title: Cassandra CAS INSERT timeouts for requests with milliseconds latency
body: <p>We are load-testing our cassandra cluster (3 nodes, replication factor 3) and started to receive occasional <em>WriteTimeoutExceptions</em> for CAS insert operations on one table:</p>
  
  <pre><code>CREATE TABLE users.by_identity (
          account ascii,
          domain ascii,
          identity text
          PRIMARY KEY ((account, domain), identity)
      );
  </code></pre>
  
  <p>We are doing inserts with <em>IF NOT EXISTS</em> clause to this table. When increasing load to > 10 inserts/s for one partition, client requests start to "time out":</p>
  
  <pre><code>com.datastax.driver.core.exceptions.WriteTimeoutException: Cassandra timeout during write query at consistency SERIAL (2 replica were required but only 1 acknowledged the write)
  </code></pre>
  
  <p>WriteType for timeouts is <em>CAS</em> and exceptions are thrown only for this table. Execution time is always &lt; 10ms. Read/write timeouts are configured to > 1000 ms on cluster and only this table is hitting the problem. </p>
  
  <p>Any ideas what might be the issue we are hitting and why are we getting timeouts for requests with milliseconds latency?</p>
  
  <p>We are on Cassandra v3.0.8 and Datastax Java driver v3.1.0. </p>