKIVO Backup & Replication in your own environment.

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