CDH 5 supports online merging of regions. HBase splits big regions automatically but does not support merging small regions automatically. To complete an online merge of two regions of a table, use the HBase shell to issue the online merge command. By default, both regions to be merged should be neighbors; that is, one end key of a region should be the start key of the other region. Although you can "force merge" any two regions of the same table, this can create overlaps and is not recommended.

The Master and RegionServer both participate in online merges. When the request to merge is sent to the Master, the Master moves the regions to be merged to the same RegionServer, usually the one where the region with the higher load resides. The Master then requests the RegionServer to merge the two regions. The RegionServer processes this request locally. Once the two regions are merged, the new region will be online and available for server requests, and the old regions are taken offline.

For merging two consecutive regions use the following command:

hbase> merge_region 'ENCODED_REGIONNAME', 'ENCODED_REGIONNAME'

For merging regions that are not adjacent, passing true as the third parameter forces the merge.

hbase> merge_region 'ENCODED_REGIONNAME', 'ENCODED_REGIONNAME', true

  Note: This command is slightly different from other region operations. You must pass the encoded region name (ENCODED_REGIONNAME), not the full region name . The encoded region name is the hash suffix on region names. For example, if the region name is TestTable,0094429456,1289497600452.527db22f95c8a9e0116f0cc13c680396, the encoded region name portion is 527db22f95c8a9e0116f0cc13c680396.