Q. What is the furthest distance that I can perform synchronous replication with?
A. There are many factors that determine the maximum distance that synchronous replication can be performed over. Remember that with synchronous replication changes have to be made on the replica before they can be committed to the source meaning the longer the latency the greater the performance of the workload will be impacted. This latency is the greatest driver for the distance supported. Typically 5ms latency is the greatest latency that can be supported for many workloads including storage replication. Based on 5ms latency we can do some basic match to work out the maximum distance that synchronous replication can be performed over:
- In an ideal world of a vacuum at light speed a 5ms latency is around 1500 km round trip (so 750 km away)
- Optical fiber reduces this around 35% then there are switches, firewalls and other equipment so maybe half that distance to 375 km
- Then there are the financials of the infrastructure which is very expensive for this type of connectivity
The end result of all of this is about 50 km is the max distance for synchronous replication in most environment. Once you cross that distance you typically will switch to asynchronous replication.