Hello @phongnamam
To your Queries, (I) Split tries to take an approximate centre point of the RowKey for the Region being split. In this manner, Yes, Region Split is a Horizontal Split. By default, each Table would have 1 Regions. You mentioned, you have 3 Tables & 5 Regions. The same is feasible as each Table have 1 Region (Total 3 Regions) & the Hbase:Meta/Hbase:NameSpace Table have 1 Region each, making total 5 Regions. If a Small Table have 5 Regions, It's likely using Split Point.
(II) When a RegionServer dies, the Regions hosted on the RegionServer are distributed across Live RegionServers & when the RegionServer comes back online, the Master assign Regions to the RegionServer based on Balancer. The Master Logs would show if Balancer is moving Regions as part of Balancing.
(III) Ideally, Masters shouldn't flip if 1 RegionServer stops. Best approach is to check the Logs of both Masters to see for any issues.
- Smarak