The Spanner paper goes through this: http://static.googleusercontent.com/media/research.google.com/en//archive/spanner-osdi2012.pdf http://static.googleusercont

The Spanner paper goes through this: http://static.googleusercontent.com/media/research.google.com/en//archive/spanner-osdi2012.pdf http://static.googleusercontent.com/media/research.google.co... Writes are implemented with pessimistic locking. A write in a Canada datacenter may have to acquire a lock in Australia as well. See page 12 of the paper for the F1 numbers (mean of 103.0ms with standard deviation of 50ms for multi-site commits).