CDAM Logic

CDAM writes the compressed sysout to a sequential dataset. Each sysout may span one or more datasets. The datasets are allocated on the unit specified in the CDAM parameters, without secondary allocation parameters. The workflow of CDAM is as follows:

• When the application program performs the first write (PUT) operation, CDAM tries to allocate a dataset on the specified (or default) unit using the specified (or default) blocks parameter and the RLSE option.
• The dataset is allocated and opened through standard system facilities, and passes through all security and allocation verifications performed in the operating system. In case of failure, normal system messages are produced and the job terminates.
• The dataset name is cataloged immediately at the time of creation. The dataset name created by CDAM conforms to the naming conventions described below.
• When CDAM uses all the available space in the first "logical" extent, it does not abend on SB37. It tries to allocate another logical extent (see below for naming conventions). A logical extent is a standard sequential file from the operating system point of view. The attempted allocation size is twice the amount of the previous logical extent size. The logical extent may be on a different disk drive in the same unit.

  If allocation fails (that is, a space in the specified size cannot be found), the CDAM attempts half of the previous size and continues to do so until the allocation succeeds. Therefore, CDAM abends on SB37 only when the last single free track in the specified unit is full, that is, there is no remaining space in the entire unit.

  In this way, CDAM makes use of otherwise unusable areas on the disks. The disk normally contains unused tracks which cannot be used by the standard dataset allocation algorithm. These areas can be used by CDAM. For example, a CDAM dataset could be allocated as follows:

Note: This example is exaggerated to illustrate the CDAM allocation algorithm.

Table 278 CDAM Dataset Allocation – Example

• When the ddname of the sysout is deallocated (usually at the end of the step), the remaining free tracks in the last allocated logical extent are released for general use (since the dataset is allocated using parameter RLSE).

When a compressed sysout is read by CDAM, the record format, block size and the record length of the dataset are retrieved. The report takes the original LRECL, BLKSIZE and RECFM specifications (from the ACB/DCB) as it had when produced by the application program.