> I stop my cassandra node and restarted it,this start and stop > resulted in decrease in the space used by cassandra by 20%. > I am not sure what is happening?Can Anyone please shower some light on this
Probably because unused sstables were deleted on start-up that were otherwise not used, and waiting on full GC to be removed. Check system.log for "deleted sstable" messages. I didn't find a wiki ref right now, but in short cassandra uses weak references to discover when an sstable is guaranteed to no longer be used, in order to perform removals of the actual sstable files. This can lead to a delay of sstable removal which will vary with whatever else is happening (the more busy the node, the more often a concurrent mark/sweep gc phase is triggered, and the more frequently obsolete sstables are deleted). -- / Peter Schuller
