IBM 5695-DF1, 5655-DB2 manual Log Writes, No Wait Force, Physical Writes

DSNB450I =DB2Z TABLESPACE = DSNDB06.SYSCOPY, USE COUNT = 0, GBP-DEP = N

DSNB452I =DB2Z	STATISTICS	FOR DATASET 1 -
DSNB453I =DB2Z	VP CACHED	PAGES -
	CURRENT	=	64	MAX	=	64
	CHANGED	=	0	MAX	=	0
DSNB455I =DB2Z	SYNCHRONOUS I/O DELAYS -
	AVERAGE DELAY =		9	MAXIMUM DELAY	=	22
	TOTAL PAGES	=	3
DSNB456I =DB2Z	ASYNCHRONOUS I/O DELAYS -
	AVERAGE DELAY =		1	MAXIMUM DELAY	=	1
	TOTAL PAGES	=	61	TOTAL I/O COUNT =		2

Figure 36. Display Buffer Pool Data Set Statistics

10.4 Log Writes

Log records are created by application programs when data is updated. Each data update requires two log records, one with the data before the update, and another with the data after the update, generally combined into one physical record.

The application program uses two methods (see Figure 37 on page 113) to move log records to the log output buffer:

•NO WAIT

•FORCE

NO WAIT

Most log records are moved to the log output buffer, and control is immediately returned to the application program. These moves are the most common. If no log buffer is available, the application must wait for one to become available. Log records moved into the output buffer by an application program appear in a DB2 PM statistics report as the number of NOWAIT requests. See C in Figure 41 on page 118.

FORCE

At commit time, the application must wait to ensure that all changes have been written to the log. In this case, the application forces a write of the current and previous unwritten buffers to disk. Because the application waits for this to be completed, it is also called a synchronous write.

Physical Writes

Figure 37 on page 113 also shows the physical writes to disk. The log records in the log output buffer are written from the output buffer to disk. DB2 uses two types of log writes: asynchronous and synchronous, which will be explained further.

These writes to the active log data set are shown by DB2 PM as E in Figure 41 on page 118.

DB2 I/O Operations 111