1) The explanation of Cassandra isn't quite correct. See http://www.datastax.com/docs/1.0/cluster_architecture/replication http://www.datastax.com/docs/1.0/clu

1) The explanation of Cassandra isn't quite correct. See http://www.datastax.com/docs/1.0/cluster_architecture/replication http://www.datastax.com/docs/1.0/cluster_architecture/replic... for details. 2) Cassandra read performance is on par with writes now: http://www.datastax.com/dev/blog/whats-new-in-cassandra-1-0-performance http://www.datastax.com/dev/blog/whats-new-in-cassandra-1-0-... 3) Your explanation doesn't make sense to me. No matter the value of W, MongoDB should make best efforts to get it to all the replicas, no? So lower W should affect availability and perhaps latency but throughput should be unaffected, given a benchmark with sufficient client threads.