In the context of VMware vSphere and ESXi hosts, a split-brain scenario refers to a situation where two or more ESXi hosts in a High Availability (HA) cluster lose communication with each other but continue to operate independently. This can lead to data inconsistencies, service disruption, and even data corruption. Split-brain scenarios typically occur when there is a network partition, and the hosts in the cluster cannot communicate with each other or the vCenter Server.

Let’s explore two examples of split-brain scenarios in ESXi hosts:

Example 1: Network Partition

Suppose you have an HA cluster with three ESXi hosts (Host A, Host B, and Host C). Due to a network issue, Host A loses connectivity to Host B and Host C, while Host B and Host C can still communicate with each other.

• In this scenario, Host B and Host C assume that Host A has failed and attempt to restart the virtual machines that were running on Host A.
• At the same time, Host A also assumes that Host B and Host C have failed and tries to restart the virtual machines running on those hosts.

As a result, the virtual machines that were running on Host A are now running on both Host B and Host C, causing a split-brain situation. The virtual machines may have inconsistent states and data, leading to potential data corruption or conflicts.

Example 2: Network Isolation

Consider a scenario where the ESXi hosts in an HA cluster are connected to two separate network switches. Due to a misconfiguration or network issue, one switch becomes isolated from the rest of the network, leading to a network partition.

• The hosts connected to the isolated switch cannot communicate with the hosts connected to the main network, and vice versa. Each group of hosts assumes that the other group has failed.
• Both groups of hosts attempt to restart the virtual machines running on the other side, resulting in a split-brain scenario.

To avoid split-brain scenarios, vSphere HA uses a quorum mechanism to ensure that the majority of the hosts in the cluster agree on the cluster’s state before triggering a failover. By default, vSphere HA requires more than 50% of the hosts to be online and in communication to avoid split-brain situations.

Additionally, vSphere HA relies on heartbeat datastores to monitor the health of the hosts and detect network partitions. If a host cannot access its designated heartbeat datastore, it will assume that a network partition has occurred, and it will not initiate a failover.

To mitigate the risk of split-brain scenarios, consider the following best practices:

1. Use redundant network connections and switches to minimize the risk of network partitions.
2. Configure proper fencing mechanisms, such as VMware’s APD (All Paths Down) and PDL (Permanent Device Loss), to ensure that hosts can properly isolate failed storage paths or devices.
3. Design your network infrastructure to avoid single points of failure and ensure that all hosts can communicate with each other and the vCenter Server.
4. Regularly monitor the health of your vSphere environment and promptly address any networking or storage issues to prevent split-brain scenarios.