Choosing dual or single OLDS logging

IMS can log information to a single data set or to two identical data sets.

Definitions: Single logging uses a single data set, dual logging uses two data sets, where both data sets are identical. Whether you use single or dual logging, IMS writes information to sets of data sets, as described in Online log data set.

Recommendation: Use dual logging whenever possible because the OLDS is of primary importance to system integrity. Specify dual logging in IMS.PROCLIB using the OLDSDEF statement.

With dual logging, IMS has two options when one of the OLDSs (of a pair) gets an I/O error. The first option is to discard the pair and switch to a good pair. This behavior is just like single logging mode.

The second option is to not have IMS discard the pair of OLDSs unless both pairs of OLDSs fail. Then, if all the good pairs of OLDSs fail, IMS can degrade to single logging mode and use the good OLDS from each pair. From this point, IMS behaves just like if it had been in single logging mode from the beginning. This option also simplifies operational procedures because, for example, you only need to run the Log Recovery utility to clean up the OLDSs when you use (single or dual) logging and a write error occurs.

Essentially, there are three logging states for IMS to run in:

Single logging, which has no redundancy and has the highest maintenance when an error occurs. When only two good OLDSs remain, IMS will terminate.
Dual logging with DEGRADE=NO, which behaves just like single logging except that data is written to two LOGS instead of one. And, like in single logging mode, if only two good pairs of OLDSs remain, IMS will terminate.
Dual logging with DEGRADE=YES. In this case, when each pair of OLDSs has at least one write error, IMS will switch to single logging mode and start logging to whichever OLDSs in the remaining pairs are good.

In all three cases, when IMS gets to the point where it only has two data sets left to write to, it terminates. If, for some reason, IMS has no good data sets to write to, it will terminate with an ABENDU0616. For more information about log errors, see IMS Version 15.3 Operations and Automation.