1. Understanding Computers
Chapter 3
Storage
Understanding Computers

2. Understanding Computers
Overview
This chapter covers:
Common characteristics of storage systems
Magnetic disk storage systems
Optical disc storage systems
Other types of storage, such as flash memory, magnetic tape, remote storage, and smart cards
Understanding Computers

3. Properties of Storage Systems: Storage Devices and Media
Involve two physical parts: a storage device and a storage medium.
Can be internal (located inside the system unit), or external (located outside the system unit).
Storage devices are identified by names and/or letters (e.g. “C” for first hard drive).
Understanding Computers

4. Properties of Storage Systems: Non-Volatility
Storage media are non-volatile:
When power to the device is shut off, data stored on the medium remains.
This is in contrast to most types of memory, which are volatile.
Understanding Computers

5. Properties of Storage Systems: Removable vs. Fixed Media
Fixed media: typically faster and less expensive.
Removable media: unlimited capacity and can be easily transported and secured.
Understanding Computers

6. Properties of Storage Systems: Random vs. Sequential Access
Random access ( direct access ): data can be retrieved in any order, independent of its physical location (most types of storage media).
Sequential access: data can be retrieved only in the same sequence in which it is physically stored (magnetic tape).
Understanding Computers

7. Properties of Storage Systems: Logical vs. Physical Representation
Logical file representation refers to the user’s view of the way data is stored (filename, folders, etc.).
Physical file representation is the actual physical way the data is stored on the storage media as viewed by the computer.
Understanding Computers

9. Understanding Computers
Magnetic Disk Systems
Magnetic disks are the most widely used storage medium in computers today.
Data is stored by magnetizing particles on the storage medium.
Two common types:
Floppy disks
Hard disks
Understanding Computers

10. Understanding Computers

11. Magnetic Disks and Drives, Cont’d.
Disk is divided into tracks, sectors, and clusters.
The disk’s file directory keeps track of the contents of the disk so files can be retrieved (by filename) at a later time.
Understanding Computers

12. Magnetic Disks and Drives
Floppy disk characteristics
Typically 3½ inches in diameter.
Typically hold 1.44 megabytes.
Inserted into floppy disk drive to be read from or written to.
Understanding Computers

13. Understanding Computers
Hard Disk Drives
Hard drive characteristics
Metal disk onto which data is stored magnetically.
Disks are usually permanently sealed inside the hard drive—allows faster speeds and storage of more data than removable systems.
Can be internal or external.
Understanding Computers

15. Hard Disk Drives, Cont’d.
Hard drive characteristics, cont’d
Organized into tracks, sections, clusters, and cylinders (the collection of tracks located in the same location on a set of hard disk surfaces).
Read/write head doesn’t touch the surface of the disk.
Bumping PC when disk is being accessed or dust or other obstacles on a hard-disk system can cause a head crash and damage the surface of the disk.
Understanding Computers

16. Hard Disk Drives, Cont’d.
Disk access time.
Factors: seek time, rotational delay, data movement time
Can use multiple partitions.
Disk cache—strategy for speeding up system performance.
Hard drive standards (EIDE, SCSI, Fibre Channel, USB) .
Understanding Computers

17. Hard Disk Drives, Cont’d.
Portable hard drive systems
Offer large storage capacities and portability.
Either entire drive or just hard disk cartridge is transported.
Understanding Computers

18. Hard Disk Drives, Cont’d.
Storage systems for large computer systems and networks
Storage servers containing racks of hard drives.
Network attached storage (NAS).
Storage area networks (SANs).
RAID (for increased performance and/or fault tolerance).
Understanding Computers

19. Understanding Computers
Optical Disc Systems
Laser beams write and read data packed at very tight storage densities, many times finer than that of a typical magnetic disk.
Are typically 4½-inch discs, but can be a variety of shapes and sizes.
Data is stored optically on a continuous spiral track.
Can be CDs or DVDs.
Understanding Computers

20. Understanding Computers

21. Read-Only Discs: CD-ROM and DVD-ROM Discs
CD-ROM discs cannot be written to or erased and typically hold 650 MB (e.g. music CDs).
DVD-ROM discs are similar to CD-ROM discs, but have higher capacity of 4.7 GB to 17 GB (e.g. movie DVDs).
Data is stored by burning pits into the disc surface that can be read using a laser beam.
Understanding Computers

22. Recordable Discs: CD-R, DVD+/-R
Recordable discs can be written to, but not erased or rewritten.
Burning a CD or DVD permanently records data onto the disc.
CD-R discs are commonly used to store data and for custom music CDs.
DVD-R/DVD+R discs are commonly used for home movies and other high-capacity applications.
Understanding Computers

23. Understanding Computers

24. Rewritable Discs: CD-RW, DVD+/-RW
Rewritable discs can be used in the same way as magnetic disks—data can be stored and erased as needed.
Phase-change technology is used to record data onto the disc using reflectivity.
Rewritable discs can be erased by reverting the reflectivity back to the original state.
Blue laser discs are an emerging higher-capacity (23.3 GB) form of optical disc.
Understanding Computers

25. Other Types of Storage Systems
Flash memory media
Chip-based storage.
Solid state storage system—has no moving parts, so are more shock-proof and portable than conventional storage systems.
Commonly used with digital cameras, digital music players, handheld PCs, notebook computers, smart phones, etc.
Understanding Computers

26. Understanding Computers

27. Other Types of Storage Systems, Cont’d
Flash memory media, cont’d.
Common forms of flash memory:
Flash memory sticks.
Flash memory cards—CompactFlash, Secure Digital (SD), MiniSD, MultiMedia (MMC), SmartMedia, and xD Picture cards.
Flash memory drives, such as USB flash drives.
Understanding Computers

28. Other Types of Storages Systems, Cont’d
Magnetic tape systems—plastic tape that is magnetized to represent data.
Used primarily for backup and archival purposes (sequential access only).
Read from and written to via a tape drive.
Most tape media are in the form of cartridge tapes, though detachable-reel tapes exist as well.  
Understanding Computers

29. Other Types of Storage Systems, Cont’d
Remote storage—storage devices that are not directly connected to your PC.
Network storage—accessible through a local network.
Online storage—accessible via the Internet and used for backup, as well as to transfer files to others.
Understanding Computers

31. Other Types of Storage Systems, Cont’d
Smart card: credit-card-sized piece of plastic that contains some type of computer circuitry.
Holds less than a few megabytes.
Commonly used to store prepaid amounts of digital cash or personal information.
Smart card readers are attached to a PC or built into a cell phone, keyboard, or other device.
Understanding Computers

33. Other Types of Storage Systems, Cont’d
Holographic storage
Uses multiple laser beams to store data in three dimensions.
Stores data in page format.
No moving parts and simultaneous access to all data on a page.
Understanding Computers

34. Comparing Storage Alternatives
Factors to consider: speed, expense, portability, storage capacity, and compatibility.
Most PC users require:
Hard drive
CD or DVD drive
Floppy drive
Additional devices (flash memory card reader, etc.) as needed for the devices being used in conjunction with the PC (e.g. digital camera)
Understanding Computers

35. Understanding Computers
Summary
Properties of storage systems
Magnetic disk systems
Optical disc systems
Other types of storage systems
Comparing storage alternatives
Understanding Computers