Format-F and format-V records: BLKSIZE specifies the maximum length, in bytes, of a data block. If the records are format-F, the block size must be an integral multiple of the record length, except for SYSOUT data sets. (See Spooling and Scheduling Data Sets.) If the records are format-V, you must specify the maximum block size. If format-V records are unblocked, the block size must be 4 bytes greater than the record length (LRECL). If you do not use the large block interface (LBI), the maximum block size is 32␠760 except for ISO/ANSI Version 3 records, where the maximum block size is 2048. You can override the 2048 byte limit by a label validation installation exit (see z/OS DFSMS Installation Exits). If you use LBI, the maximum block size is 32␠760 except on magnetic tape, where the maximum is larger. Additionally, the maximum block size when using BSAM with UNIX is 65␠535.

Extended-format data sets: In an extended-format data set, the system adds a 32-byte suffix to each block, which your program does not see. This suffix does not appear in your buffers. Do not include the length of this suffix in the BLKSIZE or BUFL values.

Compressed-format data sets: When you read blocked format-F or format-V records with BSAM or BPAM from a compressed data set with DBB compression, PDSE, or UNIX files, the records might be distributed between blocks differently from when they were written. In a compressed format data set, the BLKSIZE value has no relationship with the actual size of blocks on disk. The BLKSIZE value specifies the maximum length of uncompressed blocks.

System-determined block size: The system can derive the best block size for DASD, tape, and spooled data sets. The system does not derive a block size for BDAM, old, or unmovable data sets, or when the RECFM is U. See System-Determined Block Size for more information on system-determined block sizes for DASD and tape data sets.

Minimum block size: If you specify a block size other than zero, there is no minimum requirement for block size except that format-V blocks have a minimum block size of 8. However, if a data check occurs on a magnetic tape device, any block shorter than 12 bytes in a read operation, or 18 bytes in a write operation, is treated as a noise record and lost. No check for noise is made unless a data check occurs.