DISK SPACE MANAGEMENT

DISK SPACE MANAGEMENT 

The disk space manager is the lowest level of software in the DBMS architecture, with manages space on disk. In short, the disk space manager supports the concept of a page as a unit of data, and provides commands to allocate or deallocate a page and read or write a page. the size of a page is chosen to be the size of a disk block and pages are stored as disk blocks so that reading or writing a page can be done in one disk Input/Output. 

It is often useful to allocate a sequence of page as  a contiguous sequence of blocks to hold data that is frequently accessed in sequential order This capability is essential for exploiting the advantages of sequentially accessing disk blocks. Such  a capability, if desired, must be provided by the disk space manager to higher - level layers of the DBMS. 

Thus, the disk space manager hides details of the underlying hardware (and possibly the operating system) and allows higher levels of the software to think of the data as a collection of pages.

Track :

A disk surface contains several concentric tracks. In fact, a track contains data which can be read by signal read head without changing its position. the maximum amount of information that can be read by a single read/write head system in one revolution is determined by the track length. The track length is expressed in bytes, words or characters. Each track, in addition to the data, contents some extra information such as : the address of tracks (i.e., cylinder number and track number), block number, gap between blocks cyclic check code etc. information are used by Input/Output controls for proper processing of data.

Sectors : 

The tracks are sub-divided into smaller regions called sectors. A sector is the smallest addressable segment (part) of a track. The division of a track into sectors is done by hardware or software format operation.

Blocks 
Since the entire track length is too large for data to be transferred by a single Input/Output command, the data is stored on the track in a number of blocks of equal length A block may be equal to a sector or a number of sectors. Infact, the block size determines the basic unit of data which is read or written by a single Input/Output command. The blocks are separated by a gap (G) and this gap reduces this storage capacity of data. A small block size increases the number of gaps thereby causing wastage of storage space. Very large blocks on the other hand create problems for processor requiring larger main memory in which data is to be transferred.

In order to refer to a particular block, an address identifying the track and the position of the block on the track is constructed. This block identification address is known as a block pointer, which is donated by P. The block pointer (i.e., the pointer to a block) can be its absolute address consisting of the cylinder number, surface number, sector or block number etc.