The SESAM/SQL DBH administers a user data buffer and a system data buffer which are available for storing user data and system access data respectively. In each case, the administrative unit and access unit is one PAM block with a size of 4kBytes.
From the point of view of the DBH, the blocks in the user data buffer are all equally important, whereas the system data buffers have a hierarchical structure. The DBH therefore administers the system data buffer blocks in different priority classes according to their importance in terms of access optimization.
System administrators can set the respective buffer sizes when they start the DBH by setting the relevant DBH options accordingly.
Displacement from main memory
If there is insufficient space in the buffer to store new data, the standard procedure is for blocks to be displaced according to the LRU principle (the “least recently used” principle). The oldest blocks (i.e. the blocks that have not been accessed for the longest period of time) are always overwritten. This means that blocks that are constantly required are kept resident and given priority over other blocks.