Postgres 9.6 - Taking differenctial backup
Please don't re-invent the wheel! There are so many traps in backup up and recovering the probability you fall into them is pretty high!


  
  Directly Rsync the Postgres data directory.
  


Please don't! You'll copy unconsistent data...

I think you should consider one of these 3 tools :


barman 
backrest
probackup


Barman and probackup have real differential backups and will check backup consistency before restoring a backup. That's why I prefer one of those tools.

You can find a complete comparison in Anastasia Lubennikova's excellent conf talk Advanced PostgreSQL backup and recovery methods slides.