of the data on disk likewisewhen writing to particular logical byte position with respect to the beginning of the filethis position must be converted by the file manager into the corresponding physical location on the disk to gain some appreciation for the the approximate time costs for these operationsyou need to understand the physical structure and basic workings of disk drive disk drives are often referred to as direct access storage devices this means that it takes roughly equal time to access any record in the file this is in contrast to sequential access storage devices such as tape driveswhich require the tape reader to process data from the beginning of the tape until the desired position has been reached as you will seethe disk drive is only approximately direct accessat any given timesome records are more quickly accessible than others disk drive architecture hard disk drive is composed of one or more round plattersstacked one on top of another and attached to central spindle platters spin continuously at constant rate each usable surface of each platter is assigned read/write head or / head through which data are read or writtensomewhat like the arrangement of phonograph player' arm "readingsound from phonograph record unlike phonograph needlethe disk read/write head does not actually touch the surface of hard disk insteadit remains slightly above the surfaceand any contact during normal operation would damage the disk this distance is very smallmuch smaller than the height of dust particle it can be likened to -kilometer airplane trip across the united stateswith the plane flying at height of one metera hard disk drive typically has several platters and several read/write headsas shown in figure (aeach head is attached to an armwhich connects to the boom the boom moves all of the heads in or out together when the heads are in some position over the plattersthere are data on each platter directly accessible to each head the data on single platter that are accessible to any one position of the head for that platter are collectively called trackthat isall data on platter that are fixed distance from the spindleas shown in figure (bthe collection of all tracks that are fixed distance from the spindle is called cylinder thusa cylinder is all of the data that can be read when the arms are in particular position each track is subdivided into sectors between each sector there are intersector gaps in which no data are stored these gaps allow the read head to recognize the end of sector note that each sector contains the same amount of data because the outer tracks have greater lengththey contain fewer bits per inch than do the inner tracks thusabout half of the potential storage space is wastedbecause only the innermost tracks are stored at the highest possible data density this