VMware vSphere is one of the most widely adopted virtualization platforms, used mostly across enterprises and mid-sized businesses to run data centers and private clouds. It offers a wide range of features that simplify infrastructure management and enhance resource efficiency. However, as infrastructures grow bigger and workloads become more dynamic, manually managing compute and storage resources quickly becomes challenging.
This is where vSphere Distributed Resource Scheduler (DRS) comes in – helping clusters maintain balance, ensuring applications receive the resources they need, and reducing administrators’ manual workload.
This guide covers what vSphere DRS is, its requirements, key features, how it integrates with vSphere High Availability (HA), and the benefits DRS brings to vSphere clusters.
What Is a vSphere Cluster in VMware vSphere?
A vSphere cluster is a logical grouping of two or more ESXi hosts centrally managed by vCenter Server. When hosts are added to a cluster, their CPU and memory resources are pooled, enabling virtual machines to move seamlessly across hosts as if they were running on a single, larger system. This compute pooling is essential for features like vMotion, High Availability (HA), and VMware DRS, which rely on clusters to optimize workloads.
Clusters transform separate servers into a single shared compute pool. Instead of managing ten individual hosts, a 10-node cluster appears in vCenter as one resource entity. Workloads are automatically placed and migrated, ensuring critical applications have sufficient CPU and memory, while less critical virtual machines utilize remaining capacity. This flexibility allows organizations to scale environments, prioritize business-critical workloads, and maintain performance during spikes in demand.
What Is Distributed Resource Scheduler (DRS)?
Distributed Resource Scheduler (DRS) is a vSphere feature that automatically manages and balances compute resources across a cluster of ESXi hosts. It continuously monitors the load on each host and makes real-time placement and migration decisions to ensure virtual machines always have the resources they need.
In practical terms, this means administrators don’t have to manually track CPU and memory utilization or move workloads between hosts to avoid bottlenecks. DRS intelligently shifts VMs to maintain balance across the cluster.
For example, if one host is consistently operating at 90% CPU utilization while others in the same cluster are only at 40-50%, DRS can automatically vMotion some VMs away from the overloaded host. This prevents application slowdowns, keeps performance consistent, and reduces the risk of outages during peak demand. Similarly, when a new VM is powered on, DRS chooses the most suitable host, avoiding resource contention before it becomes an issue.
Organizations rely on DRS because it:
- Reduces manual administration by automatically balancing workloads.
- Enhances application performance by preventing resource hot spots.
- Facilitates maintenance by automatically migrating workloads during host maintenance.
- Supports scalability by adapting workload placement as the environment grows.
When to Use VMware DRS?
DRS is most effective in medium and large VMware clusters where workloads are dynamic and resource demands fluctuate throughout the day. It provides the greatest value when:
- A cluster has three or more hosts, giving the system enough flexibility to rebalance workloads.
- Applications are sensitive to performance bottlenecks and require consistent resource availability.
- The environment supports non-disruptive maintenance, allowing patching and upgrades without downtime.
- Organizations run CI/CD pipelines or seasonal workloads (for example, e-commerce peaks), where demand is unpredictable and balancing needs to be continuous.
VMware DRS Requirements and Licensing
To enable DRS, several prerequisites must be met:
vCenter Server
Clustering in vSphere is only possible through vCenter, which provides the centralized intelligence to manage and balance resources across multiple ESXi hosts.
Enterprise Plus License
DRS is not included in Standard or Essentials editions of vSphere. It requires an Enterprise Plus license, which unlocks advanced automation features.
vMotion
Since DRS relies on live migration to move VMs between hosts without downtime, a properly configured vMotion network is mandatory. This includes dedicated networking with sufficient bandwidth (commonly 10 GbE or higher in production environments) and shared storage accessible by all hosts. VMware reports that over 90% of customers who enable DRS use vMotion daily to maintain performance.
Licensing Bundles
With newer VMware offerings such as VMware Cloud Foundation (VCF) and VMware vSphere Foundation (VVF), DRS comes bundled and can be enabled at the cluster level. This simplifies adoption for organizations modernizing their licensing.
Key Features of VMware Distributed Resource Scheduler (DRS)
Distributed Resource Scheduler (DRS) comes with a set of capabilities that help administrators manage resources more efficiently. Each feature is designed to address a specific operational challenge in clustered environments:
Load Balancing
vSphere DRS continuously monitors CPU and memory usage across all hosts in the cluster. When it detects that one host is overutilized while others have spare capacity, it automatically migrates VMs to balance the workload. This prevents performance degradation and ensures fair resource allocation.
Why it matters: Without DRS, admins would have to manually track host utilization and decide when to move workloads – a time-consuming process that often happens too late.
Example: VMware reports that clusters with DRS enabled experience 20-25% fewer performance-related incidents compared to manually managed clusters.
Initial Placement
When a VM is powered on, DRS evaluates the available hosts and selects the one that can provide the best performance based on current load and configured policies.
Why it matters: Poor initial placement can lead to resource contention right from the start. DRS avoids this by making an intelligent decision upfront.
Example: If two hosts are at 60% and 30% CPU usage, a newly started VM will be automatically placed on the less loaded host.
Maintenance Mode Support
When a host needs to be patched, upgraded, or replaced, administrators can put it into maintenance mode. DRS automatically migrates (vMotion) all running VMs to other hosts in the cluster before the maintenance begins.
Why it matters: This eliminates downtime and manual VM migrations during maintenance windows.
Example: In a 12-host cluster, patching with DRS takes up to 70% less administrator time compared to manual migration.
Resource Pools
DRS allows administrators to organize virtual machines into resource pools with defined CPU and memory shares, limits, and reservations. This way, resources can be aligned with business priorities.
Why it matters: Not all workloads are equally important. Resource pools ensure critical applications always get what they need, even under contention.
Example: An ERP system might be guaranteed minimum CPU shares, while dev/test workloads receive fewer, ensuring business continuity during busy hours.
Affinity and Anti-Affinity Rules
DRS supports rules that define whether certain VMs should always run on the same host (affinity) or be kept apart (anti-affinity).
Why it matters: These rules are essential for both performance optimization and fault tolerance.
Examples:
- Two VMs in a multi-tier application (web + app) may benefit from being on the same host for faster communication.
- Redundant domain controllers should always be kept on separate hosts to avoid a single point of failure.
Predictive DRS
When integrated with vRealize Operations (vROps), DRS can use predictive analytics to anticipate future resource demand and migrate workloads before contention occurs.
Why it matters: Instead of reacting to problems after they happen, Predictive DRS proactively prepares the cluster.
Example: If analytics show that CPU demand spikes every morning at 9 a.m., Predictive DRS can redistribute workloads at 8:55 a.m., ensuring users experience no slowdowns.
High Availability (HA) vs. Distributed Resource Scheduler (DRS)
High Availability (HA) and Distributed Resource Scheduler (DRS) are often mentioned together, but they solve very different challenges inside a vSphere cluster. HA focuses on availability. If a host fails, HA automatically restarts affected virtual machines on surviving hosts to minimize downtime. DRS focuses on performance and efficiency. It continuously monitors resource usage and moves workloads to prevent hotspots during normal operations.
Here’s a side-by-side comparison:
| Feature | Purpose | Key Benefit | Example Scenario |
| HA | Ensures uptime by restarting VMs on surviving hosts if a host fails | Minimizes downtime, improves resilience | If a host with 20 VMs fails, HA automatically restarts them on other hosts within minutes |
| DRS | Ensures balanced performance by distributing workloads across hosts | Prevents hotspots, optimizes resource usage | If one host hits 90% CPU and another is at 40%, DRS moves VMs to balance the load |
| HA + DRS Together | Combines resilience with performance optimization | Better uptime, smoother operations, reduced admin work | After HA restarts VMs on another host, DRS may migrate them again to balance the cluster |
So, HA ensures recovery in case of failure, while DRS ensures ongoing performance efficiency. Together, they provide both stability and optimal performance for VMware clusters.
Advantages and Limitations of VMware DRS
Enabling Distributed Resource Scheduler (DRS) gives IT teams several benefits:
- Better performance. By continuously balancing workloads, DRS prevents hotspots and ensures applications run smoothly. VMware case studies show that enabling DRS can improve application responsiveness by up to 30% compared to manually managed clusters.
- Lower operational overhead. Administrators no longer need to constantly monitor utilization and move VMs by hand. Large environments often save dozens of admin hours per month, freeing IT staff for higher-value tasks.
- Smarter maintenance. DRS automatically migrates VMs away from a host that is placed into maintenance mode. This makes patching, upgrades, or hardware replacements far less disruptive. In large clusters, this process has been shown to cut manual effort by around 70%.
- Business alignment. Using resource pools, organizations can ensure that critical workloads, such as ERP or database systems, always get guaranteed access to CPU and memory, while dev/test systems can be deprioritized.
- Elastic growth. As new hosts are added to a cluster, DRS automatically redistributes workloads, allowing the environment to scale smoothly without requiring reconfiguration.
Despite its advantages, DRS has some constraints that must be considered:
- Cost. The feature is only available with Enterprise Plus licensing or bundled in VMware Cloud Foundation (VCF) and VMware vSphere Foundation (VVF). For smaller businesses, this cost may be prohibitive.
- Infrastructure dependencies. DRS relies on vMotion and shared storage. To work effectively, the cluster must have properly configured shared datastores and a fast, dedicated vMotion network (commonly 10 GbE or higher in production). Without these, the benefits of DRS are reduced.
High Availability in Small Clusters
In smaller VMware environments, especially 2-node clusters, High Availability (HA) typically plays a more important role than advanced load balancing. While DRS has limited impact in such setups, reliable shared storage is still essential to make HA possible.
This is where StarWind Virtual SAN provides a distinct advantage. By mirroring local disks between two hosts, it eliminates the need for an external SAN, reducing both cost and complexity. The result is a high availability VMware cluster that can withstand host failures while remaining affordable and easy to manage – a combination particularly valuable for SMB and ROBO deployments.
For larger VMware clusters, DataCore SANsymphony can serve as an advanced storage platform. It integrates smoothly with VMware, supports HA configurations, and works alongside features like DRS, making it suitable for enterprises that require flexible, software-defined storage at scale.
Conclusion
VMware Distributed Resource Scheduler (DRS) is a practical feature that delivers real benefits in medium and large vSphere clusters. By automating workload placement and balancing, it helps avoid performance bottlenecks, reduces the need for manual intervention, and keeps applications running consistently.
When used together with High Availability (HA), DRS provides both resilience and efficiency: HA reacts to failures, while DRS ensures resources are continuously optimized during normal operations. For organizations already managing clusters with vSphere, enabling DRS is a relatively simple step that improves stability, simplifies daily operations, and supports long-term scalability.
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