This preview shows page 1. Sign up to view the full content.
Unformatted text preview: s. The access time for a sector has three main components: seek time, rotational latency, and transfer time: ¯ Seek time: To read the contents of some target sector, the arm ﬁrst positions the head over the track that contains the target sector. The time required to move the arm is called the seek time. The seek time, Ì× , depends on the previous position of the head and the speed that the arm moves across the surface. The average seek time in modern drives, Ì Ú × , measured by taking the mean of several 288 CHAPTER 6. THE MEMORY HIERARCHY
thousand seeks to random sectors, is typically on the order of 6 to 9 ms. The maximum time for a single seek, ÌÑ Ü × , can be as high as 20 ms. ¯ Rotational latency: Once the head is in position over the track, the drive waits for the ﬁrst bit of the target sector to pass under the head. The performance of this step depends on the position of the surface when the head arrives at the target sector, and the rotational speed of the disk. In the worst case, the h...
View Full Document