I am trying to understand how read and write happens in HBase and how HBase does the caching. From various articles and videos, I found that a read merge happens when a read request is made to HBase. What I understood is: Whenever read request made, block cache is first checked for the data. Then memstore is checked. If data found in both block cache or memstore, data is sent to client. Else fetched from hfiles. My doubts are whether both block cache and memstore is always checked for the data? or whether memstore will be ignored if found in block cache. If memstore not checked (since found in blockcache),how will client get latest value if there was an edit in memstore? I created a new table. I added one row. I issued get command to fetch the data. I obtained the data but I didn't see any change in cache hits and reads of block cache. Why? I know there are multiple questions but all these are linked to read merge and HBase caching. I need a clarity on these concepts and I could not find any in documentation. ... View more

I have a table named 'X' and a column family 'cf. The table contains data of past 5 years. Old data are requested only few times whereas recent data are accessed frequently. I wanted to apply different storage policies for the data based on time. How can i configure ?   Also is it possible to specify different compression algorithms for hot and cold data in single column family? I am asking this because in HBase documentation, different algorithms are recommended for hot and cold data. ... View more