Scalable Database Server, HiRDB Version 8 Description
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Global buffer pre-writing is a function that loads in advance data from a specified table or index into global buffers. Figure 6-13 provides an overview of global buffer pre-writing.
Figure 6-13 Overview of global buffer pre-writing
![[Figure]](FIGURE/ZU060112.GIF)
- Explanation
- Global buffer pre-writing is not specified
When a UAP accesses HiRDB immediately after HiRDB starts, data must be read from the table (a physical I/O occurs) because the global buffers do not hold any data. When data in this table is accessed later, pages from this table that have been written into global buffers are not read from the table. However, accesses to data on other pages must still be read from the table.
- Global buffer pre-writing is specified
Table data is written into global buffers in advance, so the table can be accessed without data having to be read from the table (no physical I/O occurs). Subsequent accesses to this table do not require that data be read from the table.
- Organization of this subsection
- (1) Advantages of global buffer pre-writing
- (2) Execution method
- (3) Considerations when using
(1) Advantages of global buffer pre-writing
Because reading of table and index data from specified tables is performed in advance, the global buffer hit ratio is improved. By specifying global buffer pre-writing before online operations or other activities are started of tables or indexes that you believe will be accessed frequently, you can expect to achieve a higher buffer hit ratio.
(2) Execution method
(3) Considerations when using
You should consider the following points about using global buffer pre-writing:
- You must have more global buffers than the number of pages stored in the tables or indexes that are to be pre-read.
- If you do not have enough global buffers, older page information will be forced out of the global buffers according to the LRU management method (the oldest page in the accessed global buffers is forced out as specified by the value in the pd_dbbuff_lru_option system definition operand). This means that executing pdpgbfon is pointless if you do not have enough global buffers.
- When you specify pre-writing with the global buffer residence utility (pdpgbfon), the pre-fetch facility becomes available. This makes it possible to reduce execution time by specifying the prefetch facility when you define global buffers.
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