Cassandra vnodes: can I lower the number on slower nodes and expect rebalancing to occur automatically?

Question

I am running a small Cassandra 2.2.1 test cluster with 3 computers in it. Two of them are i7s and one is a somewhat slower i5, but I didn\'t bother when first setting things

Answer 1

I don't have sufficient reputation to comment, but to answer Thomas Browne's question: Don't do that! If you drop the tables and keyspaces, you'll lose all the data on all the nodes. What you'll want to do is stop cassandra, go to your data directory on the node you're modifying, and rm -rf * the data in that directory. Eg: rm -rf /var/lib/cassandra/*

Answer 2

A simple restart won't allow you to change the number of vnodes.

You'll need to do a nodetool decommission or nodetool removenode for the i5 node, then stop Cassandra on that node, scrub all the data, update num_tokens, then start Cassandra back up and let it rejoin the cluster as an empty new node with the reduced number of tokens. Then Cassandra will rebalance with the reduced number of tokens on the i5 box.

At least that's how it is in 2.1.9, but probably it is the same in the newer branches too.