You want to do a full table scan on your data. You decide to disable block caching to see if this
improves scan performance. Will disabling block caching improve scan performance?

A.
No. Disabling block caching does not improve scan performance.

B.
Yes. When you disable block caching, you free up that memory for other operations. With a full
table scan, you cannot take advantage of block caching anyway because your entire table won’t fit
into cache.

C.
No. If you disable block caching, HBase must read each block index from disk for each scan,
thereby decreasing scan performance.

D.
Yes. When you disable block caching, you free up memory for MemStore, which improves,
scan performance.

Explanation:
Disabling Blockcache
Do not turn off block cache (You’d do it by setting hbase.block.cache.size to zero). Currently we
do not do well if you do this because the regionserver will spend all its time loading hfile
indicesover and over again. If your working set it such that block cache does you no good, at least
size the block cache such that hfile indices will stay up in the cache (you can get a rough idea on
the size you need by surveying regionserver UIs; you’ll see index block size accounted near the
top of the webpage).
Reference: Apache HBase (TM) Configuration