Minimizing downtime during MariaDB Replication Setup
Use https://github.com/alestic/ec2-consistent-snapshot of your EBS volume using --mysql-master-status-file FILE option.

Create a new volume from that snapshot and attach to new instance or another instance you can use to copy to a remote location if needed.. The db server will only be locked for a couple seconds. I just performed this operation on a server with a 2TB server with millions of tables. 

Start the slave db with data files from other server reset the slave and change your master with the correct log position.