Sam Tunnicliffe’s describes the old and new, optimized behavior of secondary indexes writes in Cassandra 1.2:
While secondary indexes can add a lot of flexibility to the way data is modelled and accessed, they do add complexity on the server side as the indexes need to be kept in sync with the primary data. Until recently, this has led to some significant trade offs in write throughput and IO utilisation as we always had to perform a read before the write in order to update any relevant secondary indexes. In Cassandra 1.2, this area has been substantially reworked to remove the need for read-before-write. New index entries are now written at the same time as the primary data is updated and old entries removed lazily at query time. Overall, this has lead to some decent performance improvements.
Original title and link: Improving Secondary Index Write Performance in Cassandra 1.2 ( ©myNoSQL)