z/OS MVS JCL Reference
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Subparameter definition

z/OS MVS JCL Reference

Specifies the maximum length, in bytes, of a block.
The number of bytes that you specify for BLKSIZE depends on the device type, the record format for the data set and other programs that will read or write the data set. The maximum value that you can code is 2,147,483,648 (coded without the commas) or 2G. When your program uses the data set, the OPEN function may impose a smaller limit. The maximum allowed by OPEN is:
  • 2,147,483,648 for dummy data sets. Note that you cannot actually get a buffer that large.
  • 2,147,483,647 for tape that does not have ISO/ANSI Version 3 or Version 4 labels. When writing with BSAM and QSAM, the system imposes a limit that depends on the tape subsystem model. Currently the maximum for certain models is 256K. See z/OS DFSMS Using Data Sets for more details about block size limits for specific models. OPEN allows EXCP programs to have higher limits that depend on the hardware.
  • 2048 for tape that has ISO/ANSI Version 3 labels, where the minimum value for BLKSIZE is 18 bytes. To allow a block size greater than 2048, use installation exit routine IFG0193G, described in z/OS DFSMS Installation Exits.Version 4 labels do not have this restriction.
  • 32,760 for DASD, ISO/ANSI Version 4 tape labels, and other data sets.
Specifies the maximum length, in kilobytes, of a block. (1 kilobyte = 1024 bytes.) The maximum is 2097152. If you code 2097152K, the block size is the maximum: 2,147,483,648 bytes.
Specifies the maximum length, in megabytes, of a block. (1 megabyte = 1024 kilobytes.) The maximum is 2048. If you code 2048M, the block size is the maximum: 2,147,483,648 bytes.
Specifies the maximum length, in gigabytes, of a block. (1 gigabyte = 1024 megabytes.) The maximum is 2G. If you code 2G, the block size assigned is the maximum: 2,147,483,648 bytes.

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