they are not erased from disk and tape when the power is turned off in contrastram used for main memory is usually volatile -all information is lost with the power second advantage is that floppy diskscd-romsand "flashdrives can easily be transferred between computers this provides convenient way to take information from one computer to another in exchange for reduced storage costspersistenceand portabilitysecondary storage devices pay penalty in terms of increased access time while not all accesses to disk take the same amount of time (more on this later)the typical time required to access byte of storage from disk drive in is around ms ( thousandths of secondthis might not seem slowbut compared to the time required to access byte from main memorythis is fantastically slow typical access time from standard personal computer ram in is about - nanoseconds ( - billionths of secondthusthe time to access byte of data from disk drive is about six orders of magnitude greater than that required to access byte from main memory while disk drive and ram access times are both decreasingthey have done so at roughly the same rate the relative speeds have remained the same for over twenty-five yearsin that the difference in access time between ram and disk drive has remained in the range between factor of , and , , to gain some intuition for the significance of this speed differenceconsider the time that it might take for you to look up the entry for disk drives in the index of this bookand then turn to the appropriate page call this your "primary memoryaccess time if it takes you about seconds to perform this accessthen an access taking , times longer would require months it is interesting to note that while processing speeds have increased dramaticallyand hardware prices have dropped dramaticallydisk and memory access times have improved by less than an order of magnitude over the past ten years howeverthe situation is really much better than that modest speedup would suggest during the same time periodthe size of both disk and main memory has increased by about three orders of magnitude thusthe access times have actually decreased in the face of massive increase in the density of these storage devices due to the relatively slow access time for data on disk as compared to main memorygreat care is required to create efficient applications that process diskbased information the million-to-one ratio of disk access time versus main memory access time makes the following rule of paramount importance when designing disk-based applicationsminimize the number of disk accesses