Physical Characteristics of Disks

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.