CopyChunk is a rapid internal file replication feature. It is an advanced form of CIFS. It is supported on windows clients properly. Mac OS doesn't fully support this feature as of now.
So what is copychunk ?
When CopyChunk is used, clients running on Windows 7, Windows Server 2008, Windows 8, Windows Server 2012 can complete access to shared files on the server side rather than traversed through the network. In this manner, the network interaction is reduced and the client performance can be maintained.
Here is a little depiction of how it works.


