68 Storage Management with DB2 for OS/390

7.2.3 Storage Group

Because the Storage Class has guaranteedspac e,the BSDS data sets are
allocated on thedisk volumes requested by the DB2 administrator. The volumes
must belong tothe assigned Storage Group (such as: SGDBACTL), and the disk
volume must be eligiblefor SMS. For example, this can be done with the
DFSMSdss CONVERT command.
• SGDBACTL for several DB2 susbsystems
• SGDB2PLG for the DB2P subsystem

7.2.4 ACS Example

An exampleof ACS routines to allocate these SMS classes and Storage Groups
forBSDSs is s hownin Appendix B,section B.1, “BSDS and Active Logs” on page
185.
7.3 Active Logs
The activelog data sets are used for data recovery and ensure data integrityin
case of software or hardwareerrors. Active log data sets record all updates to
user and system data. If theac tivelog is not available, DB2 cannot guarantee
data integrity.
The active log data sets are open as long as DB2 is active. Activelog data sets
arereus ed when thetotal active log space is used up, but only after the active log
to be reused has been copied to an archive log.
Data Organization
The active log data sets are VSAM LDSs. Figure 20 on page 69 shows an
example definitionof an active log data set.
Performance
For DB2 subsystemswith high update transaction rates, the active logs havea
very highI /O activity(m ainlywrite I /O).The performance of the active logs has an
important impact on the overallDB2 subsystem performance. See 10.4.5,
“Improving Log Write Performance”on page 114 and 10.5.1, “Improving Log
Read Performance”on page 116 formore information.
Availability
The activelog data sets have a very high availability requirement forDB2 data
integrity.To ensure this, DB2 optionally supports two copies for each active log
data set (dual activelogs). Dualactive logs are highly recommended for DB2
production environments.
Toimprove active log availability, RAID disks or disks with dual copy can be
considered forthe ac tivelogs.
Migration
Active logdata sets should never be migrated by DFSMShsm.
Backup
Every time an active logdata sets is filled, DB2 attempts to create an automatic
backup.The backup copies of the active log data sets are thearchive log data
sets.