What is DRS (Distributed Resource Scheduler) and HA (High Availability) in VMware

DevOpsbyAshish February 20, 2025

 Both DRS and HA are vSphere features that help manage and optimize resources in a VMware cluster, but they serve different purposes.

1. DRS (Distributed Resource Scheduler)

DRS is responsible for balancing the compute resources (CPU & RAM) across ESXi hosts in a vSphere cluster. It uses vMotion to move VMs dynamically to ensure optimal resource utilization.

  • Purpose: Load balancing of workloads.
  • How it works:
    • Continuously monitors resource usage.
    • Automatically migrates VMs (using vMotion) to balance CPU & memory.
    • Can be configured as manual, partially automated, or fully automated.
  • Benefits:
    • Prevents resource contention.
    • Ensures even distribution of workloads.
    • Reduces performance bottlenecks.

2. HA (High Availability)

HA is designed to minimize downtime by restarting VMs on another ESXi host if a host failure occurs.

  • Purpose: Automatic failover & VM restart after host failure.
  • How it works:
    • Uses a master/slave architecture.
    • Detects host failures and restarts VMs on other available hosts.
    • Works with VMware heartbeat mechanism to monitor VM availability.
  • Benefits:
    • Minimizes service downtime.
    • Automatically restarts VMs after a hardware failure.
    • Works with vSphere Fault Tolerance (FT) for zero-downtime protection.

🌟 VMware DRS vs. HA 🌟

Feature🟢 DRS (Distributed Resource Scheduler)🔴 HA (High Availability)
🔍 PurposeBalances workloads dynamicallyProtects against host failures
⚡ TriggerCPU/Memory imbalanceHost failure detected
🛠️ Technology UsedvMotion (Live VM Migration) 🚀VM Restart on another host 🔄
⏳ DowntimeNo downtime (Live Move) ✅Some downtime (Restart Required) ⏳
🤖 AutomationManual / Partial / Fully AutomatedFully Automated
📊 BenefitPrevents performance bottlenecks 📈Ensures business continuity 🏢

🚀 Prerequisites to Configure DRS & HA

✅ Common Requirements:

🔹 vSphere Cluster – Group of ESXi hosts.
🔹 vCenter Server – Required to manage the cluster.
🔹 Shared Storage – NFS, iSCSI, or FC storage.

🌍 DRS-Specific Needs:

🟡 vMotion Enabled – VMkernel port & proper networking.
🟡 EVC (Enhanced vMotion Compatibility) – Helps with CPU differences.

🔴 HA-Specific Needs:

🔵 Management Network Redundancy – HA heartbeat protection.
🔵 Admission Control Configuration – Defines how many hosts can fail while keeping VMs running.


  1. DRS Preemptive vs. Non-Preemptive

    • Preemptive DRS: Moves VMs proactively before resource contention occurs.
    • Non-Preemptive DRS: Reacts only when resource usage crosses a threshold.

  2. HA Preemptive vs. Non-Preemptive

    • Preemptive HA: Proactively detects host failures and restarts VMs immediately.
    • Non-Preemptive HA: Waits until a failure is confirmed before restarting VMs.

🌟 VMware DRS vs. HA - Visual Representation

1️⃣ DRS (Distributed Resource Scheduler) - Load Balancing

📌 Scenario:

  • Host 1 is overloaded (🚨 High CPU & RAM usage).
  • Host 2 is underutilized (🟢 Free resources).
  • DRS migrates VMs using vMotion to balance resources.

📊 Diagram:


Before DRS:                      After DRS (Balanced Load):
-------------------------------------------------------
[Host 1]  🟠🟠🟠🟠🟠  (Overloaded)       [Host 1]  🟠🟠🟠  (Balanced)
[Host 2]  ⚪⚪⚪⚪⚪  (Underloaded)       [Host 2]  🟠🟠  (Now Utilizing Resources)
-------------------------------------------------------

🚀 No downtime! VMs are live-migrated without interruption.

2️⃣ HA (High Availability) - Failover Protection

📌 Scenario:

  • Host 1 fails suddenly (💥 Hardware crash).
  • HA detects the failure and automatically restarts VMs on other hosts.

📊 Diagram:


Before HA:                      After HA Recovery:
-------------------------------------------------------
[Host 1]  ❌ (FAILED)            [Host 1]  ❌ (Down)
[Host 2]  🟢🟢🟢🟢🟢 (Restarting VMs)   [Host 2]  🟢🟢🟢🟢🟢 (All VMs Running Again)
-------------------------------------------------------

🔄 Some downtime occurs (VM restart needed), but recovery is automatic!

🔄 DRS vs. HA Summary (Quick Color Guide!)

Feature🌟 DRS (Balance Load)🔥 HA (Failover Protection)
PurposeKeep workloads evenly distributed ✅Restart VMs after host failure ⚠️
Uses?vMotion 🚀HA Heartbeat Monitoring ❤️
Trigger?High resource usage 📊ESXi host failure 💥
Downtime?❌ None (Live migration)⏳ Some (VMs restart)
Who benefits?Performance Optimization ⚡Business Continuity 🔄


DevOpsbyAshish

Posted by DevOpsbyAshish