Q:

vSphere Replication Performance Impact

We're planning to move some production workloads to a UCS environment using vSAN and all flash. We are looking at vSAN with replication as we have two identical nodes in two data centres.
Is there a performance hit from using replication that would negate the benefits of  the all flash storage?
We are currently using LeftHand and the write latency introduced by a commit having o be confirmed from all nodes before the write commit is passed up to the storage consumer makes it very slow. I've tried to find documentation on this but came up short. 

A:

If you are talking about storage replication, you should check two important things: 

Storage performance - in your case it shouldn't be a problem.
Network performance between the servers.

Is there a performance hit from using replication that would negate
  the benefits of the all flash storage?

I am using StarWind VSAN and have zero issues with write performance on all flash HA config.

A:

Is there a performance hit from using replication that would negate
  the benefits of the all flash storage?

It depends on how much 'spare' resources you have left on your cluster after all your VM load. If there's lots of spare CPU/memory/network-bandwidth/disk-bandwith-and-IOPS then no you should see little if any impact during replication, essentially it'll work as a background task and use those spare resources to do so.
If you've not left adequate resources then the impact will be greater as the replication does take an amount of those resources away while it replicates.

