What are the functions of magnetic disk?

Answer 1

1. The storage capacity of a single disk ranges from 10MB to 10GB. A typical commercial database may require hundreds of disks.
2. Figure 10.2 shows a moving-head disk mechanism.
   • Each disk platter has a flat circular shape. Its two surfaces are covered with a magnetic material and information is recorded on the surfaces. The platter of hard disks are made from rigid metal or glass, while floppy disks are made from flexible material.
   • The disk surface is logically divided into tracks, which are subdivided into sectors. A sector (varying from 32 bytes to 4096 bytes, usually 512 bytes) is the smallest unit of information that can be read from or written to disk. There are 4-32 sectors per track and 20-1500 tracks per disk surface.
   • The arm can be positioned over any one of the tracks.
   • The platter is spun at high speed.
   • To read information, the arm is positioned over the correct track.
   • When the data to be accessed passes under the head, the read or write operation is performed.
3. A disk typically contains multiple platters (see Figure 10.2). The read-write heads of all the tracks are mounted on a single assembly called a disk arm, and move together.
   • Multiple disk arms are moved as a unit by the actuator.
   • Each arm has two heads, to read disks above and below it.
   • The set of tracks over which the heads are located forms a cylinder.
   • This cylinder holds that data that is accessible within the disk latency time.
   • It is clearly sensible to store related data in the same or adjacent cylinders.
4. Disk platters range from 1.8" to 14" in diameter, and 5"1/4 and 3"1/2 disks dominate due to the lower cost and faster seek time than do larger disks, yet they provide high storage capacity.
5. A disk controller interfaces between the computer system and the actual hardware of the disk drive. It accepts commands to r/w a sector, and initiate actions. Disk controllers also attach checksums to each sector to check read error.
6. Remapping of bad sectors: If a controller detects that a sector is damaged when the disk is initially formatted, or when an attempt is made to write the sector, it can logically map the sector to a different physical location.
7. SCSI (Small Computer System Interconnect) is commonly used to connect disks to PCs and workstations. Mainframe and server systems usually have a faster and more expensive bus to connect to the disks.
8. Head crash: why cause the entire disk failing (?).
9. A fixed dead disk has a separate head for each track -- very many heads, very expensive. Multiple disk arms: allow more than one track to be accessed at a time. Both were used in high performance mainframe systems but are relatively rare today.

Answer 2

Original optical disc was the CD. These CD's store data with accordence to a file system structure for either digital recorded music or plain binary data as found in a computers filesystem. Eventually, these were improved in the past five years (to 2009) with greater capacity of data for CD'S up to 750 Mega bytes and then dual(double) sided (2x 750Mb). The filesystem on CD's is not the same in actuallity as a hard drive or Floppy disc and there are various formats for CD's and DVD's. Again the same is of DVD's, but DVD physical capacity and the machinery that reads them has much greater capacity than a CD and can be a double sided if specified. DVD's and CD's have various ratings for writing data to them such as 2X , 4X ,8X and ratings for reading them with a reader only mechanism of 24X , 44X and 52Xspeeds. Another feature that is peculiar to the writing mechanism by the software for DVD or CD is filenames. These are either able to be normal the same as a computer, or a system called jolliet filenames. CD's and DVD's come in three basic types of RW(read and "writeable once") , direct RW that is the same as before but requires no disc formatting and organisation or also can overwrite more than once, RO or CDR read only pre constructed disc or CDRW DVDRW and can be written many multiple times. Another final feature is an action called "overburn" that allows e.g. a 750 Mb disc to have 775 Mb of data dangerously as a possibility at burn time if the disc is well organised and will be finalised.

Answer 3

As the name implies, the storage medium in a magnetic disk system consist of one or more disks mounted on a common spindle. A thin magnetic film is deposited on each disk, usually on both sides

Answer 4

Its advantages are that it has direct access and is the most popular media.

Answer 5

1. Have larger space.

2. More cheaper in the same Gb unit 3. Speed is faster then CD or DVD

Answer 6

common uses of magnetic disks