HBase already allows to cache blocks on flush. This is very useful for usecases where most queries are against recent data. However, as soon as their is a compaction, those blocks are evicted. It will be interesting to have a table level parameter to say "When compacting, cache blocks less than 24 hours old". That way, when running compaction, all blocks where some data are less than 24h hold, will be automatically cached.
Very useful for table design where there is TS in the key but a long history (Like a year of sensor data).