The SDB option specifies whether system-determined block size should be used to select an optimal block size for SORTOUT, SORTOFxx, SORTOFx, SORTXSUM, and SORTXDUP data sets when none is provided. This parameter will automatically provide a block size that will most efficiently utilize the space on the output device.
SDB=ON enables the use of system-determined block size for both tape and new or previously allocated but unopened DASD output data sets except in the following conditions:
- A block size is found in the JCL DCB BLKSIZE specification or, in the case of a DISP=MOD tape data set, it is derived from an available tape label.
- The output file is a VSAM data set
If the output data set is on DASD, the block size selected will be based upon the RECFM and LRECL, either specifically provided or determined from the usual analysis of SORTIN or RECORD statement attributes. For example, the block size selected for a blocked output data set assigned to a 3380 or 3390 DASD device will represent a size as close to half-track blocking as possible.
If the output file is a tape data set, the block size will be determined from the RECFM and LRECL in conjunction with the following rules:
- RECFM of F or FS: BLKSIZE=LRECL
- RECFM of FB or FBS and LABEL type is not AL: BLKSIZE=highest multiple of LRECL that is less than or equal to 32760.
- RECFM of FB and LABEL type is AL: BLKSIZE=highest multiple of LRECL that is less than or equal to 2048.
- RECFM of V, VS, D: BLKSIZE=LRECL +4
- RECFM of VB, VBS: BLKSIZE=32760
- RECFM of DB: BLKSIZE=2048
If SDB=OFF is specified, MFX will not use system-determined block size. The block size, if unavailable, will be determined from SORTIN if the SORTIN and output data set LRECLs are the same, otherwise MFX will select an appropriate blocksize.
If SDB=DISKONLY is specified, MFX will use system-determined block size only for disk output data sets.
If SDB=TAPEONLY is specified, MFX will use system-determined block size only for TAPE output data sets.
SDB=LARGE enables the use of system-determined block size for both tape and DASD output data sets, as with SDB=ON. Additionally, SDB=LARGE enables selection of a system-determined block size greater than 32760 for eligible tape output data sets if not restricted by the system BLKSZLIM value.
SDB=SMALL has the same meaning as SDB=ON.
SDB=INPUT enables the use of system-determined block size for both tape and DASD output data sets, as with SDB=ON. Additionally, if an input tape data set has a block size greater than 32760, SDB=INPUT enables selection of a system-determined block size greater than 32760 for eligible tape output data sets if not restricted by the system BLKSZLIM value. SDB=INPUT is the default.
SDB=LARGEONLY enables the use of system-determined block size for tape output data sets only, as with SDB=TAPEONLY. Additionally, SDB=LARGEONLY enables selection of a system-determined block size greater than 32760 for eligible tape output data sets if not restricted by the system BLKSZLIM value.
SDB=INPUTONLY enables the use of system-determined block size for tape output data sets only, as with SDB=TAPEONLY. Additionally, if an input tape data set has a block size greater than 32760, SDB=INPUTONLY enables selection of a system-determined block size greater than 32760 for eligible tape output data sets if not restricted by the system BLKSZLIM value.