Given a tombstoned key which is older than GCGraceSeconds, current (0.6.1) compaction implementation still requires a major compaction for the key to actually be deleted. The major compaction is required is because we must know whether there is a version of the key inside all SSTables associated with the columnfamily, including ones not involved in minor compactions. However, as we have bloom filters into each one of these SSTables, minor compaction can relatively inexpensively check for existence of this key in SSTable files not involved in the current minor compaction, and thereby delete the key, assuming all bloom filters return negative. If the filter returns positive, a major compaction would of course still be required.
For use cases like
CASSANDRA-1041 where users are strongly motivated to not do a major compaction, this seems to answer the jbellis objection :
"You don't want to skip large files in major compactions, since the definition of major is "compact everything so it is safe to remove tombstones." "
The above described improvement appears to provide "safe to remove (some) tombstones" without requiring "compact everything", and so may be a useful optimization.