External Memory Devices Overview

External Memory Devices

• Types include Magnetic Tape, Magnetic Disk, RAID, Optical Memory, and SSD.

Magnetic Tape

• Utilizes magnetizable material on a flexible tape.
• Contains parallel tracks with nine tracks storing one byte each.
• Advantages: Low cost.
• Disadvantages: Requires sequential access and manual positioning for data retrieval.

Magnetic Disk

• Comprised of circular platters coated in magnetizable material.
• Employs a conducting coil (head) for read/write operations.
• Types include Floppy disk (single head) and others with dual heads for simultaneous operations.
• Data stored in concentric rings called tracks, divided into sectors of 512 bytes each.
• Formats involve specific data patterns at sector boundaries.

Data Formatting for Magnetic Disk

• Sector components:
  • 17 Bytes for sector start
  • 7 Bytes for sector ID
  • 41 Bytes for gaps
  • 512 Bytes of actual data
  • 20 Bytes for sector end

Physical Characteristics of Magnetic Disk

• Head Movability: Fixed or movable.
• Mounting Method: Removable or non-removable.
• Number of Sides: Single-sided or double-sided.
• Platters: Single or multiple platters.

Head Mechanism in Magnetic Disk

• Consists of fixed distance to disk with physical contact.
• Air gap changes during operation for discs like Winchester disks.

Hard Disk Reading Mechanisms

• Constant Angular Velocity (CAV) and Multiple Zone Recording (MZR).
• MZR maintains constant data density, allowing higher storage capacity in inner sectors.
• CAV advantages: simplicity without complex circuitry and benefits with data balance.

RAID (Redundant Array of Independent Disks)

• Combines multiple physical disks viewed as one single logical drive.
• Distributes data among disks while using parity information for data integrity.

RAID Levels Overview

• RAID Level 0: Not a true RAID system; data distribution increases I/O transfer rates.
• RAID Level 1: Duplicates all data across two disks; enhances data recovery during disk failure.
• RAID Level 2: Includes additional disks for parity data, correcting single-bit errors with small strip sizes.
• RAID Level 3: Utilizes very small strips for high data transfer rates with parallel disk use.