When flush ‘table’ operation is triggered, all the regions belonging to that table will flush independently. Once the HFile corresponding to a region is flushed, it records the max sequence id in metadata and notifies the WAL corresponding to the regionserver. WAL maintains a mapping table for regions and their corresponding flushed sequence id’s. When the HBase cluster restarts, the hMaster will distribute flushed sequence id’s per region to the recovery threads splitting the WAL, so that they can skip the edits which have already been persisted in HFiles.
This is particularly important for clusters which frequently spin up and down, a feature of Platform-as-a-Service solutions like HDInsight.