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vCenter: create a DRS-enabled cluster

Configuring a vSphere Distributed Resource Scheduler (DRS) enabled cluster is a foundational step in building a resilient and efficiently managed…

Configuring a vSphere Distributed Resource Scheduler (DRS) enabled cluster is a foundational step in building a resilient and efficiently managed virtualized environment. DRS automates the workload balancing of virtual machines (VMs) across ESXi hosts within a cluster, ensuring optimal resource utilization and performance. This article details the process of creating such a cluster, from initial setup to advanced configuration, including prerequisites and considerations for shared storage and networking.

DRS relies heavily on vCenter Server for its orchestration capabilities and requires a minimum of two ESXi hosts. While VMware vSphere 7.0 and 8.0 share many similarities in cluster creation, specific UI elements or default settings might slightly differ. This guide focuses on vSphere 8.0, but most steps are applicable to 7.x.

Prerequisites for a DRS Cluster

Before initiating cluster creation, several components must be in place and correctly configured. Failure to meet these prerequisites will either prevent DRS from functioning or lead to suboptimal performance and stability.

  • vCenter Server: A running vCenter Server instance is mandatory. DRS is a vCenter-level feature.
  • ESXi Hosts: At least two ESXi hosts, all running the same or compatible ESXi versions (e.g., all ESXi 8.0.x or a mix of 7.0 Update 3 and 8.0 Update 1 if interoperability is explicitly supported). All hosts should be licensed appropriately for DRS (vSphere Enterprise Plus or higher).
  • Shared Storage: All ESXi hosts in the cluster must have access to the same shared datastores. This is critical for vMotion and DRS to migrate VMs between hosts without downtime. Common shared storage types include Fibre Channel (FC), iSCSI, NFS, or vSAN. Ensure consistent LUN IDs/NFS mount points across all hosts.
  • Consistent Networking: All ESXi hosts must have vMotion enabled and configured on a dedicated VMkernel adapter. Network connectivity for VM traffic should also be consistent across hosts. For robust and scalable deployments, a vSphere Distributed Switch (vDS) is highly recommended.
  • DNS Resolution: Ensure forward and reverse DNS resolution is working correctly for all ESXi hosts and the vCenter Server.
  • NTP Synchronization: All ESXi hosts and vCenter Server should be synchronized with a reliable NTP source to prevent issues with logging, authentication, and DRS/HA event timing.

Creating the DRS Cluster in vSphere Client

The core cluster creation process is performed within the vSphere Client. This involves defining the cluster and enabling key features like DRS and HA.

1. Initial Cluster Creation

  1. Log in to the vSphere Client (HTML5 interface).
  2. Navigate to the Datacenter where you wish to create the cluster.
  3. Right-click the Datacenter object and select New Cluster...
  4. In the "New Cluster" wizard:
    • Name: Provide a descriptive name for your cluster (e.g., Prod_DRS_Cluster_A).
    • vSphere DRS: Toggle this to ON.
    • vSphere HA: Toggle this to ON (highly recommended alongside DRS for automated failover).
    • vSAN: Leave this OFF unless you intend to use vSAN storage for this cluster.
    • DRS Automation: Set this to Fully Automated for most production environments. This allows DRS to automatically migrate VMs with vMotion to optimize resource utilization without user intervention. Other options include "Partially Automated" (DRS makes recommendations, but user confirms) or "Manual" (DRS only monitors).
    • Predictive DRS (vSphere 7.0+): If you have vRealize Operations Manager integrated, enable this for intelligent workload balancing based on predictive analytics.
  5. Click OK to create the cluster object.

At this point, you have an empty cluster with DRS and HA enabled.

2. Adding ESXi Hosts to the Cluster

Once the cluster is created, you need to add your prepared ESXi hosts to it.

  1. Drag and drop the ESXi hosts from the Datacenter into the newly created cluster. Alternatively, right-click the cluster and select Add Host...
  2. Follow the wizard to add each host, providing its FQDN or IP address, and root credentials.
  3. Confirm the host summary and click Finish.

As hosts are added, vCenter Server will automatically configure them for DRS and HA, including installing the HA agent (Fault Domain Manager - FDM) and enabling vMotion if not already done.

Configuring DRS Settings

After initial setup, fine-tuning DRS settings is crucial for optimal performance and resource distribution. Navigate to the cluster, then Configure > vSphere DRS.

1. DRS Automation and Migration Threshold

Under DRS Automation, verify your desired level. For "Fully Automated," consider the Migration Threshold:

  • Aggressive (5): DRS will perform more frequent migrations to achieve better resource balance. This can lead to more vMotion operations.
  • Conservative (1): DRS will only migrate VMs for significant imbalances, reducing vMotion overhead but potentially leading to higher resource contention.
  • Recommended (3): A balanced approach, suitable for most environments.

The threshold determines how sensitive DRS is to resource imbalances. A higher threshold means DRS is more aggressive in trying to balance workloads.

2. Additional DRS Options (Advanced Settings)

Click Edit on the DRS settings page, then expand Advanced Options for more granular control:

  • CPU Overcommitment Ratio (CpuOvercommitRatio): (vSphere 7.x, 8.x) Defines the maximum CPU overcommitment DRS should attempt. Default is often unlimited or a very high value. Consider lowering this if you experience performance degradation due to high CPU contention.
  • Host Defragmentation (HostDefragmentation): (vSphere 7.x, 8.x) When enabled, DRS attempts to consolidate VMs onto fewer hosts to allow some hosts to enter standby mode or be taken out of service.
  • VM Distribution (VmDistribution): (vSphere 7.x, 8.x) Controls how DRS attempts to spread VMs across hosts to improve resilience. By default, it tries to keep VMs on different hosts if possible.

Typically, default advanced options are sufficient, but specific use cases might benefit from tuning. For example, if you run latency-sensitive applications, you might set MinPartialAutomationCpu and MinPartialAutomationMemory to 100% for specific VMs to prevent DRS from moving them based on resource consumption if it means degrading performance.

DRS Rules and Groups

DRS rules allow administrators to define how VMs are placed and migrated within a cluster, overriding default DRS behavior. Navigate to Configure > Configuration > VM/Host Groups and VM/Host Rules.

1. VM/Host Groups

Create groups of VMs and/or hosts that logically belong together. This is a prerequisite for creating affinity and anti-affinity rules.

  1. Click Add... under VM/Host Groups.
  2. Choose VM Group or Host Group.
  3. Name the group (e.g., SQL_Servers_Group, Web_Servers_Group, Maintenance_Hosts).
  4. Add the respective VMs or hosts to the group.
  5. Click OK.

2. VM/Host Rules

These rules dictate placement and migration behavior:

  • Keep Virtual Machines Together: Ensures selected VMs always run on the same host. Useful for multi-tier applications with extremely low-latency requirements between components (e.g., database and application server if they cannot be split).
  • Separate Virtual Machines: Ensures selected VMs always run on different hosts. Crucial for high availability, preventing a single host failure from taking down all instances of a critical application (e.g., domain controllers, clustered application nodes).
  • Virtual Machines to Hosts: Defines a relationship between VM groups and Host groups.
    • Must run on hosts in group: Strict rule; VMs in the specified group can ONLY run on hosts in the specified host group.
    • Should run on hosts in group: Preferential rule; DRS attempts to place VMs on hosts in the specified group but can violate the rule if no other option is available or to meet other DRS objectives.
    • Must NOT run on hosts in group: Strict rule; VMs in the specified group are NEVER allowed to run on hosts in the specified host group. Useful for isolating specific workloads or keeping them away from maintenance hosts.
    • Should NOT run on hosts in group: Preferential rule; DRS attempts to keep VMs off hosts in the specified group.

Example: To ensure your critical SQL servers are never on the same host, and your test VMs are kept off production hosts:


# Create VM Groups
VM_Group_SQL: SQL_Server_1, SQL_Server_2
VM_Group_Test: Test_VM_A, Test_VM_B

# Create Host Group
Host_Group_Prod: ESXi_Prod_01, ESXi_Prod_02, ESXi_Prod_03
Host_Group_NonProd: ESXi_Test_01, ESXi_Test_02

# Create VM/Host Rules
Rule_SQL_Separate:
    Type: Separate Virtual Machines
    VMs: SQL_Server_1, SQL_Server_2

Rule_Test_Isolation:
    Type: Virtual Machines to Hosts
    VM Group: VM_Group_Test
    Operation: Should NOT run on hosts in group
    Host Group: Host_Group_Prod

vSphere Distributed Switch (vDS) Configuration

While standard switches work, a vDS provides advanced networking features essential for large or complex vSphere environments, especially with DRS. It centralizes network configuration for all hosts in the cluster.

  1. Create a vDS: Navigate to Networking > Datacenter > New Distributed Switch...
    • Name: e.g., Prod_vDS_01
    • Version: Latest compatible with your ESXi hosts (e.g., 8.0.0).
    • Number of Uplinks: Match the physical NICs you intend to dedicate per host (e.g., 2-4).
    • Network I/O Control: Enable (recommended for QoS).
    • Default port group: Uncheck "Create a default port group" if you plan to create specific ones.
  2. Add Hosts to vDS: Right-click the vDS, choose Add and Manage Hosts... > Add Hosts > New Hosts...
    • Select all ESXi hosts from your DRS cluster.
    • Configure physical adapters (uplinks): Assign physical NICs (e.g., vmnic0, vmnic1) from each host to the vDS uplinks.
    • Migrate VMkernel adapters: Migrate existing vMotion and management VMkernel adapters to new or existing distributed port groups on the vDS. Create dedicated distributed port groups for vMotion (e.g., DPortGroup_vMotion) and management (e.g., DPortGroup_Management).
    • Migrate virtual machine networking: Migrate existing VM networks from standard switches to new or existing distributed port groups (e.g., DPortGroup_Prod_VMs).

Ensure that all relevant VMkernel adapters (Management, vMotion, Storage) are migrated to the vDS and configured with appropriate VLANs and IP addresses. For vMotion, ensure it is on a dedicated network and enabled for vMotion traffic.

Resource Pools (Optional but Recommended)

Resource pools allow you to delegate resource management within a cluster, partitioning CPU and memory resources among groups of VMs. This is useful for multi-tenancy or for guaranteeing resources to critical applications.

  1. Right-click the DRS cluster and select New Resource Pool...
  2. Name the resource pool (e.g., Prod_HighPriority, Test_Dev).
  3. Configure Shares, Reservation, and Limit for CPU and Memory.
    • Shares: Relative priority. A VM in a "High" shares resource pool gets more resources during contention than one in a "Low" shares pool.
    • Reservation: Guaranteed minimum resources.
    • Limit: Upper bound on resources consumed.
  4. Drag and drop VMs into the appropriate resource pools.

Resource pools enable DRS to manage resource allocation not just at the VM level, but also at the group level, adhering to the defined shares and reservations.

Common Pitfalls and Troubleshooting

  • Shared Storage Inconsistency: If hosts do not see the same datastores with the same LUN IDs (for block storage) or mount points (for NFS), vMotion will fail, and DRS migrations will be impossible. Verify connectivity and configuration.
  • vMotion Network Issues: Incorrect IP addresses, subnet masks, VLANs, or MTU mismatches on vMotion VMkernel adapters will cause vMotion failures. Ensure dedicated vMotion networks are configured correctly on all hosts. Test vMotion manually between hosts before relying on DRS.
  • DRS & HA Interdependencies: While HA takes priority for host failures, DRS handles workload balancing. Ensure both are configured correctly. HA admission control can sometimes prevent DRS from migrating VMs if it would violate HA reservation policies.
  • DRS Rules Conflicts: Conflicting affinity/anti-affinity rules can prevent DRS from finding a suitable host for a VM or even lead to VMs being powered off (for "Must" rules). Review and validate your rules. The DRS recommendations panel can highlight rule violations.
  • Licensing: Ensure all ESXi hosts have vSphere Enterprise Plus or higher licenses to enable DRS. Hosts without the correct license will not participate in the DRS cluster.
  • Performance Issues: If DRS is too aggressive or too conservative, adjust the migration threshold. Monitor CPU/memory contention and vMotion activity. High vMotion rates without performance gains might indicate an overly aggressive threshold.
  • Predictive DRS not working: Ensure vRealize Operations Manager is correctly integrated with vCenter Server and data collection is functioning. Check the vROps and vCenter logs for connectivity issues.

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