Characteristics of Disk Drives

Characteristics of Disk

1. Track and Sectors: The disk is divided into several rings called Tracks. A Track is thus one complete rotation of the disk underneath the read/write head. The width of a track is determined by the size of the read/write head, and the distance between tracks is determined by the mechanics of the stepper motor which controls the positioning of the arm to which the read/write head is attached. Each track is subdivided into several sectors. Each sector contains a specific number of bytes or characters. Increasing the number of tracks is one way to increase the storage capacity of a disk drive.

2. Bad Blocks: The drive maintains an internal table that holds the sectors or tracks which can’t be read or written to because of surface imperfections. This table is called the Bad Block Table. It is created when the disk surface is initially scanned in a low-level format.

3. Partitions: A disk partition is a sub-division of the disk into one or more areas. Each partition can be used to hold a different Operating System. The computer system boots from the active partition and the software provided allows the user to select which partition is the active one.

4. Sector Interleave: It refers to the numbering of the sectors located in a track. An interleave has sectors numbered sequentially 0, 1, 2, 3, 4, etc. The disk drive rotates at a fixed speed of 3600 rpm which means that there is a fixed time interval between each sector.

5. Drive Controller: The drive is managed by a special peripheral card called a drive controller. It may handle multiple drives or only a single drive. The controller is often responsible for issuing commands to position the read/write head.

6. Rotation Speed: It refers to the speed of rotation of the disk. Most hard disks rotate at 3600 rpm. To increase data transfer rates, higher rotational speeds are required.

7. Low/High Formatting: Low-level formatting is placing track and sector information, plus bad block tables and other timing information on the disk. Sector interleave can also be specified at this time.
High-level formatting involves writing directory structures and file allocation tables to the disk. Often it also means transferring the boot file for the operating system onto the hard disk.

8. Access Time: Access time refers to how soon the drive makes data available once issued with the command is to read the data. Once a read command is issued, the driver must position the read/write head at the appropriate track number and wait for the correct sector to arrive.

9. Latency: It refers to the delay between the read/write request and the appearance of the required sector under the read/write head.

10. Timing Tracks: In larger drives used on mainframe computers, the disk drives often had timing tracks written. These tracks were used for alignment purposes, to ensure that the read/write head was accurately positioned over the track.