The HBase RegionServers are the processes that run on every node in the cluster. Typically, there is one RegionServer (or RS) and one HDFS DataNode per physical node in the cluster. It is possible to run more than one RegionServer per physical node, but that's not common.
Each RegionServer is responsible for a set of regions in the cluster and serves all read and write requests to them from HBase clients. A region is end-of-day persisted in one or more files on HDFS. So, to service reads and writes to a region, the RegionServer communicates with the DataNode process on the same or a different machine. The RegionServer also maintains an LRU cache of the blocks that were retrieved to speed up subsequent accesses to those ...