title: Cassandra - HDD vs. SSD usage makes no difference in throughput
body: <p><strong>The Context</strong><br>
  I'm currently running tests with Apache Cassandra on a single node cluster. I've ensured the cluster is up and running using nodetool status, I've done a multitude of reads and writes that suggest as such, and I'm confident my cluster is set up properly. I am now attempting to speed up my throughput by mounting a SSD onto the directory where Cassandra writes its data to.</p>
  
  <p><strong>My Solution</strong><br>
  The write location of Cassandra data is generally to /var/lib/cassandra/data, however I've since switched mine using cassandra.yaml to write to another location, where I've mounted my SSD. I've ensured that Cassandra is writing to this location by checking the size of the data directory's contents through <code>watch du -h</code> and other methods. The directory I've mounted the SSD on includes table data, commitlog, hints, a nested data directory, and saved_caches.</p>
  
  <p><strong>The Problem</strong><br>
  I've been using YCSB benchmarks (see <a href="https://github.com/brianfrankcooper/YCSB" rel="nofollow noreferrer">https://github.com/brianfrankcooper/YCSB</a>) to test the average throughput and ops/sec of Cassandra. I've noticed no difference in the average throughput when mounting HDD vs. SSD on the location where Cassandra writes its data to. I've analyzed disk access through <code>dstat -cd --disk-util --disk-tps</code> and found HDD caps out on CPU usage in multiple instances whereas SSD only spikes to around 80% on several occassions.</p>
  
  <p><strong>The Question</strong><br>
  How can I speed up the throughput of Cassandra using a SSD over a HDD? I assume this is the correct place to mount my SSD, but does Cassandra not utilize its extra processing power? Any help would be greatly appreciated!</p>