A pair of queues should be enough. The first queue is the one being fsynced, the second queue is open for writes. The first fsync call starts the first queue fl

A pair of queues should be enough. The first queue is the one being fsynced, the second queue is open for writes. The first fsync call starts the first queue flushing. The second fsync call marks the second queue as read to flush once the first one is done. The third has no additional effect. Writes can still proceed against the second until it actually starts flushing. At which point the queues swap and you're back where you started.