External Memory[Week 6][Rizal Damajuwananto][2311949]

“External Memory”

External Memory

External memory, such as magnetic disks and RAID systems, is a vital part of modern data storage, helping computers store, organize, and access information efficiently. Here’s a detailed explanation of key concepts:

Magnetic Disk Overview

Magnetic disks, like Hard Disk Drives (HDDs), are widely used storage devices that save data by magnetizing small sections on their surface.

Key Components:

1. Platters: Circular disks coated with magnetic material where data is stored.
2. Read/Write Head: A small device that reads or writes data by hovering very close to the platter’s surface.
3. Spindle Motor: Rotates the platters at high speeds (thousands of revolutions per minute), allowing the read/write head to quickly access data.
4. Actuator Arm: Moves the read/write head to the correct position on the platter.

How Data Is Stored:

• Tracks: Circular paths on the platter where data is recorded.
• Sectors: Subdivisions of tracks, each holding a small portion of data.
• Cylinders: Aligned tracks on multiple platters that allow simultaneous access to data.
• Gaps: Spaces between tracks and sectors to prevent interference.

Read/Write Mechanisms:

• Writing: The read/write head uses electromagnetism to change the magnetic orientation of bits, representing binary data (0s and 1s).
• Reading: The head senses the magnetic orientation of bits and translates them into binary data, which the computer processes into files, programs, or images.

Disk Performance Parameters

1. Seek Time: Time taken for the read/write head to move to the desired track.
2. Rotational Delay: Time it takes for the desired sector to align with the read/write head.
3. Access Time: Total time to position the head and start reading or writing data (seek time + rotational delay).
4. Transfer Time: Time to actually read or write the data once in position.

These parameters influence how quickly data can be accessed or stored on a magnetic disk.

RAID (Redundant Array of Independent Disks)

RAID combines multiple hard drives to improve speed, storage capacity, and reliability. Here are the main RAID levels:

RAID 0 (Focus on Speed):

• Splits (or stripes) data across multiple drives for faster access.
• No redundancy, so data is lost if a single drive fails.
• Best for: Tasks like video editing that prioritize speed.

RAID 1 (Focus on Redundancy):

• Mirrors data on two or more drives, creating identical copies.
• If one drive fails, the system continues running with the backup.
• Best for: Critical applications needing high reliability, like databases.

RAID 5 (Balance of Speed and Reliability):

• Stripes data with parity (error-checking data) across multiple drives.
• Can recover data if one drive fails.
• Best for: File servers and applications needing efficient storage.

RAID 6 (Enhanced Fault Tolerance):

• Similar to RAID 5 but with two sets of parity, allowing recovery from two simultaneous drive failures.
• Best for: High-reliability applications handling large datasets.

File Systems and Access Methods

A file system is the structure used by a computer to store, organize, and retrieve data. It determines how files are arranged and accessed on storage devices.

Access Methods:

1. Sequential Access:

• Data is accessed one after another, in order.
• Example: Tapes used for backups or archives.
• Use: Situations where storage capacity matters more than access speed.

2. Direct Access:

• Data can be read or written at any location without going through other data.
• Example: HDDs and SSDs.
• Use: Operating systems and applications needing quick access to specific data.

3. Indexed Access:

• Uses an index to locate data quickly, like a library catalog.
• Example: Databases or large storage systems.
• Use: Systems requiring fast access to non-sequential data.

Storage Hierarchy:

The file system is part of a larger memory system, ranging from high-speed temporary memory (like RAM) to slower, high-capacity storage (like hard drives). File systems ensure data is efficiently managed across these levels.

Conclusion

External memory, including magnetic disks, RAID systems, and file systems, plays a crucial role in modern data management. Magnetic disks offer structured and reliable storage, RAID enhances performance and fault tolerance, and file systems determine how data is accessed. Together, these components form the foundation of efficient and scalable storage solutions, meeting the demands of diverse applications.