sec disk drives intersector gap sector header sector data sector header sector data intrasector gap figure an illustration of sector gaps within track each sector begins with sector header containing the sector address and an error detection code for the contents of that sector the sector header is followed by small intra-sector gapfollowed in turn by the sector data each sector is separated from the next sector by larger inter-sector gap the track where the head is moving towe will consider only two numbers one is the track-to-track costor the minimum time necessary to move from track to an adjacent track this is appropriate when you want to analyze access times for files that are well placed on the disk the second number is the average seek time for random access these two numbers are often provided by disk manufacturers typical example is the gb western digital wd caviar se serial ata drive the manufacturer' specifications indicate that the track-to-track time is ms and the average seek time is ms for many yearstypical rotation speed for disk drives was rpmor one rotation every ms most disk drives today have rotation speed of rpmor ms per rotation when reading sector at randomyou can expect that the disk will need to rotate halfway around to bring the desired sector under the / heador ms for -rpm disk drive once under the / heada sector of data can be transferred as fast as that sector rotates under the head if an entire track is to be readthen it will require one rotation ( ms at rpmto move the full track under the head if only part of the track is to be readthen proportionately less time will be required for exampleif there are sectors on the track and one sector is to be readthis will require trivial amount of time ( / of rotationexample assume that an older disk drive has total (nominalcapacity of gb spread among plattersyielding gb/platter each platter contains , tracks and each track contains (after formatting sectors of bytes/sector track-to-track seek time is ms and average seek time for random access is ms assume the operating system maintains cluster size of sectors per cluster ( kb)yielding clusters per track the disk rotation rate is rpm ( ms per rotationbased on this information we can estimate the cost for various file processing operations