Buffer Manager

• Software layer responsible for bringing pages from disk to main memory and back.

• The buffer manager partitions the available memory into pages known as buffer pool.

• The pages are known as frames

• If there is a request for data and the block is already in the buffer, buffer manager returns the address of the block in main memory

• If the block is not in the buffer, the buffer manager allocates space in the buffer for the block

• Two variables for each frame are used : pin_count : the number of times that the page currently in a given frame has been requested but not released

• dirty : Boolean variable indicating whether the page has been modified since it was brought into the buffer pool

• When a page is requested the buffer manager : Checks the buffer pool to see if some frame contains the requested page and, if so, increments the pin_count (also called Pinning) of that frame.

• If the page is not in the pool, the buffer manager brings it in as follows :

• Chooses a frame for replacement, using the replacement policy, and increments its pin_count.

  • If the dirty bit for the replacement frame is on, writes the page it contains to disk

• Reads the requested page into the repla frame.

• Returns the address of the frame contair requested page to the requestor.

• Most of the systems replace the block least recently used (LRU strategy) : use past pattern of block references as a predictor of future references

• Clock replacement is another variant of LRU with less over head

• Both policies are not good for sequential scans

• FIFO

• Most Recently Used (MRU)

• Random

• Buffer manager can use statistical information regarding the probability that a request will reference a particular relation

• Buffer managers also support forced output of blocks for the purpose of recovery