If your redis initial replication fails with error like 
"5101:M 20 Feb 18:14:29.130 # Client id=4500196 addr=71.459.815.760:43872 fd=533 name= age=127 idle=127 flags=S db=0 sub=0 psub=0 multi=-1 qbuf=0 qbuf-free=0 obl=13997 oll=1227 omem=192281275 events=rw cmd=psync scheduled to be closed ASAP for overcoming of output buffer limits.

this means that slave buffer is not enough and you should increase it (at master!) with command like
redis-cli config set client-output-buffer-limit "slave 836870912 836870912 0"

more info: https://redis.io/topics/clients