VMware DRS Overview: Optimizing Resource Allocation in Your vSphere Cluster

Products

• VMware vCenter Server

Issue/Introduction

The Distributed Resource Scheduler (DRS) is a powerful feature within VMware vSphere that automates the distribution of virtual machines (VMs) across a cluster of ESXi hosts to optimize resource utilization. By intelligently balancing workloads, DRS ensures that virtual machines (VMs) run efficiently, avoiding overloading or resource contention. This article will explain how DRS works, its various automation levels, and how it helps improve your VMware environment's overall performance.

What is VMware DRS?

Distributed Resource Scheduler (DRS) is a feature within VMware vSphere clustering that automatically balances virtual machine workloads across a cluster of ESXi hosts. It continuously monitors the load on each host and VM within the cluster, and when it detects an imbalance—such as a host or VM running out of resources—DRS migrates the VM to a more suitable ESXi host. The goal is to ensure that all VMs maintain a high quality of service (QoS) by having enough CPU and memory resources to run smoothly.

How DRS Works in VMware

DRS works by checking resource utilization on a regular basis (every 5 minutes by default). It monitors the CPU and RAM usage for both VMs and ESXi hosts within a cluster. If DRS detects that a particular host is overloaded or that a VM needs additional resources, it will initiate a migration to another host with available resources.

The process is based on the following calculations:

• VM Memory Demand: DRS calculates VM memory demand by considering active memory, swapped memory, shared memory, and an additional 25% for idle consumed memory.
  • VM memory demand = Function(Active memory used, Swapped, Shared) + 25% (idle consumed memory)
• CPU Demand: DRS calculates the CPU demand based on the number of processor resources the VM is consuming, using historical data (such as maximum and average CPU values) to predict trends.

If DRS detects a resource imbalance, it triggers a live migration (vMotion) to balance the load across the cluster. This migration occurs seamlessly, without service interruption, ensuring that your environment remains responsive.

VMware DRS Automation Levels

VMware offers different automation levels for managing VM placement and migration:

1. Fully Automated

   • DRS automatically handles both initial VM placement and migration within the cluster. If a resource imbalance is detected, the system will automatically move VMs to achieve better load balancing.

2. Partially Automated

   • DRS will provide automatic recommendations for initial VM placement. However, for load balancing, the system will generate migration recommendations that must be manually applied by the administrator.

3. Manual

   • In manual mode, DRS only provides migration recommendations, which must be manually approved for execution. Initial VM placement is also manually configured.

DRS Aggression Levels

DRS provides a slider to adjust its "aggression level," which determines how aggressively it will balance resources across the cluster:

1. Most Conservative

   • DRS will tolerate minor imbalances and initiate migrations only when necessary. It’s designed to minimize VM migrations.

2. Balanced (Default)

   • This setting provides a balanced approach with an optimal number of migrations, making it the recommended setting for most environments.

3. Most Aggressive

   • DRS will initiate migrations even for minimal imbalances, leading to frequent VM migrations to ensure better resource distribution.

4. Predictive DRS

   • This advanced feature, available when integrated with VMware vRealize Operations Manager, allows DRS to predict future resource demands based on historical trends. It helps proactively migrate VMs before resource contention occurs, reducing the risk of imbalances.

Affinity and Anti-Affinity Rules

VMware DRS supports affinity and anti-affinity rules that govern the placement of VMs on ESXi hosts. These rules can be used to ensure that certain VMs are either kept together on the same host or kept apart on different hosts, based on specific requirements like availability or application needs.

VM-VM Affinity Rules:

• Keep VMs together: Ensures that selected VMs run on the same host (useful for localized network traffic or co-dependent applications).
• Separate VMs: Ensures that selected VMs are never placed on the same host (ideal for high availability scenarios).

VM-Host Affinity Rules:

• Assign VMs to a specific host or group of hosts based on specific needs, such as licensing constraints or hardware requirements.

VM Overrides for DRS

VM overrides allow for granular control over DRS settings for specific VMs. By applying overrides, administrators can configure DRS settings on a per-VM basis, overriding the global cluster settings. This is useful when certain VMs require different migration behaviors or placement preferences.

Predictive DRS

When used with VMware vRealize Operations Manager, Predictive DRS helps anticipate resource shortages before they happen. By analyzing historical performance data, it predicts when and where high resource demands will occur. This allows DRS to initiate proactive VM migrations to avoid potential issues, such as resource contention or host overload.

Distributed Power Manager (DPM)

Distributed Power Manager (DPM) works alongside DRS to optimize energy consumption. It migrates VMs to fewer hosts when resources are available, enabling the shutdown or standby of unused ESXi hosts. When resource demands increase, DPM brings hosts back online and redistributes VMs across the cluster. This feature helps reduce power consumption and operational costs.

Advantages of Using VMware DRS

The main benefits of VMware DRS include:

• Optimized Resource Utilization: Ensures that resources are balanced across the cluster, preventing any one host from becoming overloaded.
• Improved Service Quality: By maintaining balanced workloads, DRS helps ensure that VMs run smoothly and efficiently.
• Automated Management: DRS simplifies cluster management by automating VM placement and migration, reducing manual intervention.
• Energy Efficiency: Combined with DPM, DRS helps optimize power consumption by consolidating workloads and shutting down unused hosts.

Environment

VMware vCenter Server

Resolution

VMware Distributed Resource Scheduler (DRS) is an essential tool for maintaining a balanced, efficient, and high-performing VMware environment. Whether you are aiming for full automation or manual control, DRS offers flexibility in how you manage your cluster's resources. By optimizing CPU and memory allocation, and ensuring that VMs are placed on the right hosts, DRS contributes to both the stability and cost-effectiveness of your virtual infrastructure.

Additional Information

• VM-Host Affinity Rule Violation in Partially Automated DRS:

  • If DRS is set to Partially Automated, a VM-Host affinity rule violation will appear in the DRS recommendations.
  • This is not considered a DRS fault because, in partially automated mode, migrations are handled both manually and by DRS.
  • Therefore, when a rule violation occurs, it will show up as a recommendation rather than a fault.

• Rule Conflict Findings:

  • The conflict count increases if VMs or hosts are part of two different rules.
  • In such cases, the system flags the conflict because the VM/host is part of multiple conflicting rules.