Cassandra partition size and performance?

Question

I was playing around with cassandra-stress tool on my own laptop (8 cores, 16GB) with Cassandra 2.2.3 installed out of the box with having its stock configuration. I was doing exactly what was described here:

http://www.datastax.com/dev/blog/improved-cassandra-2-1-stress-tool-benchmark-any-schema

And measuring its insert performance.

My observations were:

• using the code from https://gist.github.com/tjake/fb166a659e8fe4c8d4a3 without any modifications I had ~7000 inserts/sec.
• when modifying line 35 in the code above (cluster: fixed(1000)) to "cluster: fixed(100)", i. e. configuring my test data distribution to have 100 clustering keys instead of 1000, the performance was jumping up to ~11000 inserts/sec
• when configuring it to have 5000 clustering keys per partition, the performance was reducing to just 700 inserts/sec

The documentation says however Cassandra can support up to 2 billion rows per partition. I don't need that much still I don't get how just 5000 records per partition can slow the writes 10 times down or am I missing something?

Answer 1

Supporting is a little different from "best performaning". You can have very wide partitions, but the rule-of-thumb is to try to keep them under 100mb for misc performance reasons. Some operations can be performed more efficiently when the entirety of the partition can be stored in memory.

As an example (this is old example, this is a complete non issue post 2.0 where everything is single pass) but in some versions when the size is >64mb compaction has a two pass process, that halves compaction throughput. It still worked with huge partitions. I've seen many multi gb ones that worked just fine. but the systems with huge partitions were difficult to work with operationally (managing compactions/repairs/gcs).

I would say target the rule of thumb initially of 100mb and test from there to find own optimal. Things will always behave differently based on use case, to get the most out of a node the best you can do is some benchmarks closest to what your gonna do (true of all systems). This seems like something your already doing so your definitely on the right path.