The log record header
Each logical record includes a prefix, called a log record header (LRH), which contains control 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.
The 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.
| 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 | 6 | Available |
| 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:
|
| 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 |
