Introduction to vSphere High Availability
Availability has traditionally been one of the most important aspects when providing services. When providing services on a shared platform like VMware vSphere, the impact of downtime exponentially grows and as such VMware engineered a feature called VMware vSphere High Availability. VMware vSphere High Availability, hereafter simply referred to as HA, provides a simple and cost effective solution to increase availability for any application running in a virtual machine regardless of its operating system. It is configured using a couple of simple steps through vCenter Server (vCenter) and as such provides a uniform and simple interface. HA enables you to create a cluster out of multiple ESXi or ESX
servers. We will use ESXi in this book when referring to either, as ESXi is the standard going forward. This will enable you to protect virtual machines and hence their workloads. In the event of a failure of one of the hosts in the cluster, impacted virtual machines are automatically restarted on other ESXi hosts within that same VMware vSphere Cluster (cluster).
Figure 1: High Availability in action
On top of that, in the case of a Guest OS level failure, HA can restart the failed Guest OS. This feature is called VM Monitoring, but is sometimes also referred to as VM-HA. This might sound fairly complex but again can be implemented with a single click.
Figure 2: OS Level HA just a single click
Unlike many other clustering solutions, HA is a fairly simple solution to implement and literally enabled within 5 clicks. On top of that, HA is widely adopted and used in all situations. However, HA is not a 1:1 replacement for solutions like Microsoft Clustering Services (MSCS). The main difference between MSCS and HA being that MSCS was designed to protect stateful cluster-aware applications while HA was designed to protect any virtual machine regardless of the type of application within.
In the case of HA, a failover incurs downtime as the virtual machine is literally restarted on one of the remaining nodes in the cluster. Whereas MSCS transitions the service to one of the remaining nodes in the cluster when a failure occurs. In contrary to what many believe, MSCS does not guarantee that there is no downtime during a transition. On top of that, your application needs to be cluster-aware and stateful in order to get the most out of this mechanism, which limits the number of workloads that could really benefit from this type of clustering.
One might ask why would you want to use HA when a virtual machine is restarted and service is temporarily lost. The answer is simple; not all virtual machines (or services) need 99.999% uptime. For many services the type of availability HA provides is more than sufficient. On top of that, many applications were never designed to run on top of an MSCS cluster. This means that there is no guarantee of availability or data consistency if an application is clustered with MSCS but is not cluster-aware.
In addition, MSCS clustering can be complex and requires special skills and training. One example is managing patches and updates/upgrades in a MSCS environment; this could even lead to more downtime if not operated correctly and definitely complicates operational procedures. HA however reduces complexity, costs (associated with downtime and MSCS), resource overhead and unplanned downtime for minimal additional costs. It is important to note that HA, contrary to MSCS, does not require any changes to the guest as HA is provided on the hypervisor level. Also, VM Monitoring does not require any additional software or OS modifications except for VMware Tools, which should be installed anyway as a best practice. In case even higher availability is required, VMware also provides a level of application awareness through Application Monitoring, which has been leveraged by partners like Symantec to enable application level resiliency and could be used by in-house development teams to increase resiliency for their application.
HA has proven itself over and over again and is widely adopted within the industry.
