How the log is structured

Use this topic to understand the terminology used to describe log records.

Each active log data set must be a VSAM linear data set (LDS). The physical output unit written to the active log data set is a 4 KB control interval (CI). Each CI contains one VSAM record.

Physical and logical log records

One VSAM CI is a physical record. The information logged at a particular time forms a logical record, with a length that varies independently of the space available in the CI. So one physical record might contain:

Several logical records
One or more logical records and part of another logical record
Part of one logical record only

The term log record refers to the logical record, regardless of how many physical records are needed to store it.