Data syncronization

I am planning to provision a medium scale NAS server at my site and at the DR location. I expect this to be around 20Tb. Mostly this will have a fileserver type workload, with few updates to existing files. My issue is how to keep the 2 locations in sync. While CPU is not a big concern, bandwidth may become a problem but the biggest worry is latency - I want to ensure that the DR site is as up-to-date as practical, and would prefer something where I can measure the amount of data waiting to be replicated. While I could simply run unison or rsync on the filesystem(s) it is my understanding that this simply polls for updates to replicate rather then capturing enqueueing the remote writes. DRBD would be more efficient in terms of CPU and bandwidth - but I can't see how I would be able to have the DR filesystem "live" (writeable) at the same time as the production site (and I don't see how to measure the lag). Should I be looking at AFS / intermezzo? Any pointers to case studies would be appreciated.