IBM 5655-DB2, 5695-DF1 manual Data Management Threshold

Figure 24 shows the prefetch quantity as a function of the page size and the buffer pool size. For certain utilities (REORG, RECOVER), the prefetch quantity can be twice as much.

Table 24. Number of Pages Read Asynchronously in One Prefetch Request

Page Size	Bufferpool Size	Prefetch Quantity
	(buffers)	(pages/request)
4K	<224	8
	224-999	16
	>=1000	32
8K	<113	4
	113-499	8
	>=500	16
16K	<57	2
	57-249	4
	>=250	8
32K	<100	2
	>=100	4

From the DB2 PM accounting trace, the average number of pages read in prefetch operations by an application program can be calculated. The average number of pages read is the total number of pages read in prefetch operations (E in Figure 33 on page 109) divided by the sum of prefetch operations (B, C, D in Figure 33), that is:

Average pages read by one prefetch operation = E / (B+C+D).

The DB2 PM statistics report calculates these values for every type of prefetch operation. Figure 34 on page 110 shows the average pages read by sequential prefetch (K), by list prefetch (L) and by dynamic prefetch (M). These numbers apply to the whole DB2 subsystem, while the accounting report numbers normally refer to one plan.

10.2.6 Data Management Threshold

The data management threshold (DMTH) is set by DB2 at 95% of each virtual buffer pool. The DMTH is maintained and checked independently for each individual virtual buffer pool.

This threshold is checked before a page is read or updated. If the threshold has not been exceeded, DB2 accesses the page in the virtual buffer pool once for each page, no matter how many rows are retrieved or updated in that page. If the threshold has been exceeded, DB2 accesses the page in the virtual buffer pool once for each row that is retrieved or updated in that page. Reaching this threshold has a significant effect on processor usage and performance.

106Storage Management with DB2 for OS/390