𝘇/𝗢𝗦 𝟯.𝟮 𝗗𝗲𝗳𝗲𝗰𝘁 : 𝟭𝟯 𝗦𝘁𝗲𝗽𝘀 𝗞𝗲𝗽𝘁 𝗔𝗽𝗽𝗲𝗻𝗱𝗶𝗻𝗴 𝗥𝗲𝗰𝗼𝗿𝗱𝘀 𝘁𝗼 𝘁𝗵𝗲 𝗗𝗮𝘁𝗮𝘀𝗲𝘁, 𝗯𝘂𝘁 𝗢𝗻𝗹𝘆 𝟳𝟬𝗠 𝗼𝗳 𝘁𝗵𝗲 𝗘𝘅𝗽𝗲𝗰𝘁𝗲𝗱 𝟱𝟵𝟰𝗠 𝗥𝗲𝗰𝗼𝗿𝗱𝘀 𝗔𝗿𝗲 𝗣𝗿𝗲𝘀𝗲𝗻𝘁

A batch job was executed to consolidate data from the most recent 13 generations of a GDG, representing approximately 13 months of historical information.

The process consists of 13 sequential steps, with each step appending its output records to a single output dataset. The dataset was allocated on a single DASD volume.

Dataset attributes:

• RECFM: FB
• LRECL: 21
• BLKSIZE: 27,993

The number of records appended by each processing step is shown below. STEP100 created the dataset, and all subsequent steps appended records to the same dataset using DISP=MOD.

Step	Records Written
STEP100	37,923,095
STEP110	44,967,733
STEP120	57,202,964
STEP130	44,714,138
STEP140	43,599,256
STEP150	43,838,386
STEP160	45,313,346
STEP170	46,349,138
STEP180	45,525,353
STEP190	46,162,534
STEP200	46,518,402
STEP210	45,979,524
STEP220	46,381,066

Total records written from STEP100 to STEP220 : 594,474,935

But the Final dataset had only 70,325,745 records.

 

Dataset Track Limitation

The output dataset was allocated as a basic-format sequential dataset, which is limited to a maximum of 65,535 tracks per volume.

But z/OS 3.2 allowed dataset to grow beyond 65,535 tracks per volume. This is a defect.

 

When the dataset grows beyond this limit:

 

1. The system can no longer correctly address the entire dataset.
2. It effectively treats the dataset as if it contains only the records corresponding to the overflowed tracks at the beginning of the file.
3. Existing records are overwritten as additional data is appended.
4. Consequently, part of the previously accumulated data is lost.

First Overflow Event (After STEP130)

The number of tracks is calculated as: (Number of Records × LRECL) / (2 × Block Size)

Step	Records Written	Running Record Count	Calculated Tracks
STEP100	37,923,095	37,923,095	14,225
STEP110	44,967,733	82,890,828	31,092
STEP120	57,202,964	140,093,792	52,549
STEP130	44,714,138	184,807,930	69,321

At the completion of STEP130, the dataset reaches 69,321 tracks, exceeding the maximum addressable limit of 65,535 tracks.

Overflow tracks: 69,321 - 65,535 = 3,785 tracks

System now thinks the dataset has 3785 tracks instead of 69,331 tracks.

3785 tracks equates to first 10,090,810 records written by STEP100. 

At this point, the system begins reusing existing track addresses, causing subsequent writes to overwrite data already present in the dataset

The process then continues appending additional data:

Step	Records Written	Running Record Count	Calculated Tracks
STEP100	10,090,810		3,785
STEP140	43,599,256	53,690,066	20,139
STEP150	43,838,386	97,528,452	36,582
STEP160	45,313,346	142,841,798	53,579
STEP170	46,349,138	189,190,936	70,965

 

At the completion of STEP170, the dataset again exceeds the track limit.

 

Overflow: 70,965 − 65,535 = 5,430 tracks

 

5,430 tracks corresponds to approximately 14,475,047 records, consisting of:

• First 10,090,810 records written from STEP100  
• First 4,384,237 records written from STEP140

Subsequent write operations reuse track addresses that have already been used, causing newly written records to overwrite previously stored data.

Processing then continues with the next set of steps:

Step	Records Written	Running Record Count	Calculated Tracks
STEP100	10,090,810		3,785
STEP140	4,384,237		1,645
STEP180	45,525,353	60,000,400	22,506
STEP190	46,162,534	106,162,934	39,821
STEP200	46,518,402	152,681,336	57,270
STEP210	45,979,524	198,660,860	74,517

At the completion of STEP210, the dataset exceeds the addressable limit for a third time.

 

Overflow tracks: 74,517 - 65,535 = 8,982 tracks

8,982 tracks corresponds to approximately 23,944,679 records, consisting of:

• First 10,090,810 records written from STEP100  
• First 4,384,237 records written from STEP140
• First 9,469,632 records written from STEP180

The final processing step STEP220 then appends an additional 46,381,066 records.

 

Final Dataset Contents

 

After all overflow events and the completion of the final processing step, the dataset contained records originating from only a subset of the processing steps.

 

Step Contribution Retained in Final Dataset	Record Count
STEP100	10,090,810
STEP140	4,384,237
STEP180	9,469,632
STEP220	46,381,066
Total Records in Final Dataset	70,325,745

 

Conclusion

 

The multiple overflow events caused entire sections of data written by several intermediate processing steps to be lost. As a result, the final dataset did not contain a continuous history of all records written during the job execution.  

The detailed issue outlined above was successfully resolved after implementing the z/OS 3.2 maintenance update (RSU2604/PUT2601).