Cassandra issues

[Arthur Smirnoff]

I know that Cassandra has a linear scalability. Recently I've increased the number of nodes in my cluster from 3 to 6.
So as I have more nodes, I've also increased the replication factor from 2 to 4.
I've checked the metrics and I don't see any significant increase in write throughput.

[Tim Grant]

You haven't provided any metrics, however, it looks like there is some misunderstanding of the concept from your side.
With еру additional nodes, you'll get a linear increase in write throughput only if you have the replication factor unchanged.
Otherwise, you need to decrease write consistency level.
See more details at:
https://stackoverflow.com/questions/24587869/understand-cassandra-replication-factor-versus-consistency-level
http://docs.datastax.com/en/cassandra/3.0/cassandra/dml/dmlConfigConsistency.html
https://www.slideshare.net/planetcassandra/201404-cluster-sizing
Also, if you have difficulties with this you can ask these guys from Russia: https://www.issart.com/en/. I've contacted them several times for some issues.