We only allow one flush_tier operation at a time in the system. If an application calls flush_tier while a prior operation is still in progress—including asynchronous data copies to the cloud—the new flush_tier will block in __flush_tier_wait.

We should consider whether applications would benefit from being able to specify a maximum wait time (including zero) and have the call return EBUSY if it waits that long without being able to start work.