Stretched clusters extend the Virtual SAN cluster from a single site to two sites for a higher level of availability and intersite load balancing. Stretched clusters are typically deployed in environments where the distance between data centers is limited, such as metropolitan or campus environments.
Storage Based Policy Management (SPBM) is the mechanism by which data protection and data placement are controlled for vSAN. Two policy settings are key to assigning protection and placement specifically.
Site disaster tolerance
This policy setting determines if data is mirrored between the two sites, or which side it will be placed on if not mirrored.
- None - Standard Cluster - This setting is used when not using a stretched cluster at all.
- Dual site mirroring - This setting allows for the loss of an entire data center site, by mirroring the data to both sites. This is what most people think of with a stretched cluster.
The following two settings are often used for applications that handle their own replication. An example would be setting one virtual machine to be pinned to one site, and another to the second. Common applications that can cluster themselves include Exchange DAG, SQL AlwaysOn, Oracle Dataguard, etc.
- None - keep data on preferred. This setting is useful if you want to "pin" the data to the preferred site.
- None - keep data on Non-preferred. This setting pins data to the opposite site.
Lastly, comes an option not commonly used.
None - Stretched cluster. This option will keep the virtual machine components on a single side of the stretched cluster but will do so with a bit of a random distribution. The vSAN object manager will make attempts to keep them on the same site, but beyond that, it will largely look at the free capacity within a site and use that as its guiding logic to randomly place virtual machines across sites. This setting might make sense for a larger quantity of applications that lack the need for survivability of loss of either data center. In general, most people build stretched clusters explicitly to protect most workloads from datacenter failure, but for a set of capacity hungry but lower priority virtual machines this policy may make sense.
Failures to tolerate
This policy specifics the data protection that will happen within a site. Note this raid protection level will be mirrored on both sites if a stretched cluster policy was previously selected. In this example, RAID-1 mirroring would result in 4 copies of data being created (2 at each site).
Do note that the minimum number of hosts to reach the protection level (4 hosts for RAID 5, 6 hosts for RAID 6) must be maintained within each of the two sites.
For more information on vSAN stretched clusters see the vSAN stretched cluster guide.