There is a note in chapter 4.4.3. Data Replication [1]:

Synchronization occurs in parallel with current network traffic so this won't cause any blocking on current clients. 

which is not fully correct. In fact clients are blocked for duration of {{ initial-replication-sync-timeout }} when backup is started. After this timeout clients are unblocked even if synchronization is not completed.

[1] https://access.stage.redhat.com/documentation/en/red-hat-jboss-enterprise-application-platform/version-7.0/configuring-messaging/#data_replication