used internally by the phone company let' say an entry is stored as record requiring bytes the result is file size of , which is , , bytes or megabytes we'll assume that on the target machine this is too large to fit in main memory but small enough to fit on your disk drive thusyou have large amount of data on your disk drive how do you structure it to provide the usual desirable characteristicsquick searchinsertionand deletionin investigating the answersyou must keep in mind two facts firstaccessing data on disk drive is much slower than accessing it in main memory secondyou must access many records at once let' explore these points very slow access computer' main memory works electronically any byte can be accessed just as fast as any other bytein fraction of microsecond ( millionth of secondthings are more complicated with disk drives data is arranged in circular tracks on spinning disksomething like the tracks on compact disc (cdor the grooves in an old-style phonograph record to access particular piece of data on disk drivethe read-write head must first be moved to the correct track this is done with stepping motor or similar deviceit' mechanical activity that requires several milliseconds (thousandths of secondonce the correct track is foundthe read-write head must wait for the data to rotate into position on the averagethis takes half revolution even if the disk is spinning at , revolutions per minuteabout more milliseconds pass before the data can be read once the read-write head is positionedthe actual reading (or writingprocess beginsthis might take few more milliseconds thusdisk access times of around milliseconds are common this is something like , times slower than main memory technological progress is reducing disk access times every yearbut main memory access times are being reduced fasterso the disparity between disk access and main memory access times will grow even larger in the future one block at time when the read-write head is correctly positioned and the reading (or writingprocess beginsthe drive can transfer large amount of data to main memory fairly quickly for this reasonand to simplify the drive control mechanismdata is stored on the disk in chunks called blockspagesallocation unitsor some other namedepending on the system we'll call them blocks the disk drive always reads or writes minimum of one block of data at time block size variesdepending on the operating systemthe size of the disk driveand other factorsbut it is usually power of for our phone book examplelet'