问题
I'm carrying some experiments on the performance of Cassandra. I have a single cluster of 8 nodes. While increasing the replication factor from 1 to 8, i noticed that the overall throughput decreases. I'm using consistency level ONE for both reads and writes. I found that these are not the expected results. Anyone explain why this is happening?
回答1:
This should be completely expected. Consistency level one will keep the performance of your reads as basically the same (if not improved since their are more possible machines to serve each read) but your writes will be doing significantly more work.
Every write to the cluster, regardless of consistency level, will end up causing a write to every replica node for that data. The consistency level only determines when the write will be acknowledged to the client as having been completed. This does not mean you can avoid the performance penalty of having done all those additional writes. So you can imagine that by increasing your replication factor to 8 causes every write to now do 8 times the work which explains your performance changes.
来源：https://stackoverflow.com/questions/27084341/effects-of-increasing-the-replication-factor-on-the-performance-of-cassandra