Magnetic Disk

What is a Magnetic Disk and How it works. Types of Magnetic Disk with data organisation.

1. PRESENTATION ON MAGNETIC DISK

2. Magnetic disk : A magnetic disk is the primary storage disk in a computer. A magnetic disk is a used to read from and write data to the disk. The data on a magnetic disk are read and written using a magnetization process. It is covered with a magnetic coating and stores data in the form of tracks and sectors . Hard disks, zip disks and floppy disks are common examples of magnetic disks.

3. Magnetic disk diagram:

4. Magnetic Disk continuous… A disk is a circular platter constructed of non magnetic material, called the substrate. Disk substrate coated with magnetizable material (iron oxide) Substrate used to be aluminium or aluminium alloy material. Benefits of glass substrate: Improve surface uniformity Increase reliability Reduction in surface defects Reduced read/write errors

5. Magnetic Disk Magnetic Read and Write Memory: Magnetic disks remain the most important component of external memory Both removable and fixed, or hard disks are used in systems ranging from personal computers to mainframes and supercomputers. Data are recorded on and later retrieved from the disk via a conducting coil named the head . In many systems, there are two heads, a read head and a write head.

6. Magnetic Disk Magnetic Read and Write Memory: The write mechanism the electricity flowing through a coil produces a magnetic field. Electric pulses are send to the write head and the resulting magnetic patterns are recorded on the surface below, with different patterns for positive and negative currents. The read mechanism exploits the fact that a magnetic field moving relative to a coil produces an electrical current in the coil. When the surface of the disk passes under the head, it generates a current of the same polarity as the one already recorded. The read head consists of a partially shielded magneto resistive (MR) sensor. The MR material has an electrical resistance that depends on the direction of the magnetization of the medium moving under it.

7. Magnetic Disk Data Organization and Formatting: The head is a relatively small device capable of reading from or writing to a portion of the platter This gives rise to the organization of data on the platter in a concentric set of rings, called tracks . Each track is the same width as the head. There are thousands of tracks per surface.

8. Data Organization and Formatting Disk Velocity: Increase spacing between bits in different tracks. The information can then be scanned at the same rate by rotating the disk at fixed speed, known as the constant angular velocity(CAV) The disk is divided into a number of pie-shaped sectors and into a series of concentric tracks. The advantage of using CAV is that individual blocks of data can be directly addressed by track and sector. The disadvantage of CAV is that the amount of data that can be stored on the long outer tracks is the only same as what can be stored on the short inner tracks.

9. Continuous…. To increase density, modern hard disk systems use a technique known as multiple zone recording , in which the surface is divided into a number concentric zones(16 is typical) Within a zone, the number of bits per track is constant Zones further from the center contain more bits(more sectors) than zones closer to the center. This allows for greater overall storage capacity at the expense of some what more complex circuitry.

10. Magnetic Disk Physical Characteristics:

11. Head Motion Fixed / Movable head Disk: Fixed head One read write head per track Heads mounted on fixed ridged arm Movable head One read write head per side Mounted on a movable arm

12. Disk Portability Removable or Not: Removable disk Can be removed from drive and replaced with another disk Provides unlimited storage capacity Easy data transfer between systems Non removable disk Permanently mounted in the drive.

13. Platters Multiple Platter: One head per side Heads are joined and aligned Aligned tracks on each platter form cylinders. Data is striped by cylinder Reduces head movement Increase speed(transfer rate)

14. Head Mechanism Floppy disk: 8’’, 5.25’’, 3.5’’ Small capacity Up to 1.44Mbyte Slow Universal Cheap Obsolete

15. Winchester Hard Disk(1): Developed by IBM in Winchester(USA) Sealed unit One or more platters(disks) Heads fly on boundary layer of air as disk spins Very small head to disk gap Getting more robust.

16. Winchester Hard Disk(2): Universal Cheap Fastest external storage Getting larger all the time 250 Gigabyte now easily available

17. Disk Performance Parameters: Seek time Moving head to correct track (Rotational) latency Waiting for data to rotate under head Access time=Seek + Latency Transfer rate