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Embed code for: Disk Filing System- Extended Notes
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Disk Filing System
A hard disk uses read/write heads to load and/or store information. Every read/write head hovers over the disk at a fraction of a millimeter. If it does happen to touch, then the head scratches the disk, disk crash, making it useless. To increase the amount of storage space in the drive, most drives have multiple platters and multiple read/write heads.
Data is stored on the surface of a platter in tracks, sectors, clusters and cylinders.
Concentric circles on one side of the disk
V- shaped radials cutting through different tracks and containing a fixed number of bytes
Is where a track and a sector meet
Sectors which are grouped together
Same numbered tracks of every platter are aligned on top of each other
Hard disks use two different techniques for reading/writing data.
1: The Movable Head
These came very close to the speed of the fixed head. In this system there is only one head for every side of the platter. This head is attached to a movable arm that allows data to be found almost instantly.
2: The Fixed Head
Every track on the disk has a read/write head positioned directly over it. This avoids the need for the head to move when searching making them very fast. However these were very expensive and ended up being replaced by the,
When data is stored on a disk it is stored in groups of sectors called blocks which are addressed every time the data needs to be accessed. Before a disk can start being used to store data it needs to contain a map of all available sectors and locations (just like a telephone directory). This is necessary for the computer to find where the data is stored. Without it is equivalent to a postman who forgets were streets and roads are and keeps on going round and round in the hope that he finds (by luck) the location he needs to pick up a parcel
Some examples of such disk maps or filing systems (Microsoft based) are the FAT (File Allocation Table) and NTFS (New Technology Files System). These are found on the disk and help control the disk’s volume. They are set up during the formatting of a disk. When a file is stored, it is broken into parts and stored in various sectors randomly. An entry is created in the file allocation system basically recording where the first part of the file is stored and subsequently linking to the rest of the file parts. The file allocation directory needs to be kept up to date in order for all recent and older data to be retrieved efficiently. Corrupt file allocation systems can lead to data loss. This can be where viruses attack.
Disks are usually accessed in blocks of sectors rather than bytes. A block is usually 1024 Bytes. Disks access data directly. This means that data is found and retrieved as the head moves directly to the location where the data is stored. Sequential filing/access systems like magnetic tapes are much slower as the head has to access one location on disk after another before the correct place in memory is found. Disk access time varies from device to device and refers to the time it takes to:
Access the FAT to find location of data
Move the head over the location
Read or write to that location.
Everyday computer users cannot really see how the file allocation system works. It is the job of the operating system (Windows, Android, etc) to provide an interface which allows users to store their files in an organised fashion. Programs as file managers or Windows Explorer allow users to create folder and sub-folders in particular drives in a hierarchical fashion. When a user needs to save something s/he first decides in which drive s/he will save the data to. The drive contains the directory of folders and sub-folders stored in it. The directory is the path where all the folders and sub-folders are kept. This makes storage and retrieval of folders and files easier from the user side as it provides order (especially in a GUI) at a glance. This structure is also stored in the FAT.
Disk Filing System Extended Notes – Ms Natasha