# HBase Read Merge

the KeyValue cells corresponding to one row can be in multiple places, row cells already persisted are in Hfiles, recently updated cells are in the MemStore, and recently read cells are in the Block cache. So when you read a row, how does the system get the corresponding cells to return? A Read merges Key Values from the block cache, MemStore, and HFiles in the following steps:

1. First, the scanner looks for the Row cells in the Block cache - the read cache. Recently Read Key Values are cached here, and Least Recently Used are evicted when memory is needed.
2. Next, the scanner looks in the MemStore, the write cache in memory containing the most recent writes.
3. If the scanner does not find all of the row cells in the MemStore and Block Cache, then HBase will use the Block Cache indexes and bloom filters to load HFiles into memory, which may contain the target row cells.

![](https://1182370643-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LsbZSe32ZgrQ1gzZoPR%2F-LsbZZk03RhW1ap7TKI8%2F-LsbZ_6xvQ0r0nPXlKzD%2FHBase%20Read%20Merge%201.png?generation=1572621933709862\&alt=media)

there may be many HFiles per MemStore, which means for a read, multiple files may have to be examined, which can affect the performance. This is called read amplification.

![](https://1182370643-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LsbZSe32ZgrQ1gzZoPR%2F-LsbZZk03RhW1ap7TKI8%2F-LsbZ_6zAfihb4CNXbYG%2FHBase%20Read%20Merge.png?generation=1572621933773131\&alt=media)


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