Disk

Secondary Storage

Magnetic Disk Storage

Improving Disk Drive Performance

Personal Computer Storage

Optical Storage

Data Organization

File Organization

Processing Data into Information

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Secondary Storage

⊕ Benefit

♦ Economy

♦ Reliability

♦ Convenient

⊕ Magnetic Tape

♦ Forms (inch in Width)

    1/2 inch-wide tape wound on reel

    1/4 inch-wde tape in cartridges andcassettes

♦ Amount of Data measure by density

    character per inch (cpi)

    byte per inch (bpi)

♦ Data Representation

    tracks or channels (9 tracks)

    one crose section represents one EBCDIC (8+P) character

♦ Magnetic Tape Unit (Tape Drive)

    read/write head

    erase head

    supply reel and take-up tape

♦ Blocking

    Interblock gap (IBG) or Interrecord gap (IRG)

♦ Digital Audio Tape (DAT) and Helical Scan Recording

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Magnetic Disk Storage

⊕ Hard Disk

    Faster, expensive, large capacity

♦ Traditional

♦ Winchester

    sealed module, chaper, more convenient.

    used by all PC's

⊕ Diskette (Floppy Disk)

    Slower, cheap, small capacity

⊕ Data Organization On Disks

♦ Sector Method

    Data are referred by surface #, track #, and sector number

♦ Cylinder Method

    all the vertical tracks ofone cylinder are accessible by the read/write heads wit one positioning of the access arms mechanism.

⊕ Disk Access to Data

♦ Seek time

♦ Head Switching

♦ Rotation Delay

♦ Data Transfer

♦ Average access time 9ms to 40ms (could be slower)

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Improving Disk Drive Performance

⊕ Caching

♦ use cache to stored used data in main memory for later reuse

♦ Hit rate (Hit ratio)

♦ Data Synchronization (data sync)

    write-through

    write-back

⊕ Zone Recording

⊕ Data Compression

⊕ Using several disk together

♦ Disk Mirroring

♦ RAID (Redundant Array of Inexpensive Disks)

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Personal Computer Storage

⊕ Diskettes (Floppy Disks)

    3.5"/1.44M, 5.25"/1.2M

⊕ Hard Disks

    5.25", 3.5" Wenchester (Full Height, Half Height, Quater Height)

    540Mbyte, 1 Gbyte, cost several thousand dollors NT.

    For Notebook PCs, 1.8", 1.3" (Kitty Hawk, 20Mbyte)

⊕ Removable Storage

⊕ Hard Cards

⊕ RAM disks

⊕ Flash Memory

⊕ Flash Memory (行動碟）

⊕ Backup System

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Optical Storage

⊕ How it work?

♦ Write Operation

    Use powerful laser beam to hit up a spot on the surface

♦ Read Operation

    Use a laser beam to light up the spot to read back the data by detecting the difference in reflection

    Mostly Read-only

♦ Write-once, read-many (WORM)

♦ CD-ROM (Compact Disk read-only memory)

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Data Organization

⊕ Character

♦ a letter, number, or special character

⊕ Field

♦ a set of related characters

⊕ Record

♦ a collection of related fields

⊕ File

♦ a collection of related records

⊕ Database

♦ a collection of interrelated files stored together with minimum redundancy

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File Organization

⊕ Sequential File Organization

♦ records are organized in sequential order by key.

⊕ Direct File Organization

♦ records are organized randomly

♦ Use Hashing techniques

⊕ Indexed File Organization

♦ Combination of both

♦ indexes are sequentially ordered

♦ data record are randomly stored

⊕ Why is tape still needed?

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Processing Data into Information

⊕ Batch Processing

⊕ Online Transaction Processing (OLTP)

♦ Real-time vs. Batch Processing

♦ Online vs. Offline Processing

♦ Timesharing System

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