Items dispatched to the queue run at background priority; the queue is scheduled for execution after all high priority queues have been scheduled and the system runs items on a thread whose priority is set for background status. Such a thread has the lowest priority and any disk I/O is throttled to minimize the impact on the system.
那Disk I/O Throttle做什么用呢?按照上面这段描述,Disk I/O会impact system performance。
Throttles request bandwidth by limiting the packet size and adding a delay for each chunk read from the upload stream.
When uploading over a 3G or EDGE connection, requests may fail with "request body stream exhausted". Setting a maximum packet size and delay according to the recommended values (`kAFUploadStream3GSuggestedPacketSize` and `kAFUploadStream3GSuggestedDelay`) lowers the risk of the input stream exceeding its allocated bandwidth. Unfortunately, there is no definite way to distinguish between a 3G, EDGE, or LTE connection over `NSURLConnection`. As such, it is not recommended that you throttle bandwidth based solely on network reachability. Instead, you should consider checking for the "request body stream exhausted" in a failure block, and then retrying the request with throttled bandwidth.
@param numberOfBytes Maximum packet size, in number of bytes. The default packet size for an input stream is 16kb.
@param delay Duration of delay each time a packet is read. By default, no delay is set.