External Memory

Magnetic Disk Overview

A magnetic disk, like a hard disk drive (HDD), is a data storage device using a magnetic coating to store information.

Key Parts of a Magnetic Disk:

Platters: Circular disks coated with magnetic material. Data is stored here.

Read/Write Head: Hovers above the platter surface, reading and writing data.

Spindle Motor: Spins the platters at high speeds for quick data access.

Magnetic Read and Write Mechanisms

Writing Data:

• Magnetic Patterns: The read/write head creates a magnetic field to alter magnetization.
• Binary Data: Magnetization direction represents binary data (0s and 1s).
• Electromagnetism: The write head contains an electromagnet to control data patterns.

Reading Data:

• Sensing Magnetic Fields: The read head detects magnetic orientations.
• Electrical Resistance: Changes in magnetization induce resistance changes, translating into binary data.
• Digital Data Translation: Binary data is sent to the computer’s processor for interpretation.

Data Organization and Formatting:

• Tracks: Concentric rings on the platter where data is recorded.
• Sectors: Subdivisions of tracks, typically 512 bytes each.
• Inter-sector and Inter-track Gaps: Prevent errors due to misalignment or interference.
• Cylinders: Data access across multiple platters at the same physical position.

Disk Performance Parameters:

• Seek Time: Time to position the head at the track.
• Rotational Delay: Time for the sector to align with the head.
• Access Time: Sum of seek time and rotational delay.
• Transfer Time: Time to read or write data once in position.

Redundant Array of Independent Disks (RAID):

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RAID 0

• No Redundancy: Data loss if a disk fails.
• Data Striping: Faster access by splitting data across disks.
• Best for: Applications needing speed and capacity over reliability.

RAID 1

• Mirrored Disks: Data duplicated for redundancy.
• Best for: Applications needing reliability and quick recovery.

RAID 5

• Data Striping with Parity: Balances speed and fault tolerance.
• Best for: Applications needing balanced speed, reliability, and efficient storage.

RAID 6

• Double Parity: Can handle failure of up to two disks.
• Best for: Applications requiring high reliability.

File System and Access Methods

• Sequential Access: Data read in order. Example: Tapes.
• Direct Access: Data read from any location. Example: Hard drives.
• Indexed Access: Uses an index to locate data quickly. Example: Library catalog.

Storage Hierarchy: File systems manage data across different memory levels, from registers to secondary storage.

File Organization Techniques: File systems use sequential, indexed, or direct-access methods to optimize data retrieval.