Hard Drive Sectors

The basic unit of data storage on a hard disk is a sector. Each sector is made up of three parts namely, the sector header, the data area and the Error Correcting Code (ECC). The information used by the drive and controller is stored in sector header, whereas recorded user data is stored in data area. The Error Correcting Code (ECC) field has codes related to data area. Codes help to check and correct errors that would have been, introduced into the data.

Each sector stores 512 bytes of data in a hard disk with default values. In an optical disk, each sector holds 2048 bytes. Ipods usually contains sectors holding 2048 and 4096 bytes. The term ‘sector’ is a mathematical term, described as pie shaped angular section of a circle. It has two radii from the centre and a corresponding arc. A cluster consists of one or more consecutive sectors. Group of sectors form a circle, which is nothing but a track. Group of tracks form a surface of disk platter.

Previously, number of sectors per track was same throughout. Now, it varies due to the advancement. Previously, hard disks had only single sided platters but now we have platters having tracks on both the sides. These days multiple platters are also available. Number of sectors available is an exponent of 2 like, 1 (2°=1), 2, 4, 8, 16 etc sectors. It cannot be 10, 12 and so on.

To optimize file storage, cluster size is changed. Clusters, with large size reduces the occurrence of fragmentation but increases the possibility of unused space in clusters.

Let us assume a scenario wherein, contiguous clusters are not available to store data. Contiguous refers continuous or adjacent to each other. After writing the data in available clusters, the remaining data has to be written either somewhere else on the same disk or on the same cylinder wherever the clusters are available. This decreases the performance because of the overhead involved in the movement of the head to retrieve the file from different addresses. This non-contiguous storage of files is fragmentation. Fragmentation affects performance.

Fragmentation can be reduced to some extent by using clusters of size larger than one sector. Disk sectors are labeled by using the factory track positioning data. Sector structure depends on the model and its manufacturer. Tracks at the centre of the disk are densely populated than at the end or outside part. Bad sectors check is performed using the command - chkdsk. Using good antivirus, cleaning the system regularly, performing chkdsk when prompted etc, should avoid bad sectors.

Some of the technical terms related to hard drive are as follows:

Capacity total data, which can be stored on a hard drive
Transfer rate amount of data read or written from the disk per unit of time, expressed in bits per second
(Rotational delay)
The duration between the time the disk finds the track and the time it finds the data
Average access time The duration, the disk takes to provide the required data from the moment it got an order to do so.
Radial density number of tracks per inch (tpi).
Linear density number of bits per inch (bpi) on a given track
Surface density it is the ratio between the linear density and radial density
Rotational speed the speed at which the platters rotate, expressed in rotations per minute (rpm) 

Speeds of hard drive, range from 7200 to 15000 rpm. If the drive rotates faster, then the transfer rate is higher. But, the fast rotating drives gets heated up faster and makes more noise.

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