Re: FTP From Tape With Small Lrecl
- From: gilmap@xxxxxxxxxxxx
- Date: 17 Oct 2005 09:25:35 -0700
In a recent note, Bruce Black said:
> Date: Mon, 17 Oct 2005 10:51:58 -0400
>
> >At one time there was a definite problem with tape if the LRECL was less than 1
> 8 bytes, as the data was treated as noise errors.
> >
> The limitation was on blocksize, not LRECL.
>
> I thought that noise records no longer applied to cartridge tapes, but
> the IBM "Using Data Sets" manual still has this text
>
> "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. "
>
However, elsewhere in the same publication:
# 3.1.6.3 "z/OS V1R7.0 DFSMS Using Data Sets"
3.1.6.3 Using Magnetic Tape
Note that there is no minimum requirement for block size. However,
in nonreturn-to-zero-inverted mode, if a data check occurs on a
magnetic tape device, any record shorter than 12 bytes in a read
operation will be treated as a noise record and lost. No check
for noise will be made unless a data check occurs.
Apparently other sections of the same document have not been
updated to account for the dwindling prevalence of NRZI and the
advent of other formats.
> Data checks are much rarer on cartridge drives, so this rule may rarely
> apply, if it really does apply to cartridges.
>
But, what's a "data check"? A phrase search of the same publication
reveals no definition, cross-reference, or glossary entry. Intuitively,
I'd expect it to mean an inconsistency detected not by hardware, but
by software, such as length read inconsistent with RDW, or not a
multiple of LRECL; i.e. the block appears physically OK, but contains
invalid data.
-- gil
--
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