Specific Buffer Replacement Policies in DBMS

In this article, we are going to discuss Buffer replacement policies also we will cover some specific features for each policy. Let’s discuss one by one.

DBMS cache will hold the disk pages that contain information currently being processed in main memory buffers. If all buffers in the DBMS cache are copied and new disk pages are required to be loaded into main memory from disk, a new page replacement policy is needed to select particular buffers to be replaced.

Some page replacement policies that have been developed specifically for database systems are the following.

1. Domain Separation (DS) Method :
   DBMS contains many disk pages like index pages, data file pages, log file pages, etc. DBMS cache is divided into separate domains(set of buffers). Each domain handles one type of disk pages, and page replacements within each domain are handled via the basic LRU (Least Recently Used) page replacement. It is a static algorithm and does not adapt to dynamically changing loads. Several variations which add dynamic load – balancing features are proposed in this.

   For example, GRU (Group LRU) gives each domain a priority level and selects pages from low priority domain whereas other methods dynamically change no.of buffers in each domain based on current workload.

2. Hot Set Method :
   This is useful in queries that have to scan a set of pages repeatedly, such as where a join operation is performed using the nested-loop method. If inner loop file is loaded completely into main memory buffers without replacement (the hot set), the join will be performed efficiently because each page in the outer loop file will have to scan all records in the inner loop file to find join matches.

   This method determines for each database processing algorithm set of disk pages that will be accessed repeatedly, and it does not replace them until their processing is completed.

3. The DBMIN Method :
   This uses a model known as QLSM (Query Locality Set Model), which predetermines the pattern of page references for each algorithm for a particular type of database operation. This method calculates locality set using QLSM for each file instance involved in query. Then it allocates appropriate no.of buffers to each file instance involved in query-based on the locality set for that file instance.