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2.3.2 Mass Storage

  • How Mass Storage Devices Differ from RAM

  • Disk Drive Reliability

  • Optical Media: CDs versus DVDs

  • Magnetic Media

  • Optical versus Magnetic

  • Solid State

  • Comparing Storages

How Mass Storage Devices Differ from ram

Mass storage devices (magnetic disks, optical disks, and magnetic tape) have slow access times and low transfer rates. They are also located far from the processor. These mass storage devices are not even on the motherboard. (Sometimes they are not even in the same box as the motherboard!) But, mass storage technologies also have several important advantages:

  • They are nonvolatile—meaning that information is not lost when power is turned off.

  • They have huge capacities, measured in billions or even trillions of bytes.

  • Their cost per bit stored is far lower than RAM.

  • In some cases, they use removable media that can be popped into a drive, used as needed, and then taken out of the drive, or mailed to a friend. Several newer removable disk technologies have appeared since the 3.5-inch floppy (1.44 MB). The Iomega Zip drive uses removable disks with capacities of 100 to 750 MB, and the CD Read/Write drive uses writable optical disks with 650 MB or 700 MB of storage capacity. These days, the compact flash drives (also called thumb drives) which plug into USB ports have taken over the duties of the floppy or Zip drives. The memory capacity of these flash drives range from 128 MB to 4 GB. On the higher end, we now have external hard drives with 600 GB storage capacity. These types of drives are currently popular for making backups of a system's main hard drive, or for transferring large data files from one site to another. On the optical side, we now have dual layer writable DVD disks which have a storage capacity as high as 8.5 GB. As hard drive capacity increases and optical disk technology matures, creating backups on external hard drives and re-writable optical disks has become a popular alternative.

Disk Drive Reliability

The disk drive marketplace is highly competitive because there is a huge consumer demand for disks with high capacities, fast transfer rates, and low costs. In order to meet these goals, disk manufacturers are continually advancing the state of the art for magnetic coatings, high-strength magnets (used in the motors that drive disk drive arms), and ultra-clean manufacturing technology.

In order to achieve high data transfer rates, a hard disk drive has to spin very fast: from 4,000 to 15,000 rpm (revolutions per minute). Also, magnetic patches that store data bits on the hard disk tend to be small and packed closely together. Therefore, in order to read these tiny little bits spinning at such high speed accurately, the read/write head has to be very, very close to the disk surface. The head rides on a cushion of air, and the distance between it and the disk surface is far less than the width of a human hair—less than the size of a single dust particle. In fact, if a head encounters a dust particle sitting on the surface of a disk while the disk is spinning at several thousand rpm, the head will crash into the disk, damaging itself and the magnetic coating on the disk. It is important to back up all data onto tape or another disk regularly.

A common specification for disk drive reliability is mean time between failures (MTBF), the average time (in hours) a device is expected to function before failing. Typically, disk drives for PCs have MTBF ratings of about 500,000 hours, 57 years. However, MTBF is a theoretical estimate because drives have not existed long enough for their life expectancy to be verified. The MTBF rating should be used in conjunction with service life. Service life is the amount of time before failures occur due to increased wear and tear of the component devices. It is how long the manufacturer predicts the disk would last. When shopping for disks, use the service life as a more accurate estimate for how long the disk will last before failures occur.