Seagate ST3500630A Serial ATA Native Command Queuing (670K, PDF) - Page 3

Serial ATA Native Command Queuing sides of the discs, is collectively called a cylinder. Thus, data is laid out across the discs sequentially in cylinders starting from the outer diameter of the drive. One of the major mechanical challenges is that applications rarely request data in the order that it is written to the disc. Rather, applications tend to request data scattered throughout all portions of the drive. The mechanical movement required to position the appropriate read/write head to the right track in the right rotational position is non-trivial. The mechanical overheads that affect drive performance most are seek latencies and rotational latencies. Both seek and rotational latencies need to be addressed in a cohesive optimization algorithm. The best known algorithm to minimize both seek and rotational latencies is called Rotational Position Ordering. Rotational Position Ordering (or Sorting) allows the drive to select the order of command execution at the media in a manner that minimizes access time to maximize performance. Access time consists of both seek time to position the actuator and latency time to wait for the data to rotate under the head. Both seek time and rotational latency time can be several milliseconds in duration. Earlier algorithms simply minimized seek distance to minimize seek time. However, a short seek may result in a longer overall access time if the target location requires a significant rotational latency period to wait for the location to rotate under the head. Rotational Position Ordering considers the rotational position of the disk as well as the seek distance when considering the order to execute commands. Commands are executed in an order that results in the shortest overall access time, the combined seek and rotational latency time, to increase performance. Native Command Queuing allows a drive to take advantage of Rotational Position Ordering to optimally re-order commands to maximize performance. Seek Latency Optimization Seek latencies are caused by the time it takes the read/write head to position and settle over the correct track containing the target Logical Block Addressing (LBA). To satisfy several commands, the drive will need to access all target LBAs. Without queuing, the drive will have to access the target LBAs in the order that the commands are issued. However, if all of the commands are outstanding to the drive at the same time, the drive can satisfy the commands in the optimal order. The optimal order to reduce seek latencies would be the order that minimizes the amount of mechanical movement. One rather simplistic analogy would be an elevator. If all stops were approached in the order in which the buttons were pressed, the elevator would operate in a very inefficient manner and waste an enormous amount of time going back and forth between the different target locations. As trivial as it may sound, most of today's hard drives in the desktop environment still operate exactly in this fashion. Elevators have evolved to understand that re-ordering the targets will result in a more economic and, by extension, faster mode of operation. With Serial ATA, not only is re-ordering from a specific starting point possible but the re-ordering scheme is dynamic, meaning that at any given time, additional commands can be added to the queue. These new commands are either incorporated into an ongoing thread or postponed for the next series of command execution, depending on how well they fit into the outstanding workload. To translate this into HDD technology, reducing mechanical overhead in a drive can be accomplished by accepting the queued commands (floor buttons pushed) and re-ordering them to efficiently deliver the data the host is asking for. While the drive is executing one command, a new command may enter the queue and be integrated in the outstanding workload. If the new command happens to be the most mechanically efficient to process, it will then be next in line to complete. 3