Creating indexes takes extra space (does in MySQL, PGSQL, etc too).
https://issues.apache.org/jira/browse/CASSANDRA-749 has quite a bit of detail about how the secondary indexes currently work.
Before 0.7, actually we can create an extra ColumnFamily as an secondary index, if we need.
I was wondering whether the secondary index mechanism in 0.7 just likes creating an extra ColumnFamily as an index.
The difference is only that users don't take care of the maintainence of the secondary index, instead the system will maintain it.
Hence, the secondary index is also a redundant copy, which costs extra space as the same as creating an extra ColumnFamily as an index.
Am I right?