In what situation do we need to enable IN_MEMORY option?

[ChingShen <ch...@gmail.com>]

Hi,
I have a question about IN_MEMORY option.

http://hadoop.apache.org/hbase/docs/current/api/org/apache/hadoop/hbase/io/hfile/LruBlockCache.html
Contains three levels of block priority to allow for scan-resistance and
in-memory families. A block is added with an inMemory flag if necessary,
otherwise a block becomes a single access priority. Once a blocked is
accessed again, it changes to multiple access. This is used to prevent scans
from thrashing the cache, adding a least-frequently-used element to the
eviction algorithm.

As above mentioned, that contained single and multiple priority, but why do
we still need MEMORY priority?
What's different in memory and multiple priority? any detail?

Shen