# Virtual Machine High Availability (VM HA)

## What is Virtual Machine High Availability (VM HA)

Virtual Machine High Availability (VM HA) is a core <code class="expression">space.vars.product\_name</code> capability that automatically detects physical host failures within a virtualized cluster and restarts the affected VMs on other healthy hosts in that same cluster.

Platform9 VM HA delivers a safety net for production workloads being deployed on virtualized clusters. By turning on VM HA for your cluster, you can ensure that the VMs running on this cluster will remain operating across host failures.

## How VM HA Works

The process is designed to be automatic and requires minimal manual intervention during a failure event:

1. **Continuous Host Monitoring:** <code class="expression">space.vars.product\_name</code> constantly monitors the health and responsiveness of all hypervisor hosts participating within an HA-enabled cluster.
2. **Failure Detection:** If a host stops responding (due to hardware failure, operating system crash, or certain network isolation scenarios), the system detects the failure.
3. **Automatic VM Recovery:** Once a host failure is confirmed, which involves both the management plane and cluster hosts to confirm failure, VM HA automatically initiates the process of restarting the VMs that were running on the failed host. These VMs are powered on using available resources on the remaining healthy hosts within the cluster.

## Key Concepts and Requirements

Understanding these concepts is helpful when working with Platform9 VM HA:

* **Clusters:** VM HA operates at the virtualized cluster level. You need to turn it on as a cluster level property.
* **Shared Storage:** This is a critical prerequisite. For VMs to be successfully restarted on a different host, their virtual disks must reside on storage that is accessible to all potential failover hosts within the cluster (e.g., SAN, NAS, or other supported shared storage solutions). VMs using only host-local storage generally cannot be automatically recovered by HA on another host.
* **Host Requirements:** VM HA requires a minimum number of healthy hosts in a cluster to function correctly. A minimum of four hosts is required for HA activation. (Support for two-node clusters is coming in the June release of <code class="expression">space.vars.product\_name</code>).
* **Configuration:** VM HA is a configurable option when creating or managing a cluster within the <code class="expression">space.vars.product\_name</code> interface (API/CLI or UI). It’s often enabled by default for new clusters.


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