"Some users logged issues about a large number of IOPS during heavy write loads. To address this we implemented a write ahead log, or WAL. No migration is required to take advantage of this feature and it is available as soon as you upgrade. It’s worth noting here that if you are testing a high write load scenario, you will get occasional pauses of up to 3 seconds during WAL flushes (depending on hardware and schema). Set your timeouts accordingly. We’ll work on smoothing this out over time in future 0.9 point releases."

This is a pretty major change and a big caveat!