Thanks for your response I think the reference you are talking about is
http://wiki.apache.org/cassandra/MemtableSSTable
Regards,
Anurag

On Sun, May 8, 2011 at 11:01 AM, Peter Schuller <peter.schuller@infidyne.com> wrote:
>           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