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BIM-EPIC Job Control Disk Processing

Chapter 1. Introduction 1-43

1. BIM-EPIC locates the dataset. For cataloged datasets, it finds the dataset’s location in the

DSN Catalog. For Start Track 1 datasets, it has to read each VTOC in the pool until it finds

the first extent of the dataset.

2. It updates the VSE Label Area, creating an extent and assigning the disk volume.

Cataloged dataset processing requires a great deal less overhead than Start Track 1 processing.

Although BIM-EPIC has to write to the DSN Catalog each time it accesses a cataloged

dataset, it uses many fewer I/O operations than would be required to read all the VTOC entries

found in your disk pool. We strongly recommend using cataloged datasets instead of Start

Track 1 datasets in most cases. We also recommend that you use Start Track 1 datasets only

in disk pools that contain fewer than three volumes.

Before we can discuss close processing for disk datasets, you need to understand space holder

records.

Each time BIM-EPIC allocates an extent for a dataset, it writes a VTOC entry called a space

holder record to prevent the same space from being used when allocating another BIM-EPIC

dataset. The VTOC entry for the space holder record contains the name of the dataset with a

right parenthesis substituted for the first letter: )ATASET.NAME for DATASET.NAME.

The entry is written with today’s date as its expiration date.

If QDAS is running, no space holder record is created. Instead, BIM-EPIC reserves the space

in the QDAS map.

When a program requests close processing for its datasets, some records can be left in the

buffer. These are written to disk when VSE closes the dataset. In some cases there are too

many records in the buffer to fit in the space remaining in the current extent, and another

extent is necessary to accommodate the excess.

When BIM-EPIC closes a disk dataset, it takes the buffer into account. It determines from the

buffer size and from the amount of space left in the extent whether another extent may be

necessary. If another extent is needed, BIM-EPIC makes the allocation.

If VSE runs out of room when writing the buffered records, it checks the VSE Label Area for

another extent. It will find the extent that BIM-EPIC has placed there. VSE then reads the

VTOC, finds an expired dataset there, and rewrites the VTOC entry with the real dataset

name.

If the extent isn’t necessary, the VTOC entry remains as a space holder record. Since the

entry indicates the dataset is expired, the space can be used for any uncontrolled dataset at any

time, and BIM-EPIC reuses space holder extents on an as-needed basis.

You may see space holder records in your VTOC listings. You may also see an extra extent in

the DSN Catalog entry for the version. Neither of these adversely affects processing.

When closing an output disk dataset:

On Input

Space Holder

Records

Close Processing

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Memorex LOCK - VERSION 2-24 Spezifikationen Seite 57