Disk Management In Operating System

 Disk Management 

• Disk is a secondary storage device that is used to store data. 
• The external devices such as hard disk, magnetic disks and floppy disks stores data in the sectors. 
• These devices provide the data to user programs by means of I/O requests. 
• Disk Management is an important function of operating system and includes disk buffering and management of swap area and disk cache. 

Disk Structure 

• Disks provide a means to store a large amount of information for modern computer. 
• A disk is usually divided into tracks, cylinder and sectors. 
• Below fig shows the various components of a disk system. 
• Data is stored on a series of magnetic disks called platters. These disks are connected by a common spindle that spins at very high speed (some spindle rotate at speeds of 3600 revolutions per minute). 
• The two surfaces of a platter are covered with a magnetic material similar to that on a magnetic tape. Information is recorded magnetically on the platters. 
• The surface of a platters is logically divided into circular rings called tracks. Thus tracks are concentric rings that are centred on spindle. These tracks are further subdivided into sectors. The sectors are of fixed size. The set of tracks that are at one arm position from cylinder. 
• A cylinder contains one track per platter surface. 
• There may be thousand of concentric cylinders in a disk drive and each track may contain hundred of sectors. 
• Sector store fixed amount of data as a sequence of bits. A typical sector usually stores 512 or 1024 bytes of user data. In addition, it can have sector management information such as error detections and correction code. 
• A sector is identified by its cylinder number, track number within the cylinder and the ordinal position number within the track. 
• Cylinders are normally numbered starting with zero consecutively from the outer most cylinder to the innermost cylinder. 
• Tracks are numbered starting with zero consecutively from the top track to the bottom track. 
• The various terms that are associated with the access of a disk block are :- 

• Sectors are numbered starting with zero consecutively anti - clockwise starting from a reference position. Thus each sector is uniquely identified by a triple (cylinder number, track number, sector number). 
• The data is accessed read or written by a series of read - write heads, one head per disk surface. 
• A read - write head can access only data immediately adjacent to it. Therefore, before data can be accessed, the portion of the disk surface form which the data is to be read or written must rotate until it is immediately under the read - write head. 
• All the read - write heads are attached to a single boom or moving arm assembly. The boom moves in and out to position a head on a desired track. 

1. Seek Time :- it is the time required to move a head to the required track. Seek time is typically 5 to 25 ms on an average in modern disks. 
2. Latency Time :- It is the amount of time it takes the portion of disk on which data record is stored (the first sector of the block) To spin under the read/write head. 
3. Transfer Time :- The Time spent in actually moving data to /from the disk surface. It is usually determined by the amount of information to be read, the number of bytes per track and rotation speed. 
4. The Total Access Time is the sum of seek time, latency time and transfer time. Therefore, in order to access a particular record of data the operations performed are : 

1. First, boom must be moved to appropriate cylinder or track. (seek time) 
2. Then the portion of the disk on which the data record is stored must rotate until it is immediately under the read - write head. (latency time) 
3. Then the record, which is of arbitrary size, must be spin by read - write head. (transmission time)