See HDFS-13150 for the full design.
This JIRA is targetted at the NameNode-side changes to enable tailing in-progress edits via the RPC mechanism added in HDFS-13608. Most changes are in the QuorumJournalManager.
[Edit Tail Fast Path Pt 2] Add ability for JournalNode to serve edits via RPC
[Edit Tail Fast Path] Allow SbNN to tail in-progress edits from JN via RPC
[Edit Tail Fast Path Pt 4] Cleanup: integration test, documentation, remove unnecessary dummy sync