Using the general situation where a total I/O profile requires 100,000 IOPS, of which 70% are write, and 30% are read, in a stretched configuration, the write I/O is what is sized against for intersite bandwidth requirements.

With stretched clusters, read traffic is, by default, serviced by the site on which the VM resides.

This concept is called Read Locality in which the required bandwidth between two data sites (B) is equal to Write bandwidth (Wb) * data multiplier (md) * resynchronization multiplier (mr):

B = Wb * md * mr 

The data multiplier is comprised of overhead for vSAN metadata traffic and miscellaneous related operations. VMware recommends a data multiplier of 1.4. The resynchronization multiplier is included to account for resynchronizing events. It is recommended to allocate bandwidth capacity on top of required bandwidth capacity for resynchronization events. Making room for resynchronization traffic, an additional 25% is recommended.

Example workload of 10,000 writes per second to a workload on vSAN with a “typical” 4 KB size write would require 40 MB/s, or 320 Mbps bandwidth. 

B = 320 Mbps * 1.4 * 1.25 = 560 Mbps.

Including the vSAN network requirements, the required bandwidth would be 560 Mbps.