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Understanding Computers 1
Chapter 3
Storage
Understanding Computers 2
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).
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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.
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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.
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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
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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.
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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
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.
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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.
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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
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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
Summary
Properties of storage systems
Magnetic disk systems
Optical disc systems
Other types of storage systems
Comparing storage alternatives
