BUFFER MANAGEMENT

BUFFER MANAGEMENT

The buffer manager is the software layer that is responsible for bringing pages from physical disk to main memory as needed. The buffer manages the available main memory by dividing the main memory into a collection of pages, which we called as buffer pool.  The main memory pages in the buffer pool are called frames.

                                                     Page Requests from Higher Level

Buffer Management in a DBMS

• Data must be in RAM for DBMS to operate on it!
• Buffer manager hides the fact that not all data is in RAM.

  

       The goal of the buffer manager is to ensure that the data requests made by programs are satisfied by copying data from secondary storage devices into buffer. Infact, if a program performs reading from existing buffers. similarly, if a program performs an output statement: it calls the buffer manager for output operation - to satisfy the requests by writing to the buffers. Therefore, we can say that input and output operation occurs between the program and the buffer area only.

if an input operation does not find the requested  page in the buffer pool, then the buffer manager (software) will have to do a physical transfer the page from the secondary memory (disk) to a free block in buffer pool and then make the requested page placed in the buffer pool, that is, available to the program requesting the original input operation. A similar scenario will take place in the revers order for an output operation. That is, the buffer manager (software) makes a new empty buffer a available to the program for outputting the page. If there is no space in the buffer pool then the buffer manager (software) physically transfer one of the page from buffer pool to the disk (secondary memory) to provide the empty space in the buffer pool for the output operation of the program to display the page in the buffer pool. 

In addition to the buffer pool itself, the buffer manager maintains same block keeping information and two variable for each frame in the pool ; pin - count and dirty. The number of times the page is requested in the frame - each time the pin - count variable is incremented for that frame (because that page is in this frame). For satisfying each request of the user ; the pin - counter variable is decremented each time for that frame. Thus, if a page is requested the pin - count is incremented ; if it fulfills the request the pin - count is decremented. In addition to this, if the page has been modified the Boolean variable; dirty is set as 'on'. Otherwise 'off '.