The following figure shows the principle algorithm and data structure HBase works on, that is, LSM-tree, and the way of merging, and precedes the explanation:

HBase stores file using LSM-tree, which maintains data in two separate parts that are optimized for underlying storage. This type of data structure depends on two structures, a current and smaller one in memory and a bigger one on the persistent disk, and once the part in memory becomes bigger than a certain limit, it is merged with the bigger structure that is stored on the disk using a merge sort algorithm and a new in-memory tree is created for newer insert requests. It transforms random data access into sequential data access, which improves read performance, and merging is a background process, which does not affect the foreground processing.