Table of Contents
Introduction
Ensuring high availability (HA) and efficient resource management is crucial in a virtualized environment. VMware vSphere offers two powerful features High Availability (HA) and Distributed Resource Scheduler (DRS) to meet these goals. This guide will walk you through setting up and configuring HA and DRS, with a focus on advanced options such as Admission Control and Proactive HA.
What is VMware HA?
VMware HA is like a safety net for your virtual machines. Imagine you have several physical servers (called hosts) running virtual machines (VMs). If one of these servers’ crashes or stops working, the VMs on it would normally go down too. That can cause big problems for your business.
With VMware HA:
If one host fails, VMware HA automatically restarts those VMs on another healthy host in the cluster. This happens quickly and with little downtime, so your services keep running smoothly. You don’t have to manually move or restart anything HA does it for you
Why it’s important
VMware HA is important because it helps keep your applications and services online, even if a hardware failure occurs. By automatically restarting virtual machines on another available host in the cluster, it eliminates the need for manual intervention and significantly reduces recovery time. This automation not only saves valuable time for IT teams but also ensures that critical business operations continue with minimal disruption. As a result, VMware HA helps businesses avoid costly downtime and maintain high levels of availability and reliability in their virtualized environments.
Key Benefits:
- Reduces manual intervention during hardware failure
- Supports automated VM restart
- Works well with other features like DRS and vMotion
What is VMware DRS?
VMware DRS (Distributed Resource Scheduler) is a smart feature that helps your virtual environment run smoothly. Imagine you have several computers (called ESXi hosts) working together in a group (a cluster), and each one is running multiple virtual machines (VMs). Sometimes, one computer might be doing too much work while others are sitting idle.
DRS watches these computers and their workloads. If one computer is too busy and another has more room, DRS automatically moves some virtual machines to the less busy computer. It does this using a feature called vMotion, which moves the VMs without shutting them down.
Key Benefits:
- Optimized resource distribution
- Minimizes performance bottlenecks
- Can be manual, partially automated, or fully automated
Step-by-Step: Create a Cluster with HA and DRS Enabled
Prerequisites:
- vCenter Server installed
- At least two ESXi hosts
- Shared storage (e.g., iSCSI, NFS, vSAN)
Steps:
Login to vCenter Server:
Open your web browser and go to the vCenter address (like https://vcenter.vmorecloud.com). Enter your username and password to log in.
Go to Hosts and Clusters:
Once you’re inside vCenter, look at the left-hand side and click on “Hosts and Clusters.” This is where you manage your physical servers (ESXi hosts) and virtual machines.
Create a New Cluster:
Right-click on your Datacenter name (this is usually at the top level) and choose “New Cluster.”
Give It a Name & Enable Features:
A setup wizard will open. Type a name for your new cluster (for example, “Production-Cluster”).
✅ Check the boxes for:
- vSphere DRS (to balance workloads)
- vSphere HA (for automatic VM restart if a host fails)
Add Your ESXi Hosts to the Cluster:
After the cluster is created, right-click on it and choose “Add Host.” Enter the IP address or name of each ESXi host and login details. Repeat for all hosts.
Review & Finish:
Go through the summary page to confirm your settings. Click “Finish” when you’re done. Your cluster is now ready with HA and DRS enabled!
How to Configure VMware HA (High Availability)
Once you’ve enabled HA in your cluster, you’ll want to adjust a few settings to make sure it behaves the way you want when something goes wrong.
🖥️ Host Monitoring – What It Does
When you enable HA on a cluster in VMware vSphere, it starts monitoring the health and availability of all ESXi hosts in that cluster. Think of it like a watchdog service that’s constantly checking if your physical servers (ESXi hosts) are still functioning.
If one host stops responding maybe because of a hardware failure or power loss HA will detect it and automatically restart the VMs that were running on that host on another healthy one.
If one of your ESXi hosts crashes due to:
- hardware failure
- power outage
- network disconnection
then HA steps in. It notices that the host is unresponsive and takes action by:
- Identifying which VMs were running on the failed host.
- Finding a healthy ESXi host in the cluster that has enough free resources (CPU, RAM).
- Restarting those affected VMs on the healthy host.
This helps ensure minimal downtime and keeps your critical services running without manual intervention.
🧠 VM Monitoring – What It Does
This feature focuses on the virtual machines (VMs) themselves—not the host. It watches for heartbeat signals from VMware Tools inside each VM. If the heartbeat stops for a while (maybe the VM has frozen or crashed), VMware HA restarts just that VM to bring it back online. It’s like a built-in safety net to make sure your applications are always running.
✅ Why These Settings Matter:
- They help keep your services running even if a server fails or a VM crashes.
- They automate recovery, so you don’t have to restart things manually.
- They reduce downtime, which keeps your users happy and your business running smoothly.
How Admission Control in VMware HA?
Think of it like a safety limit
When you enable High Availability (HA), VMware needs to make sure there’s enough room (CPU and memory) left on other hosts in case one of your ESXi hosts fails. That’s where Admission Control comes in.
What It Does:
Admission Control blocks new virtual machines from being powered on if doing so would prevent HA from being able to restart all VMs in the event of a host failure.
How it Works (in simple terms):
Let’s say you have 3 ESXi hosts and 30 VMs running. Admission Control ensures that if 1 host fails, the remaining 2 can handle all 30 VMs. If you try to power on more VMs than that capacity, it won’t let you to protect HA capability.
Options You Can Choose:
- Host Failures Cluster Tolerates (Most common)
- Example: You set this to 1. It reserves resources so the cluster can survive 1 host failure.
- Percentage of Cluster Resources Reserved
- You set aside a percent (e.g., 25%) of CPU/memory.
- Dedicated Failover Hosts
- You reserve specific hosts just for failover.
🚨 What is Isolation Response?
🧰 Imagine a host is still running—but disconnected
Sometimes a host loses its connection to the rest of the cluster (e.g., due to network issues), but it hasn’t crashed. VMware HA must decide what to do in this “isolation” scenario.
👇 Isolation Response Options:
- Power Off and Restart VMs (Recommended for most setups)
- HA powers off the VMs on the isolated host and restarts them elsewhere.
- Leave VMs Powered On
- VMs keep running, but this can cause data conflicts.
- Shutdown and Restart VMs
- Tries to shut down gracefully before restarting on another host.
🧠 Tip:
Choose “Power Off and Restart” unless you have shared storage and advanced clustering apps that can handle split-brain situations.
Configuring VMware DRS
Automation Levels:
Manual: You make all decisions yourself. DRS gives placement and migration suggestions, but you must take action manually. This is ideal for testing or environments where automatic movement is risky.
Partially Automated: Initial VM placement is automatic when powering on. Migration suggestions are made, but you must approve them. This suits medium-trust environments or periods with strict change control.
Fully Automated: DRS automatically places VMs at power-on and migrates them as needed to balance the load. Perfect for production clusters with consistent workloads and trusted configuration.
Migration Threshold:
Set the threshold to control sensitivity (1 = Conservative, 5 = Aggressive).
Affinity Rules:
Affinity Rules in VMware DRS are used to control the placement of virtual machines to meet specific workload or compliance requirements.
- VM-VM Affinity: ensures that certain virtual machines are always placed on the same host. This is useful when VMs need low-latency communication or are part of a tightly coupled application.
- VM-Host Affinity: ties specific VMs to specific ESXi hosts. This is typically used for licensing compliance, performance optimization, or regulatory requirements, ensuring those VMs only run on designated hardware.
| DRS Level | Initial VM Placement | VM Migration (Load Balancing) | Admin Control |
|---|---|---|---|
| Manual | You decide | You decide | 100% manual |
| Partially Automated | Automatic | You decide | 50% manual |
| Fully Automated | Automatic | Automatic | 0% manual |
Advanced Option: Proactive HA
Proactive HA works with hardware monitoring tools (like Dell OpenManage or HPE iLO). It detects hardware issues before they cause failure.
How it Works:
vCenter receives a degradation alert from the hardware. It moves VMs off the affected host.
The host is quarantined or placed in maintenance.
Requirements: Compatible hardware monitoring plugin installed in vCenter.
Pro Tip: Proactive HA is great for preventing issues, but you need a vendor-specific health provider plugin.
Conclusion
Configuring VMware HA (High Availability) and DRS (Distributed Resource Scheduler) is essential for maintaining a resilient and balanced virtual environment. These tools work together to minimize downtime, improve performance, and ensure workload continuity in the face of host failures or resource contention. With advanced features like Admission Control, which prevents resource overcommitment, and Proactive HA, which anticipates hardware issues before they impact workloads, administrators can proactively safeguard uptime. Additionally, DRS ensures optimal resource distribution across the cluster, automatically migrating virtual machines to maintain performance equilibrium. By leveraging these capabilities, organizations can build a more efficient, fault-tolerant, and self-healing infrastructure that aligns with modern IT demands.
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