How the LWT- Light Weight Transaction is working when we use IF NOT EXIST?

The question is that, When we use

    INSERT INTO USERS (login, email, name, login_count)        
    values ('jbellis', 'jbellis@datastax.com', 'Jonathan Ellis', 1)        
    IF NOT EXISTS    

in IF NOT EXIST exactly which columns are compared together? primary key(partition-key + clustering-key)? or just partition-key?

Here is a diagram of the 4 phases of LWT: http://www.slideshare.net/doanduyhai/cassandra-introduction-nantesjug/89

The original blog post is here: http://www.datastax.com/dev/blog/lightweight-transactions-in-cassandra-2-0

exactly which columns are compare together? primary key? partition-key?

The whole primary key is checked on disk. In the example it is a simple primary key so it's also the partition key, e.g. login column