title: how to understand "Estimated droppable tombstones”?
body: <p>I have output from sstablemetadata for cassandra, i understand this Estimated droppable tombstones is an estimation of tombstones, but what exactly this number mean?</p>
  
  <pre><code>[root@cass04 ~]# du -sh /mnt/cassandra/data/Telesto/DeviceImpressions-53cf1590475911e5bad7894bc771451d/* | sort -h | tail -6  
  26G /mnt/cassandra/data/Telesto/DeviceImpressions-53cf1590475911e5bad7894bc771451d/Telesto-DeviceImpressions-ka-9310-Data.db
  99G /mnt/cassandra/data/Telesto/DeviceImpressions-53cf1590475911e5bad7894bc771451d/Telesto-DeviceImpressions-ka-8374-Data.db
  113G    /mnt/cassandra/data/Telesto/DeviceImpressions-53cf1590475911e5bad7894bc771451d/Telesto-DeviceImpressions-ka-8714-Data.db
  170G    /mnt/cassandra/data/Telesto/DeviceImpressions-53cf1590475911e5bad7894bc771451d/Telesto-DeviceImpressions-ka-9063-Data.db
  201G    /mnt/cassandra/data/Telesto/DeviceImpressions-53cf1590475911e5bad7894bc771451d/Telesto-DeviceImpressions-ka-8146-Data.db
  271G    /mnt/cassandra/data/Telesto/DeviceImpressions-53cf1590475911e5bad7894bc771451d/Telesto-DeviceImpressions-ka-8084-Data.db
  [root@cass04 ~]# du -sh /mnt/cassandra/data/Telesto*/* | sort -h | tail -6 | awk '{print $2}' | xargs /usr/local/apache-cassandra/tools/bin/sstablemetadata | grep tombstones^C
  [root@cass04 ~]# du -sh /mnt/cassandra/data/Telesto/DeviceImpressions-53cf1590475911e5bad7894bc771451d/* | sort -h | tail -6 | awk '{print $2}' | xargs /usr/local/apache-cassandra/tools/bin/sstablemetadata | grep tombstones
  Estimated droppable tombstones: 6.137201062686473E-5
  Estimated droppable tombstones: 1.1680085943591365E-4
  Estimated droppable tombstones: 6.626254059536159E-5
  Estimated droppable tombstones: 5.116100385316167E-5
  Estimated droppable tombstones: 0.8704887039387946
  Estimated droppable tombstones: 0.10260068095210549
  </code></pre>