Logical organization of hard disk

Logical organization of hard disk
Sectors and Clusters
A sector, being the smallest physical storage unit on the disk, is almost always 512 bytes in size
because 512 is a power of 2 (2 to the power of 9). The number 2 is used because there are two
states in the most basic of computer languages - on and off.
Each disk sector is labelled using the factory track-positioning data. Sector identification data is
written to the area immediately before the contents of the sector and identifies the starting address
of the sector.
The optimal method of storing a file on a disk is in a contiguous series, i.e. all data in a stream
stored end-to-end in a single line. As many files are larger than 512 bytes, it is up to the file
system to allocate sectors to store the file’s data. For example, if the file size is 800 bytes, two 512
k sectors are allocated for the file. A cluster is typically the same size as a sector. These two
sectors with 800 bytes of data are called two clusters.
They are called clusters because the space is reserved for the data contents. This process protects
the stored data from being over-written. Later, if data is appended to the file and its size grows to
1600 bytes, another two clusters are allocated, storing the entire file within four clusters.
Figure 3-2 Sectors and Clusters
If contiguous clusters are not available (clusters that are adjacent to each other on the disk), the
second two clusters may be written elsewhere on the same disk or within the same cylinder or on a
different cylinder - wherever the file system finds two sectors available. A file stored in this
non-contiguous manner is considered to be fragmented. Fragmentation can slow down system
performance if the file system must direct the drive heads to several different addresses to find all
the data in the file you want to read. The extra time for the heads to travel to a number of
addresses causes a delay before the entire file is retrieved.
Cluster size can be changed to optimize file storage. A larger cluster size reduces the potential for
fragmentation, but increases the likelihood that clusters will have unused space. Using clusters
larger than one sector reduces fragmentation, and reduces the amount of disk space needed to store
the information about the used and unused areas on the disk.
Most disks used in personal computers today rotate at a constant angular velocity. The tracks near
the outside of the disk are less densely populated with data than the tracks near the center of the
disk. Thus, a fixed amount of data can be read in a constant period of time, even though the speed
of the disk surface is faster on the tracks located further away from the center of the disk..
Modern disks reserve one side of one platter for track positioning information, which is written to
the disk at the factory during disk assembly. It is not available to the operating system. The disk
controller uses this information to fine tune the head locations when the heads move to another
location on the disk. When a side contains the track position information, that side cannot be used
for data. Thus, a disk assembly containing two platters has three sides that are available for data.