The log record header

Each logical record includes a prefix, called a log record header (LRH), which contains control information.

Begin program-specific programming interface information.The first segment of a log record must contain the header and some bytes of data. If the current physical record has too little room for the minimum segment of a new record, the remainder of the physical record is unused, and a new log record is written in a new physical record.

Start of changeThe log record can span many VSAM CIs. For example, a minimum of nine CIs are required to hold the maximum size physical log record of 36,000 bytes. Only the first segment of the record contains the entire LRH; later segments include only the first two fields. When a specific log record is needed for recovery, all segments are retrieved and presented together as if the record were stored continuously. End of change

Table 1. Contents of the log record header for 10-byte format
Hex offset Length Information
00 4 Length of this record or segment
04 2 Length of any previous record or segment in this CI; 0 if this is the first entry in the CI.
06 1 Flags
07 1 Release identifier
08 1 Resource manager ID (RMID) of the Db2 component that created the log record
09 1 Flags
0A 16 Unit of recovery ID, if this record relates to a unit of recovery; otherwise, 0
1A 16 Log RBA of the previous log record, if this record relates to a unit of recovery; otherwise, 0
2A 1 Length of header
2B 1 Available
2C 2 Type of log record
2E 2 Subtype of the log record
30 12 Undo next LSN
3C 14 LRHTIME
4A Start of change6End of change Available
Table 2. Contents of the log record header for 6-byte format
Hex offset Length Information
00 2 Length of this record or segment
02 2 Length of any previous record or segment in this CI; 0 if this is the first entry in the CI. The two high-order bits tell the segment type:
B'00'
A complete log record
B'01'
The first segment
B'11'
A middle segment
B'10'
The last segment
04 2 Type of log record
06 2 Subtype of the log record
08 1 Resource manager ID (RMID) of the Db2 component that created the log record
09 1 Flags
0A 6 Unit of recovery ID, if this record relates to a unit of recovery; otherwise, 0
10 6 Log RBA of the previous log record, if this record relates to a unit of recovery; otherwise, 0
16 1 Release identifier
17 1 Length of header
18 6 Undo next LSN
1E 8 LRHTIME

End program-specific programming interface information.