What is vSphere High Availability (HA) and How It Works

If you want to build a 24/7 IT infrastructure, it is essential to ensure the availability of your virtual machines during the business hours. This article will give you’re an overview of vSphere High Availability (HA).


By Crystal / Updated on February 2, 2023

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What is vSphere High Availability

Virtualization has certainly changed the way many enterprises organize their business. When virtual machines are running, it is essential to ensure the availability of them during the business hours. Therefore, VMware provide the clustering feature to ensure high availability.

VMware vSphere High Availability (HA) is a utility included in vSphere suite that provides high availability for virtual machines. It pools the virtual machines and the hosts they reside on into a cluster for monitoring, automatically restart failed virtual machines on alternative host servers to reduce application downtime.


VMware first introduced vSphere HA in Virtual Infrastructure 3 in 2006 and has continued to develop and support the feature. So far vSphere has provided essential failover protection for many enterprises. Next, this article will give you an overview of how vSphere HA works and its specific features.

How vSphere High Availability works

vSphere HA uses a utility called the Fault Domain Manager agent to monitor ESXi host availability and to restart failed VMs. When configuring vSphere HA, you define a group of servers as a HA cluster, and the Fault Domain Manager runs on each host within the cluster.

A single host is automatically elected as the primary host. The primary host communicates with vCenter Server and monitors the state of all protected virtual machines and of the secondary hosts.

Failover cluster

Different types of host failures are possible, and the primary host uses network and datastore heartbeating, a periodic message that indicates the host is running status, to determine the type of failure. After that, the primary host must distinguish between a failed host and one that is in a network partition or that has become network isolated, and appropriately deal with the failure.

In the case of a VM failure in which the host server continues to run, vSphere HA restarts the VM on the original host. If an entire host fails, the utility restarts all affected VMs on other hosts in the cluster.

VMware vSphere High Availability features

VMware vSphere High Availability mostly handle the following 3 scenarios:

  • ESXi host monitoring: if an ESXi host fails, VMs that were running on that host are automatically restarted on other ESXi hosts.
  • Guest OS monitoring: if the VM Monitoring option is enabled and the VM stops sending heartbeats, the guest OS is reset. The VM stays on the same ESXi host.
  • Application monitoring: the agent on an ESXi host can monitor heartbeats of applications running inside a VM. If an application fails, the VM is restarted, but it stays on the same host. This type of monitoring requires a third-party application monitoring agent and VMware Tools.

Create vSphere backup to protect critical VMs from accidental loss

vSphere High Availability can recover the business running on virtual machines in time to reduce downtime. However, even the perfect cluster configuration cannot completely prevent unexpected data loss. With vSphere 6.5 become the last release that includes vSphere Data Protection, VMware has decided to exit the data protection market and focus its investments on VMware vSphere Storage APIs.

That means you may need to migrate your backup policies to other third-party software. If you still haven’t chosen one, here I introduce you to a VMware backup software AOMEI Cyber Backup, it enables you to backup multiple VMs in VMware vSphere within 3 simple steps. And it offers you the following benefits:

Agentless Backup: create complete and independent image-level backup for VMware ESXi and Hyper-V VMs.
Support Free ESXi: AOMEI Cyber Backup support both paid and free versions of VMware ESXi.
Flexible vSphere Backup: backup large numbers of VMs managed by vCenter Server, or multiple VMs on standalone ESXi host.
Automated Execution: schedule to automate backups daily, weekly, and monthly with email notifications.
Role Assignment: allows one administrator to create sub-accounts with limited privileges.

AOMEI Cyber Backup

AOMEI Cyber Backup supports VMware vSphere/ESXi 6.0 and later versions. And simplifies the process of backing up VMs into 3 simple steps. Next, I will show you how to backup multiple VMware ESXi VMs via AOMEI Cyber Backup. You can click the following button to download the free trial.

Download Free trialVMware ESXi & Hyper-V
Secure Download

*You can choose to install this VM backup software on either Windows or Linux system.

3 simple steps to create a vSphere backup task

1. Bind Devices: Access to AOMEI Cyber Backup web client, navigate to Source Device > VMware > + Add VMware Device to Add vCenter or Standalone ESXi. And then click > Bind Device.

Add VMware ESXi host

2. Create Backup Task: Navigate to Backup Task > + Create New Task, select the Device Type as VMware ESXi Backup, then you can configure the Task NameDeviceTargetSchedule and Backup Cleanup.

  • Device: you can select multiple VMs from added vCenter Server or standalone ESXi host.
  • Target: select to back up to a local path, or to a network path.
  • Schedule: choose to perform full, differential or incremental backup, and automate execution according to the frequency you specified.
  • Backup cleanup: configure a retention policy to auto delete old backup files and save storage space.

Backup schedule type

3. Run Backup: Click Start Backup and select Add the schedule and start backup now, or Add the schedule only.

Start Backup

Created tasks will be listed and monitored separately for restoring, progress checking and schedule changing.

When you want to restore from a backup, you can choose to Restore to original location or Restore to new location

Restore to new location: Create a new VM in the same or another datastore/host directly from the backup, saves the trouble of re-configuring the new VM.

Restore to new location


If you are looking for a way to failover and reduce downtime, then vSphere High Availability may be just what you need. In this article, I briefly introduced what is vSphere High Availability, how vSphere HA works, and its specific features.

Except for High Availability, vSphere also provide Fault Tolerance (FT) and Redundancy for you to build a 24/7 IT infrastructure and disaster recovery plan. Check more details in High Availability vs Fault Tolerance vs Redundancy.

Crystal · Editor
Crystal is an editor from AOMEI Technology. She mainly writes articles about virtual machine. She is a positive young lady likes to share articles with peolpe. Off work she loves travelling and cooking which is wonderful for life.