Replicas are master/slave, and continues snapshotting is asynchronous. On top of that, we can write a write ahead transaction log. If you put snapshotting and write ahead logs on different I/O subsystems you only produce sequentual writes, which guarantees that you get the most of your IO. Of course, that's on a single box.

For our distributed cluster we have the option of writing the write ahead log to the network "log servers" so you can do it even faster. In this case once ones K out of N log servers receive a log record we acknowledge the transaction. Then eventually the log servers flush it to disk. In this case you are able to write very quickly (you can spin off as many log servers as you like), you store your data in memory in one copy and you're durable.