vSphere High Availability explained

A highly available system is one that is continuously operational for an optimal period of time. There are multiple ways to achieve high availability for systems, such as using HA applications, redundant NICs, server clusters, redundant power supplies, etc. You can also achive high availability at the virtualization layer. In vSphere, a feature called vSphere High Availability is used to provide high availability at the virtualization layer.

vSphere HA protect against the following types of failures:

• ESXi host failure – if an ESXi host fails, VMs that were running on that host are automatically restarted on other ESXi hosts.
• Guest OS failure – 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 failure – 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.

vSphere HA is configured on a cluster. A cluster is a collection of ESXi hosts configured to share their resources. Up to 32 ESXi hosts and 4000 VMs per cluster are supported.