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Normal and segmented logical log record formats

As BMC AMI Log Master for Db2 creates a logical log, it can encounter log records that are too long to write in the product’s default output record format of variable length, blocked (VB).

This situation occurs when all of the following conditions exist:

  • The log records reflect activity from table spaces with a page size of 32 KB

  • The log records reflect updates to the database (update log records contain both a before image and an after image of the row)

  • The total length of any update log record that the product writes (including the logical log header, the before image, and the after image) is greater than the system-determined block size of the device where the product writes the logical log

The situation is more likely to occur when you set the EXPANDVAR keyword to YES. The product detects when any update log records exceed the length restrictions of the VB format. If such records exist, the product automatically divides the record into multiple segments and writes those segments to a variable length, blocked file. When Log Master reads logical logs created by a 2.2.00 or later version of the product, it reassembles the segments into a complete record before it processes them.

You can use the following methods to determine if a logical log contains segmented logical log records:

  • In the logical log control file, the Logical Log Data Set record contains a field named SEGMENTED. This field contains a Y if the logical log data file contains any segmented logical log records. For more information, see Logical log data set record (DLDS).

  • In the logical log data file, each record begins with a logical log header. This header contains a field named TOTALSEGS. If the logical log record takes up more than one segment, this field contains a number greater than 1. For more information, see Data change record (LLDF).

To understand how the product divides a single record into segments, first consider the basic layout of a logical log record, shown in the following figure:

Log Master can divide a logical log record that reflects an update into one, two, or three segments. The following figure shows how the product divides the record into multiple segments and how the three indicator fields in the logical log header describe the segments.

Segmented logical log records

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