Compactions were introduced in Chapter 2 . In this chapter, I will discuss compactions in more detail. Each flush of the MemStore generates a StoreFile, which is façade on an HFile. The MemStore size at which a flush is performed is set in hbase.hregion.memstore.flush.size, which is 128MB by default. The MemStore size is checked at a frequency set in hbase.server.thread.wakefrequency, which has a default of 10,000 ms. If the number of StoreFiles in a store exceed some limits/thresholds, the files are compacted into larger StoreFiles. Compaction is the process of creating a larger StoreFile (HFile) file by merging smaller StoreFilefiles.