What is Buffer Management in DBMS?

A database buffer is a section in the main memory that is used for the temporary storage of data blocks while moving from one location to another. A copy of the disk blocks is kept in a database buffer. However, the data on the disk can have an older version than the one in the database buffer as the database buffer is the one that gets affected first by the changes. When necessary, we can write the data block back to the disk memory.

What is a Buffer Manager?

Consider the following points to understand the functioning of buffer management in DBMS:

• A buffer manager in DBMS is in charge of allocating buffer space in the main memory so that the temporary data can be stored there.
• The buffer manager sends the block address if a user requests certain data and the data block is present in the database buffer in the main memory.
• It is also responsible for allocating the data blocks in the database buffer if the data blocks are not found in the database buffer.
• In the absence of accessible empty space in the buffer, it removes a few older blocks from the database buffer to make space for the new data blocks.
• If the data blocks to be removed have been recently updated then the changes are copied/written to the disk storage, else they are simply removed from the database buffer.
• If a user requests one of these removed data, the buffer manager copies/reads the data blocks present in the disk storage to the database buffer and returns the requested block's address from the main memory.
• Programs that might interfere with requests from the disks and the database buffer can't see what the buffer manager is doing inside as it acts as a VM in the system.

Methods

The buffer manager applies the following techniques to provide the database system with the best possible service:

Buffer Replacement Strategy

If there is no space for a new data block in the database buffer, an existing block must be removed from the buffer for the allocation of a new data block. Here, the Least Recently Used (LRU) technique is used by several operating systems. The least recently used data block is taken out of the buffer and sent back to the disk. The term Buffer Replacement Strategy refers to this kind of replacement technique.

Pinned Blocks

When a user needs to restore any data block from a system crash or failure, it is crucial to limit the number of times a block is copied/written to the disk storage to preserve the data. The majority of the recovery systems forbid writing blocks to the disk while a data block update is taking place. Pinned Blocks are the data blocks that are restricted from being written back to the disk. It helps a database to have the capability to prevent writing data blocks while doing updates so that the correct data persists after all operations.

Forced Output of Blocks

Sometimes we may have to copy/write back the changes made in the data blocks to the disk storage, even if the space that the data block takes up in the database buffer is not required for usage. This method is regarded as a Forced Output of Blocks. This method is used because system failure can cause data stored in the database buffer to be lost, and often disk memory is not affected by any type of system crash or failure.

Learn more

To learn about File Organization in DBMS, visit the link:

• File Organization in DBMS