Comparision of access time of storage devices

If you have ever used an old PC (5 years or more) then you may have noticed sometimes that although there may be sufficient RAM still programs run slow even if the processor is very capable of running that program. This may happen due to a slow Hard disk drive. Usually programs run in the memory and data in memory can be access very quickly.But when program requires too many disk accesses for data stored on hard drive then it takes much more execution time.

Access time for some devices are given below:-
                                                 Access time(millisecond)                  Burst Read Speeds
     IDE drives (5 years old)                    6-8 ms                                      66 MB/s
     SATA Hard disks                               3-6 ms                                    150 MB/s (SATA 1)
     USB flash drives                             ~0.5 ms                                      48 MB/s (USB 2.0)
     Solid State drives                        0.5-0.1 ms                                    300 MB/s (SATA 2)

Compare this with Access time for main memory RAM and you will see huge differences. Although latency depends on RAM speeds they are in the range 1 to 50 ns(nano seconds).

                                   1 ms = 10^-3 s and 1 ns= 10^-9 s.

So RAM is 10⁶ times quicker than Hard drives. Lower access time of flash memory can be used to enhance PC performance using Windows Ready boost feature.