IMS 15.5 - Diagnosis - Log analysis (database related)

Log record analysis

For all changes, write a copy of the segment before it is changed as well as a copy of the segment after it is changed, if applicable. This process not only facilitates backout and recovery, but it also is useful for diagnosis.

Analyzing log records is helpful whenever you suspect bad data or a pointer problem. Determine where the error is by referring to error messages or to the contents of the dump. When you identify the location of the problem, use the File Select and Formatting utility (DFSERA10) to print the log records for the block in error. Refer to the following table to interpret the contents of the log records. You can determine what changes to the data have been made, and in what sequence the changes were made. This information is helpful in identifying the source of the error.

Sometimes, the error is caused by an internal IMS problem; other times, the error results from incorrect data that is entered by a user or by an application.

To obtain a complete listing of all control blocks, DB, DC, and log records, assemble module DFSADSCT.

CICS® puts a header on log records. To obtain the log records when running with CICS, the DD statement pointing to the CICS journal must specify DCB=RECFM=VB. This allows the File Select and Formatting utility to remove the header.

Example of log analysis

An abend is issued against a database. You have used other diagnostic tools to analyze the call. Now you must look at the database. Follow these steps when looking at the database:

Analyze the buffer to identify what seems to be wrong. (See the following figure.) The first indication that something is wrong is usually found in the buffer.
Look at the changes to that buffer (block) on the log.
Determine if the bad data is actually on the database.
If required, determine if the image copy is propagating the bad block.

The following figure shows the general areas of database analysis: Application, Buffer, Database, Image Copy, and Log.

This figure shows the flow from Image Copy (on the far right) to Database, to Buffer (Block), to Log, to Application using DL/I Interface.

Database change log record DSECT

You can use the following table to assist you in the analysis of output from log record type X'50'.

If any differences are detected in the mapping of the DSECT, you can obtain a current copy by assembling the macro ILOGREC.

Table 1. Database change log record DSECT
Offset	Field	Length	Description
DLOGB	DSECT
00	DLENGTH	2	Length of log record
02	DLOGZZ	2	Zeros for QSAM
04	DLOGCODE	1	Log record type
05	DLOGSCDE	1	Log record subrecord (X'50' X'51' X'52')
06	DLOGPSTN	2	PST number
08	DLOGRTKN	16	Recovery token
18	DLOGSTCK	8	CPU store clock (STCK)
20	DLOGVIMS	1	DLOG IMS Version/Release: X'82' Version 8 or later
28	DLOGDBF1	1	Flag 1 X'80' Record written during backout X'40' Record from DB/DC X'20' Record from batch region X'10' New date/time from DFSFTIM0 X'08' Commit each GU call (Mode=SNGL) X'04' First log record this sync interval X'02' First log record of a segment X'01' Last log record of a segment
29	DLOGDBF2	1	Flag 2 X'80' Database is nonrecoverable X'40' KSDS ERASE prohibited X'20' Bit map update for lock tracking X'08' PHIDAM primary index; no REDO X'04' DLOGSEQ has update sequence number X'02' OLR non-backoutable; cursor not active yet X'01' OLR ITASK
2A	DLOGDBOR	1	Database organization X'70' DEDB direct organization X'40' DL/I HDAM database X'20' DL/I HIDAM database X'10' Data entry database (DEDB) X'08' Primary or secondary index database X'04' HISAM or SHISAM database
2B	DLOGDSOR	1	Data set organization X'80' VSAM access method X'40' OSAM access method X'08' Entry sequenced data set X'04' Key sequenced data set
2C	DPGMNAME	8	PSB name
34	DDBDNAME	8	Database name
3C	DDSID	1	Data set ID (DCB number) X'80' When this high order bit is on, then this DCB number represents one of the M-through-V or Y data sets. When this high order bit is off, then this DCB number represents one of the A-through-J or X data sets.
3D	DDSID2	1	For ARID
3E	DLOGSLVL	1	Database share level (for DBRC-registered databases)
3F	DLOGCALL	1	Describe DL/I call issued by application program X'80' ISRT call X'40' REPL call X'20' DLET call X'10' ROLL/ROLB/ROLS call (backout)
40	DLOGRBA	4	OSAM RBN or VSAM RBA (LRECL)
44	DLOGBLK0	2	Offset of RBA within block
48	DLOGSEQ	4	Update the sequence number when X'04' flag is on in DLOGDBF2
4C	DLOGXTOF	2	Database extension section offset (not used)¹
4E	DLOGDSOF	2	Data sharing section offset¹
50	DLOGIDOF	2	RACF® userid offset¹
52	DLOGTKOF	2	Tracking (XRF) section offset¹
54	DLOGDLOF	2	DL/I call section offset (not used)¹
56	DLOGKYOF	2	Key data section offset¹
58	DLOGSPOF	2	Space management section offset¹
5A	DLOGUNOF	2	UNDO data offset¹
5C	DLOGREOF	2	REDO data offset¹
60	DDATE	4	Date in the format `YYYYDDDF`
64	DTIME	6	Time in the format `HHMMSSTHMIJU`
6A	DZONE	2	Offset to local time
Data sharing section (DLOGDSHUR DSECT)
00	DLOGDSSN	4	Data set sequence number (DSSN)
04	DLOGLSN	6	Lock sequence number (LSN)
0A	DLOGUSID	4	Update Set ID (USID)
RACF/SIGNON userid (DLOGID DSECT)
00	DLOGUSER	8	RACF userid
Buffer and lock tracking for DL/I in XRF-capable systems (DLOGTRCK DSECT)
00	DLOGPOOL	2	Pool size for buffer tracking
02	DLOGBUFF	2	Buffer number for buffer tracking
04	DLOGHASH	4	Root hash value
08	DLOGLOCK	4	Lock value
0C	DLOGLFL1	1	Change logger lock flag X'80' Log record is for root segment X'40' Log record is for dependent segment X'20' Bypass reacquiring restart locks X'10' Get bid lock on DDATAID X'08' Function is erase X'04' Index maintenance X'02' Organization is SHISAM X'01' Hash is for logical parent
0D	DLOGLFL2	1	Reserved
0E	DLOGDBDN	8	DBD name
16	DLOGSKID	4	Task ID
KSDS key data section (DLOGKEY DSECT)
00	DLOGKYF1	1	X'40' KSDS key X'20' Key is being erased
02	DLOGKLEN	2	Length of key
04	DLOGKDAT	variable	Key data
Space management section for HD inserts and deletes (DLOGSPCE DSECT)
00	DLOGSPF1	1	Space management flags X'40' Demand space request X'20' Get free space request (ISRT) X'10' Free space request (DLET)
02	DLOGSOFF	2	Offset of space management request
04	DLOGSLEN	2	Length of space management request
UNDO/REDO data section (DLOGDATA DSECT)
00	DLOGDFLG	1	X'80' Last data element in this section X'40' Data is compressed using z/OS® services
01	DLOGDFUN	1	Describe physical function being logged by this request X'80' Physical insert X'40' Physical replace X'20' Physical delete X'10' Space management create X'08' Free space element
02	DLOGDOFF	2	Offset of data in buffer
04	DLOGDLEN	2	Length of data (DLOGDDAT)
06	DLOGDDAT	variable	Variable length data
00		2 variable	Compressed data format in DLOGDDAT Expanded data length Compressed data
	DBCKCHN	6	Back chain²
	DBLGSEG	8	Logical logger sequence number²
Notes: To find each section, add the offset to the beginning of the log record. The log back chain and logical logger sequence number are at the end of the log record.