VMware High Availability vs Fault Tolerance vs Disaster Recovery

Published on October 30, 2022

VMware offers several great products with a key focus on keeping your workloads online. In this post, we’ll discuss which technologies we can utilize to maintain high levels of availability and zero downtime, as well as our options when a disaster strikes.

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VMware’s High Availability Solution Included With vSphere

Both the Standard and Enterprise Plus licensed versions of VMware’s vSphere solution include High Availability options.

High Availability kicks in when an ESXi host fails. When this happens many things occur behind the scenes but the main point here is that virtual machines which were running on the failed host, are automatically rebooted onto other hosts within the same cluster.

This feature is not enabled by default, however, you need to have created a cluster containing more than 1 ESXi host and enabled the High Availability option in the configuration settings of the cluster.

vSphere High Availability Configuration is Simple

There are a series of configurable options for vSphere High Availability that are designed to be easy and quick to roll out. 

• Failure Response. Should an ESXi host fail, you can choose to restart VMs onto other operational hosts in the cluster or turn off the feature altogether

• Response for Host Isolation. If a host becomes isolated from other ESXi hosts on the network but is still online, you can choose to do nothing, power off and restart VMs on other hosts or attempt a clean shutdown of the VMs and restart them on other hosts.

• Datastore with PDL. vSphere can also monitor datastore health and perform actions accordingly. If a datastore is detected to be completely offline (Permanent Device Loss) then vSphere can attempt to restart virtual machines on other hosts which still have access to the datastore.

• Datastore with APD. Should virtual machines be running on an ESXi host that suffers an All Paths Down event (APD), then similar options are av

  ailable to configure as with PDL. Here you can choose how long to wait before vSphere responds to the APD event. This option is available because APD issues can recover automatically.

VM Monitoring. Virtual Machines Monitoring is interesting — it allows you to automatically reboot virtual machines if the VMware tools' heartbeat has not been received by the host in a configurable period. This can be useful if a virtual machine becomes unresponsive or has a BSOD event. VM Monitoring should be configured with care to prevent false positives. What is VMware Fault Tolerance?

HA is a great vSphere feature, but a reactive one. You'll be able to maintain reasonable virtual machine availability but depending on your infrastructure, virtual machines could take up to 15 minutes or more to reboot. This is where the power of fault tolerance comes in.

VMware’s Fault Tolerance solution creates a copy of a virtual machine on another host, ideally in another location on separate storage. Everything that happens within the virtual machine including all storage, memory pages, and CPU cycles is transferred rapidly to the secondary virtual machine, maintaining an exact replica of the primary at all times.

Should the host running the primary virtual machine suffer an outage, the secondary virtual machine is instantly made primary and continues as if nothing happened.

What is vSphere Fault Tolerance Useful for?

This kind of technology is best suited for applications that do not have built-in clustering support. Most often this is ideal for legacy applications that require the highest levels of availability but cannot be re-developed to achieve high expectations of availability.