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.