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From C.B. <>
Subject Adding more nodes causes performance problem
Date Mon, 09 Feb 2015 01:54:03 GMT
I have a cluster with 3 nodes, the only keyspace is with replication factor of 3,
the application read/write UUID-keyed data. I use CQL (casssandra-python),
most writes are done by execute_async, most read are done with consistency
level of ONE, overall performance in this setup is better than I expected.

Then I test 6-nodes cluster and 9-nodes. The performance (both read and
write) was getting worse and worse. Roughly speaking, 6-nodes is about 2~3
times slower than 3-nodes, and 9-nodes is about 5~6 times slower than
3-nodes. All tests were done with same data set, same test program, same
client machines, for multiple times. I'm running Cassandra 2.1.2 with default

What I observed, is that with 6-nodes and 9-nodes, the Cassandra servers
were doing OK with IO, but CPU utilization was about 60%~70% higher than

I'd like to get suggestion how to troubleshoot this, as this is totally against
what I read, that Cassandra is scaled linearly.

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