Have you considered timing the average wait time for fsync() and continuously adjusting a timer to fsync() every N milliseconds based on this data? You could qu

Have you considered timing the average wait time for fsync() and continuously adjusting a timer to fsync() every N milliseconds based on this data? You could queue all of your response-to-client messages to wait on this fsync(), and do tens of these per second, even on a slow drive. This technique can potentially get client responses down to fractions of a second, even in high throughput scenarios.