Disk access time Smallest unit of information that can be read from or written to disk is a sector Block or page is a logical unit read from or written to by O/S   Block consists of a contiguous sequence of sectors Access time : Time to access a particular sector   Seek time : Time to position arm heads over cylinder containing the target sector   Typical seek time: 8 ms Rotational latency : (Average) time to rotate r/w head to the first bit of the sector   (1 / 2)(1 / rpm)(60 s / 1 min) Transfer time : Time to read bits from the sector   (1 / (#sectors / track))(60 / rpm)