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Write-Validity-Check Option (OPTCD=W)
You can specify the write-validity-check option in either the DCB
parameter of the DD statement or the DCB macro. OPTCD
=
W causes the
3480 to operate in tape-write-immediate mode.
For buffered tape devices, write validity check delays the device-end
interrupt until the data is physically on tape. When you use the write
validity check option, you do not benefit from the performance advantage of
buffering. This causes the maximum I/O rate to be reduced to about 10
blocks per second.
The OPTCD
=
W parameter of the DCB can be used to request that data
buffering in the control unit be bypassed when writing data. It ensures that
each block of data is physically written on the tape and read-back checked,
before the control unit gives write· completion indication to the processor.
(The read-back check is done by the hardware while the data is being
written).
Generally, you use the write-validity-check option to make the application's
write operations go directly to the tape instead of storing the data in the
control unit buffer. OPTCD
=
W is generally only meaningful with BSAM
and EXCP.
Coding the Block Size and Logical Record Length Operands
The 3480 provides good performance for all block sizes, but is designed to
provide optimal performance for block sizes in the range of 8 to 32
kilobytes.
Constructing a Data Control Block
The access method support that exists for data sets on magnetic tape is used
for the 3480. QSAM, BSAM, and EXCP are supported. The default mode
used is buffered data transfer for both read and write.
Macro Instructions Available by Access Method
MACRO
EXCP
BSAM
QSAM
INSTRUCTION
DCB
X
X
X
DCBD
X
X
X
MSGDISP
X
X
X
NOTE
X
POINT
X
SYNCDEV
X
X
X
8
IBM 3480 Magnetic Tape Subsystem User's Reference
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