The buffer pool is a mechanism that conserves I/O when multiple users are accessing information from the same disk blocks. The buffer pool has a predefined size. Once the buffer pool becomes full, buffers are replaced on a least recently used (LRU) basis. Since sequential readers of the database access so many different buffers, they sometimes monopolize the buffer pool. That is, sequential readers of the database cause many shared buffers to be replaced by the buffers most recently used by the sequential reader.

Consequently, you can request some number of buffers in the buffer pool to be private read-only buffers. Private read-only buffers do not participate in the LRU replacement algorithm of the general shared buffer pool.

Applications that read many records in a short time, such as applications that generate reports or lists, should use private read-only buffers. Private read-only buffers prevent applications from quickly using all the public buffers and depriving buffers from other users. When an application is using private read-only buffers, updates are performed correctly using the public buffers. Therefore, an application performing many read operations but only a modest amount of updates might also benefit from using private read only buffers.

When a sequential reader is using private read-only buffers and needs a buffer to perform a read operation, and the buffer is already in the private read-only buffer pool, the database engine marks the buffer as most recently used (MRU) and uses it. If the buffer is not already in the private read-only buffer pool, the sequential reader takes a buffer from the LRU chain and puts it in the private read-only buffer pool. If the sequential reader has exhausted its quota of private read-only buffers, a private read-only buffer is replaced. The sequential reader maintains a list or chain of all its private buffers and uses a private LRU replacement mechanism identical to the public-shared buffer pool LRU replacement algorithm.

All users, regular and sequential, have access to all buffers in the buffer pool (public or private). If a regular user needs a block found in a private buffer pool, the buffer is removed from the sequential readers list of private buffers and is put back into the LRU chain as the most recently used buffer. In addition, if a sequential read user needs to update a private read-only buffer, it is removed from the sequential reader's private buffer pool and put into the general shared buffer pool as most recently used.

Sequential reads use an index and require that index blocks be available in memory because they are used repeatedly. Therefore, you want to request enough private read-only buffers to hold all of the index blocks needed to retrieve a record. To determine how many private read-only buffers to set, count the number of tables that you read and determine the indexes you use. Then, determine the number of levels in the B-tree (balance tree) of each index and add 1 (for the record blocks). For example, request at least five private read-only buffers if you have a report that reads the Customer table using the Cust-Name index, and the Cust-Name index has four B-tree levels.

If you do not know the number of levels in your index, you can generally request six private read-only buffers and get a good result. If you perform a join and are reading from two tables simultaneously, request 12. If the system is unable to allocate the requested number of private read-only buffers, a message is written to the database log.

You can request a number of private read-only buffers using the Private Buffers (-Bp) startup parameter. When you use the -Bp startup parameter the request remains active for the entire session unless it is changed or disabled by an application. Each user of private read-only buffers reduces the number of public buffers (-B).

You can also request a number of private read-only buffers from within an ABL or SQL application by setting a value in the _MyConn-NumSeqBuffers field of the _MyConnection virtual system table (VST). Since _MyConnection is an updatable virtual system table, private read-only buffers can be dynamically requested and released in the application. For a description of the _MyConnection VST, see VirtualSystem Tables.

The following ABL code example demonstrates how to turn private read-only buffers on and off:

The following example demonstrates how to turn private read-only buffers on and off using an SQL statement: