Magnetic Media and Storage Devices

Magnetic Media and Magnetic Storage Devices

One of the oldest methods for electronic data storage involves using magnetism, dating back to the nineteenth century. Current magnetic media operate on the principle that an iron oxide coating can be magnetized to represent a binary 1 and demagnetized to represent a binary 0. The small size of each magnetized area allows for the storage of vast amounts of data. A key advantage is the permanence of the iron oxide's magnetic state unless overwritten. The most common devices utilizing these magnetic properties are magnetic tape drives and hard disk drives (HDDs).

Magnetic Tape Drives

A magnetic tape consists of a thin strip of plastic coated with a magnetic layer (iron oxide). Data is read from or written to the tape by a read/write head in a magnetic tape storage device. Data is stored as magnetized areas (representing a 1) or demagnetized areas (representing a 0). Data is accessed serially, meaning it can only be read in the order it was written. This makes magnetic tape unsuitable for real-time or online applications due to slow data access speeds but well-suited for offline or batch processing. Despite this, magnetic tapes are still used in large industrial or university computers because of their vast storage capacity.

Uses of Magnetic Tape

Applications using batch processing (e.g., clearing bank checks, utility billing (gas, electricity, water), and producing payslips), where specific processing order and data access speed are not essential.
Backup media for storing vast amounts of data.
Long-term archiving of data due to their huge data storage capacities and stability.

Advantages of Magnetic Tapes

Generally less expensive (per byte) than hard disk drives.
Robust technology that does not deteriorate much over time.
Huge data storage capacity.
Fast data transfer rate (but slow data access time).

Disadvantages of Magnetic Tape

Very slow data access times due to serial access, requiring all preceding data to be read before the required data is found; although the data access time is slow the data transfer rate is still high.
Updating requires another tape to store the final updated version, necessitating a master tape (original) and a transaction tape (containing all changes) to produce a new master tape. This updating process is slow and prone to errors.
Susceptible to magnetic fields; strong magnets (e.g., in loudspeakers) can corrupt data.

Hard Disk Drives (HDD)

HDDs are a common method for storing data on computers. Data is stored digitally on the magnetic surfaces of hard disks (platters). An HDD often contains multiple platters depending on its capacity. Read/write heads access all platter surfaces (typically two surfaces per platter store data). These heads move quickly, traversing from the center to the edge of the disk (and back) up to 50 times per second. Data on an HDD is read using direct access, meaning earlier data does not need to be read first. The HDD works as follows:

Actuators move the read/write heads (voice coils are used, similar to electromagnets in speakers).
A read/write arm swings the read/write head across the platter, which rotates at up to 10000 rpm (revolutions per minute).
Each read/write head contains a tiny magnet for reading data on the platter.
Platters are made from glass, ceramic, or aluminum coated in iron oxide.
There are two read/write heads per platter (one for the top and one for the bottom surface).
Data is stored in concentric, circular tracks, each broken up into sectors.
A file allocation table (FAT), stored on the HDD, maps the sectors. When the computer needs to store new data, it consults the FAT to find free sectors and moves the read/write heads to the correct location, speeding up the writing process.

Latency

While HDDs have faster data access times than magnetic tape, small delays still occur. Many applications require constant read/write head movements to access data blocks, increasing the significance of latency. Latency is the time it takes for a specific data block on a track to rotate to the read/write head. Users may notice latency effects, especially with multiple open applications, indicated by messages like "Please wait" or "Not responding." HDDs can be fixed or portable.

Uses of Fixed Hard Disk Drives

Store the operating system, system software, and working data/files.
Store applications software.
Used in real-time systems (e.g., robots, chemical plant control) and online systems (e.g., booking airline tickets, automatic stock control using EPOS).
Used in file servers for computer networks.

Advantages of Fixed Hard Disk Drives

Very fast data transfer rate and fast access times to data.
Very large memory capacities.

Disadvantages of Fixed Hard Disk Drives

Can be easily damaged (e.g., head crash if the computer is not shut down correctly).
Many moving parts can affect reliability.
Read/write operation can be quite noisy compared to solid-state drives.