Physical and logical log records

The VSAM control interval (CI) provides 4096 bytes to hold Db2 information. That space is called a physical record. The information that is to be logged at a particular time forms a logical record, whose length varies independently of the space that is available in the CI.

Begin program-specific programming interface information.One physical record can contain several logical records, one or more logical records and part of another logical record, or only part of one logical record. The physical record must also contain 37 bytes of Db2 control information if the log record is in 10-byte format, or 21 bytes of Db2 control information if the log record is in six-byte format. The control information is called the log control interval definition (LCID).

Figure 1 shows a VSAM CI containing four log records or segments, namely:

  • The last segment of a log record of 768 bytes (X'0300'). The length of the segment is 100 bytes (X'0064').
  • A complete log record of 40 bytes (X'0028').
  • A complete log record of 1024 bytes (X'0400').
  • The first segment of a log record of 4108 bytes (X'100C'). The length of the segment is 2911 bytes (X'0B5F').
Figure 1. A VSAM CI and its contents
Begin figure description. This figure depicts a VSAM control interval that holds multiple logical log records. End figure description.

The term log record refers to a logical record, unless the term physical log record is used. A part of a logical record that falls within one physical record is called a segment.End program-specific programming interface information.