A COBOL job that writes variable-length QSAM records can waste output buffer space when every block is cut for the largest possible record. The AWO compiler option applies APPLY WRITE-ONLY behavior to eligible files, so the buffer is written only when the next record will not fit. On the right file shape, that can reduce EXCPs and improve batch run time.
What is the COBOL AWO compiler option?
AWO stands for APPLY WRITE-ONLY. IBM documents that it activates APPLY WRITE-ONLY processing for physical sequential files with variable blocked format. The default is NOAWO.
In practical terms, AWO helps COBOL use output blocks more efficiently for QSAM files that have variable-length blocked records. It is a file-output option, not a general CPU tuning switch for every program.
AWO vs NOAWO
| Option | Behavior | Good fit |
|---|---|---|
NOAWO |
Does not add implicit APPLY WRITE-ONLY to eligible files. |
Programs that do not write variable blocked QSAM output, or programs where existing behavior must stay unchanged. |
AWO |
Applies APPLY WRITE-ONLY processing to eligible files. |
Programs that write blocked variable-length QSAM files, especially when record sizes vary a lot. |
How APPLY WRITE-ONLY changes buffering
Without APPLY WRITE-ONLY, the buffer can be written when there is not enough space left for the maximum-size record. With APPLY WRITE-ONLY, the buffer is written only when the next actual record does not fit in the unused part of the buffer.
That difference matters when output records vary in size. If many records are much smaller than the maximum record length, the program can fit more records into each block and issue fewer I/O calls.
Eligible file shape
AWO matters for physical sequential QSAM files with blocked variable-length records. It does not help a fixed-block file in the same way, and it is not meant for VSAM output.
SELECT OUT-FILE ASSIGN TO OUTDD.
FD OUT-FILE
RECORDING MODE IS V
BLOCK CONTAINS 0 RECORDS.
01 OUT-REC.
05 OUT-ID PIC X(10).
05 OUT-TEXT PIC X(500).The file must still be defined correctly in JCL. Check RECFM=VB, LRECL, and block-size rules with the storage and operations standards used at your site.
How to specify AWO
The exact place depends on your compile procedure. Some shops set compiler options in cataloged procedures or build tools. For a source-level example, the option can appear in a CBL statement.
CBL AWO
IDENTIFICATION DIVISION.
PROGRAM-ID. WRITVB.
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
SELECT OUT-FILE ASSIGN TO OUTDD.Keep the compile listing or build record with the change. AWO is easy to forget because it changes generated file handling code without changing the visible WRITE statement.
Performance expectation
IBM performance guidance says AWO can save time because fewer calls are made to data management services for input and output handling. IBM also gives examples where programs writing variable-length files ran faster with fewer EXCPs. Treat those examples as proof that the option can matter, not as a promise for every program.
The biggest gains usually come from output files with large variation in record length. If every output record is close to the maximum length, there may be little room to improve block usage.
AWO and BLOCK0
BLOCK0 can make more physical sequential files use system-determined block sizes when the FD does not specify a block size. IBM notes that when BLOCK0 is in effect, AWO might apply to more files because more files may become blocked.
That interaction is useful, but it also means the change deserves testing. A compile-option change can affect more files than the one the developer had in mind.
When to avoid AWO
Avoid treating AWO as a blanket answer for all I/O performance problems. It is useful only for a specific file pattern. Be careful when a job depends on immediate write behavior, custom recovery assumptions, or exact timing of output availability during execution.
If the written record must be forced out as soon as possible, IBM performance guidance recommends avoiding AWO. That is uncommon for ordinary batch output files, but it can matter for special operational files.
Review checklist before changing AWO
- Confirm the file is physical sequential QSAM output.
- Confirm the records are variable-length and blocked.
- Check whether record sizes vary enough to benefit from better buffer use.
- Review
BLOCK0because it can increase the files where AWO applies. - Compare EXCP count, CPU time, and elapsed time before and after the compile change.
- Keep the compiler listing and JCL evidence with the change record.
Common mistakes
Using AWO on the wrong file type
AWO is not a VSAM tuning option and does not help ordinary fixed-block files in the same way. Start by checking the FD and JCL.
Ignoring the block-size setup
AWO works with blocked variable-length output. If the file is not blocked, review BLOCK CONTAINS, BLOCK0, and the JCL before expecting a result.
Comparing only elapsed time
Elapsed time can move because of system load. Compare EXCPs and CPU time too, especially when the job runs in a busy batch window.
Related Mainframe Forum guides
For nearby topics, read COBOL fixed vs variable-length records, COBOL file organization, COBOL WRITE statement, COBOL BLOCK0 compiler option, and COBOL performance tuning.
External references
IBM documents performance-related compiler options, APPLY WRITE-ONLY buffer behavior, and Enterprise COBOL option comparison.
FAQ
What does AWO do in COBOL?
AWO applies APPLY WRITE-ONLY processing to eligible physical sequential files with variable blocked records.
What is the default for AWO?
IBM documents the default as NOAWO, meaning the compiler does not implicitly apply APPLY WRITE-ONLY to eligible files.
When does AWO help most?
AWO helps most when a program writes blocked variable-length QSAM records and the actual record sizes vary enough to improve buffer use.
Does AWO apply to VSAM files?
No. The AWO compiler option is for eligible physical sequential QSAM files, not VSAM files.
