To clarify what Lars said: We can do custom encoding of the key values in HFile blocks (FAST_DIFF, etc) in cache as well as on disk. We can also or instead do whole block compression using the usual suspects (gzip, snappy), but only as part of reading or writing HFile blocks "at the HDFS level".
 
NEW: Monitor These Apps!
elasticsearch, apache solr, apache hbase, hadoop, redis, casssandra, amazon cloudwatch, mysql, memcached, apache kafka, apache zookeeper, apache storm, ubuntu, centOS, red hat, debian, puppet labs, java, senseiDB