Enterprise COBOL for z/OS, Version 4.2, Programming Guide

To open a file that has never contained records (an empty file), use a form of the OPEN statement.

Depending on the type of file that you are opening, use one of the following statements:

• OPEN OUTPUT for ESDS files.
• OPEN OUTPUT or OPEN EXTEND for KSDS and RRDS files.
  (Either coding has the same effect.)
  If you coded the file for random or dynamic access and the file is optional, you can use OPEN I-O.

Optional files are files that are not necessarily available each time a program is run.
You can define files opened in INPUT, I-O, or OUTPUT mode as optional by defining them with the SELECT OPTIONAL clause in the FILE-CONTROL paragraph.

Initially loading a file sequentially:
Initially loading a file means writing records into the file for the first time.
Doing so is not the same as writing records into a file from which all previous records have been deleted.
To initially load a VSAM file:

1. Open the file.
2. Use sequential processing (ACCESS IS SEQUENTIAL).
   (Sequential processing is faster than random or dynamic processing.)
3. Use WRITE to add a record to the file.

Using OPEN OUTPUT to load a VSAM file significantly improves the performance of your program.
Using OPEN I-O or OPEN EXTEND has a negative effect on the performance of your program.

When you load VSAM indexed files sequentially, you optimize both loading performance and subsequent processing performance, because sequential processing maintains user-defined free space.
Future insertions will be more efficient.

With ACCESS IS SEQUENTIAL, you must write the records in ascending RECORD KEY order.

When you load VSAM relative files sequentially, the records are placed in the file in the ascending order of relative record numbers.

Initially loading a file randomly or dynamically:
You can use random or dynamic processing to load a file, but they are not as efficient as sequential processing.
Because VSAM does not support random or dynamic processing, COBOL has to perform some extra processing to enable you to use ACCESS IS RANDOM or ACCESS IS DYNAMIC with OPEN OUTPUT or OPEN I-O.
These steps prepare the file for use and give it the status of a loaded file because it has been used at least once.

In addition to extra overhead for preparing files for use, random processing does not consider any user-defined free space.
As a result, any future insertions might be inefficient.
Sequential processing maintains user-defined free space.

When you are loading an extended-format VSAM data set, file status 30 will occur for the OPEN if z/OS DFSMS system-managed buffering sets the buffering to local shared resources (LSR).
To successfully load the VSAM data set in this case, specify ACCBIAS=USER in the DD AMP parameter for the VSAM data set to bypass system-managed buffering.

Loading a VSAM data set with access method services:
You can load or update a VSAM data set by using the access method services REPRO command.
Use REPRO whenever possible.

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