Tape Data Sets

The system can determine the optimum block size for tape data sets. The system sets the block size at OPEN on return from the DCB OPEN exit and installation DCB OPEN exit if:

The block size in DCBBLKSI is zero (or DCBEBLKSI if using LBI).
The record length is not zero.
The record format is fixed or variable.
The tape data set is open for OUTPUT or OUTIN.
The access method is BSAM or QSAM.

Rule: For programming languages, the program must specify the file is blocked to get tape system-determined block size. For example, with COBOL, the program should specify BLOCK CONTAINS 0 RECORDS.

The system-determined block size depends on the record format of the tape data set. Table 1 shows the block sizes that are set for tape data sets.

Table 1. Rules for setting block sizes for tape data sets or compressed format data sets
RECFM	Block Size Set
F or FS	LRECL
FB or FBS (Label type=AL Version 3)	Highest possible multiple of LRECL that is ≤ 2048 if LRECL ≤ 2048
	Highest possible multiple of LRECL that is ≤ 32␠760 if LRECL > 2048
FB or FBS (Label type=AL Version 4 or not AL)	Not tape or not LBI: highest possible multiple of LRECL that is ≤ 32␠760 LBI on tape: Highest possible multiple of LRECL that is ≤ the device's optimal block size
V (not AL)	LRECL + 4 (LRECL must be less than or equal to 32␠756)
VS (not AL)	LRECL + 4 if LRECL ≤ 32␠756
	32␠760 if LRECL > 32␠756
VB or VBS (not AL)	Not tape or not LBI: 32␠760 LBI on tape: Device's optimal block size
D (Label type=AL)	LRECL + 4 (LRECL must be ≤ 32␠756)
DBS or DS (Label type=AL Version 3)	2048 (the maximum block size allowed unless an installation exit allows it)
D or DS (Label type NL or NSL or label type=AL Version 4)	LRECL + 4 (LRECL must be ≤ 32␠756)
DB or DBS (Label type NL, or NSL, or AL Version 4)	32␠760
DB not spanned (Label type=AL Version 3)	2048 if LRECL ≤ 2044
	DCBBLKSI = 32␠760 if LRECL > 2044 (you have the option, for AL Version 3, to accept this block size in the label validation installation exit)
Label Types: AL = ISO/ANSI labels NL = no labels NSL = nonstandard labels SL = IBM standard labels Not AL = NL, NSL, or SL labels RECFM Allowances: RECFM=D is not allowed for SL tapes RECFM=V is not allowed for AL tapes