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68 Storage Management with DB2 for OS/390
7.2.3 Storage Group
Because the Storage Class has guaranteed space, the BSDS data sets are
allocated on the disk volumes requested by the DB2 administrator. The volumes
must belong to the assigned Storage Group (such as: SGDBACTL), and the disk
volume must be eligible for 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 example of ACS routines to allocate these SMS classes and Storage Groups
for BSDSs is shown in Appendix B, section B.1, “BSDS and Active Logs” on page
185.
7.3 Active Logs
The active log data sets are used for data recovery and ensure data integrity in
case of software or hardware errors. Active log data sets record all updates to
user and system data. If the active log is not available, DB2 cannot guarantee
data integrity.
The active log data sets are open as long as DB2 is active. Active log data sets
are reused when the total 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 definition of an active log data set.
Performance
For DB2 subsystems with high update transaction rates, the active logs have a
very high I/O activity (mainly write I/O). The performance of the active logs has an
important impact on the overall DB2 subsystem performance. See 10.4.5,
“Improving Log Write Performance” on page 114 and 10.5.1, “Improving Log
Read Performance” on page 116 for more information.
Availability
The active log data sets have a very high availability requirement for DB2 data
integrity. To ensure this, DB2 optionally supports two copies for each active log
data set (dual active logs). Dual active logs are highly recommended for DB2
production environments.
To improve active log availability, RAID disks or disks with dual copy can be
considered for the active logs.
Migration
Active log data sets should never be migrated by DFSMShsm.
Backup
Every time an active log data sets is filled, DB2 attempts to create an automatic
backup. The backup copies of the active log data sets are the archive log data
sets.