Hard Disk Drive Design and Technology
Magnetic Hard Disk Drive
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- Introduction
- Basic principles of magnetic recording
- Basic drive design
- Basic drive concepts
- Cache or Buffer
- Formatting
- File system
- Drive interfaces
- Drive installation
Introduced in 1957, the IBM-made random access method of accounting and control (RAMAC) was the first disk drive. It consisted of 50 magnetic disks of 24-inch in diameter and rotating at 1200 RPM (rotations per minute).
Two air-bearing supported magnetic heads accessed all 50 disks. The storage capacity of this system was 5 MB with the data rate of 12.5 kB/s, and the system was rented to the end user for $130 a month. The system used Aluminum sliders, Mu-metal heads, had 20 tracks per inch and 0.002 Mb/in2, and the slider / disk spacing was about 20 micrometers (micro-meters = 20,000 nm).
About 40 years later, the IBM Travelstar 6GT drive has 3 thin-film 2.5 inch disks and 6 ceramic sliders with the heads working on magneto-resistive principles and flying over the disk surface at only about a few dozens nm. The track density is 16,000 TPI (track per inch) and the areal density is 4.1 GB/in2. The drive's 33.3 MB/sec data transfer rate benefits from the Ultra DMA interface. In fact, this drive is not the newest and the best, it just was randomly chosen to make one point - there has been a huge progress in the field of hard disk drive (HDD) technology in the 40 years, and the rate of this progress is just increasing year after year.
The HDD is used by a computer to store operating system (OS) and the user's data. Fierce competition between the drive manufacturers has pushed the cost of one MB of data to a very small number of $1 to $2 per MB making a HDD of several GB in capacity relatively inexpensive and easily affordable by almost anyone. The HDD is one of the most important component of the modern PC: no application will run reasonably without the hard drive. The HDD works using the technology called magnetic recording, the principles of which will be discussed in the next chapter.
Basic principles of magnetic recording
This figure illustrates a writing sequence:
- The drive channel electronics receive data in binary form from the computer and converts them into a current in the head coil. The current in the coil reverses at each 1 and remains the same at each 0.
- This current interaction with the media results in magnetization of the media, which direction depends on the current direction in the coil.
The reading process includes excitation of the current in the head coil when the head "senses" changes in the magnetic flux. The read voltage pulses at the flux transitions are then translated into sequences of bits equal to 0 and 1. The so-called Wallace's spacing loss factor postulates that the loss of magnetic signal power will be proportional to the media - head separation. This requires magnetic heads to fly as close to the disk surface as possible, which forces modern heads to fly at a few nanometers only (compare this to 20,000 nanometers back in 1957!).
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