How much free space in the disk is needed for elasticsearch?


I am using elasticsearch 0.19.9 for our log realtime analytics. I should
set up log retention period according to the disk usage in the
elasticsearch cluster but how much free space is needed by the percentage?

The reason why I am asking this question is, for cassandra, we need to
leave 50% free disk space for index compaction. Is this necessary for
elasticsearch too? Otherwise, can I keep index until the disk is full by

Thank you
Best, Jae