Informational Guides

What is VMware vSphere HA ?

Every organization uses servers for their products or services but at the time of disaster or any of the fault event it is necessary to have the availability of the server so that user or client can not face the problem regarding the services. In virtual Environment’s it is necessary for the IT team to provide the availability of the servers to provide the 99.99% Uptime. Nowdays most of the companies use VMware vSphere for the creation & management of the servers. So in order to overcome the downtime & increase the availability VMware has introduced the feature called High Availability. Further we will be using HA instead of the full work High Availability.

First we need to know what is HA (High Availability) & How it works ?
HA simply means to provide the availability of the virtual machines in case of the ESXi host failures or crash. HA monitors the ESXi hosts in order to protect the virtual machines. HA works on the cluster where all the ESXi hosts are added into a cluster. You need to enable the HA on the cluster settings in order to use the HA to protect the virtual machines.

HA provides the simple method for the availability of the virtual machines. HA works on the cluster created inside the datacenter in vCenter Server. So you can easily enable HA on the cluster with few clicks. Cluster contains the groups of ESXi hosts. In the event of failure or crash of ESXi host HA will occur & it will restart the virtual machine on the other available hosts so that the virtual machines can be available if the host completely fails. In order to achieve the availability HA will provide you with the minimum downtime is only the restart of the virtual machine which only takes few minutes.

As you can see in the Above Image is that there are 3 ESXi hosts are there which are in HA enabled Cluster but first host is failed because of some of the reason so the available virtual machines on that host will be automatically restarted on the other hosts by HA.

As of now you are aware about the what is HA & How it works. Now we will go into some deep information about the High Availability.

HA works on the master & slave architecture. when you enable HA on the cluster then election process occurs between all the hosts in the cluster & one host which has large number of datastore mounted has an chance to become an master server. Once the election process completes there will be one master server & other esxi are considered as the slave server. If the master server goes down or crashes then the new election process will occur.

Master host monitors the ESXi hosts & Virtual Machines in the cluster in order to protect them. If one of the ESXi host fails then HA will restart the virtual machines on to the another ESXi hosts. HA can also monitor the single virtual machine or application inside the virtual machine. If HA detects the virtual machine fail then it will try to restart the virtual machine onto the same ESXi hosts first.

HA uses network heartbeat as well as datastore heartbeat for more resiliency. If the Master server detects the network connectivity is lost with any of the ESXi host it will try using the datastore heartbeat to find out if the ESXi host failed or is network isolated. Based on that HA will take the appropriate action.

This is an overview of vSphere HA for more detailed Information on the vSphere High Availability you can refer to the VMware Documentation.

Checkout our post on How to configure vSphere HA ?


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Mayur Parmar

Hi I am Mayur Parmar. Independent Author & Founder of Mastering VMware. I am working in IT industry for more than 4 Years with knowledge of VMware , Microsoft Server's, Linux Server's.

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