VMware introduced vSAN stretched clusters in vSAN v6.1. Although, vSAN can be thought of primarily serving a lot of local site use cases, there are some specific use cases where disaster and downtime avoidance is a key requirement. That’s where vSAN stretched clusters find their existence today.
A vSAN stretched cluster basically refers to a deployment where you set up a vSAN cluster with two active-active data sites. An identical number of hosts is distributed evenly between the two sites. The sites are connected with a high bandwidth, low latency link.
The third site hosts the vSAN witness host, which is connected to both of the active-active data sites. The sole purpose of the witness host is to provide cluster quorum during data site failure events. A special witness appliance is available for use as the witness host. Connectivity between the witness site and each of the data sites can be a low bandwidth, high latency link.
This article aims only to discuss various use cases of vSAN stretched clusters which might give you a fair idea of where the focus is, if you have not yet put your hands dirty on them yet
- Protects virtual machines across data centers, not just racks: A stretched cluster extends across more than one site
- Planned Maintenance: Planned maintenance of one site without any service downtime with ability to migrate applications back after maintenance is complete
- Automated Recovery: Automated initiation of virtual machine restart or recovery with very low RTO of majority of unplanned failures