Version 1.1 of Cassandra introduced live traffic sampling, which allows you to measure the performance of a node without it really joining the cluster: http://www.datastax.com/dev/blog/whats-new-in-cassandra-1-1-live-traffic-sampling

That page mentions that you can change the compaction strategy through jmx if you want to test out a different strategy on your survey node.

That's great, but it doesn't give you a complete view of how your performance would change, since you're not doing reads from the survey node. But what would happen if you used jmx to change the compaction strategy of a column family on a single *production* node? Would that be a safe way to test it out or are there side-effects of doing that live?

And if you do that, would running a major compaction transform the entire column family to the new format?

Finally, if the test was a success, how do you proceed from there? Just change the schema?