Other DB2 Components

Other DB2 Components

You are near the end of your excursion behind the scenes of DB2. Before you finish, however, you should know about two other DB2 components that operate behind the scenes: the Boot Strap Data Set (BSDS) and DB2 logging.

The BSDS is a VSAM KSDS data set utilized by DB2 to control and administer the DB2 log data sets. It is an integral component of DB2, controlling the log data sets and managing an inventory of those logs. The BSDS is also used to record the image copy backups taken for the SYSIBM.SYSCOPY DB2 Catalog table. Because SYSIBM.SYSCOPY records all other DB2 image copies, another location must be used to record image copies of the SYSIBM.SYSCOPY table.

DB2 logs every modification made to every piece of DB2 data. Log records are written for every INSERT , UPDATE , and DELETE SQL statement that is successfully executed and committed. DB2 logs each updated row from the first byte updated to the end of the row. These log records are written to the active logs. DB2 usually has two active log data sets to safeguard against physical DASD errors. The active logs must reside on DASD. (They cannot reside on tape.) The active log data sets are managed by DB2 using the BSDS.

NOTE

For tables defined using the DATA CAPTURE CHANGES option, an UPDATE causes DB2 to log the entire updated row, even if only one column is changed.

As the active logs are filled, DB2 invokes a process called log offloading to move the log information offline to archive log data sets (see Figure 23.6). This process reduces the chances of the active logs filling up during DB2 processing, which would stifle the DB2 environment. DB2 can access archive logs to evoke table space recovery. The BSDS manages and administers the archive logs.

Figure 23.6. DB2 log offloading.