Concept
Windows Vista introduced a number of new I/O functions to the Microsoft Windows line of operating systems. They are intended to shorten the time taken to boot the system, improve the responsiveness of the system, and improve the reliability of data storage. Vista modifies the behavior of asynchronous I/O operations. With the new asynchronous I/O APIs, a thread, different from the one that issued the I/O request, can be notified when the operation completes. With this, a single thread can issue all the I/O requests, and then switch to a different worker thread. If this thread is the one that handles the data after the I/O request completes, then a thread-switch, which causes a performance hit, may be avoided. Windows Vista also introduces synchronous I/O cancellation. During a synchronous I/O request, the application is blocked until the request is serviced or fails. In Windows Vista the application may issue a cancellation request. Applications that cancel the operation on user feedback may prefer to enable user feedback during the time the issuing thread is suspended for usability. Windows Vista also implements I/O scheduling as prioritized I/O. Disk I/O requests in Windows Vista are assigned priorities; a higher priority request is given preferential treatment, over a request that has a lower priority, during the execution of the request. Windows Vista defines five priority classes – Very Low, Low, Normal, High and Critical. By default I/O requests are assigned Normal priority. Windows Vista also allows reservation of bandwidth on a per-application basis during disk access; this aims to guarantee the required throughput rate to the application when it accesses the disk. Both these features are used by Windows Media Player with respect to media playback. Disk Defragmenter, SuperFetch, Windows Defender, Windows Search, and applications that run at startup all use prioritized I/O. Prior to Windows Vista, all I/O requests were capped at 64 KB; thus larger operations had to be completed in chunks.