I'm planning to stretch our SP farm. I'm going to move 1 WFE and 1 APP to another data center. Distance between data centers less than 10 km.
Based on this article there are strong requirements:
For a stretched farm architecture to work as a supported high-availability solution, the following prerequisites must be met:
There is a highly consistent intra-farm latency of <1ms one way, 99.9% of the time over a period of ten minutes. (Intra-farm latency is commonly defined as the latency between the front-end web servers and the database servers.)
The bandwidth speed must be at least 1 gigabit per second.
The question is - What if sometimes latency between remote WFE and DB server will be 3 or 4ms? Several times during 10 min period the latency was 3 ms.
What it will cause? Only low performance or something more?