By what factor does the performance (read queries/sec) increase when a machine is added to a cluster of machines running either:
Google's research paper on Bigtable suggests that "near-linear" scaling is achieved can be achieved with Bigtable. This page here featuring MySQL's marketing jargon suggests that MySQL is capable of scaling linearly.
Where is the truth?
Having built and benchmarked several applications using VoltDB I consistently measure between 90% and 95% of additional transactional throughput as each new server is added to the cluster. So if an application is performing 100,000 transaction per second (TPS) on a single server, I measure 190,000 TPS on 2 servers, 280,000 TPS on 3 servers, and so on. At some point we expect the server to server networking to become a bottleneck but our largest cluster (30 servers) is still above 90%.