[Info-vax] Getting tape drive errors make no sense
George Cornelius
cornelius at encompasserve.org
Thu Oct 31 16:02:52 EDT 2013
glen herrmannsfeldt wrote:
> Ken Fairfield <ken.fairfield at gmail.com> wrote:
>
>>On Friday, October 25, 2013 12:33:26 PM UTC-7, tadamsmar wrote:
>>
>>>I tried to copy a large save-set (a full disk backup) from tape to a
>>>large disk. That is I am just trying to copy the file from tape to
>>>disk, not trying to restore a volume.
>>
>>And what is the block size of the saveset you're trying to copy to disk?
>>If you wonder why I'm asking, this could very well be your problem.
>
>
> I am not sure what you mean, but I have seen SCSI tape drives
> reblock data. They can be set to always write with a fixed block
> size like 1K, or they can be set to write each block to the
> appropriate size, sometimes called variable length.
>
> If it is set different than you expect, the results can be
> surprising.
He was probably referring to the 32767 byte limit on RMS
record size, something that interferes with copying, say,
a tape with blocksize 65024 to an RMS file. And, of
course, the other drawback of copying to an RMS file:
that VMS BACKUP's bad block handling is not understood
by a COPY command, so there will be no attempt to
deal with bad blocks which were successfully rewritten
on a second or later attempt (hint: for reliability
reasons, BACKUP does _not_ backspace and overwrite - at
least as long as the /INTERCHANGE qualifier is not
present).
With regard to reblocking by SCSI drives, it can be done
in a transparent way - I believe DLT's do this - so that
the O/S does not have to know it is happening; but if there
are drives or controllers that do this in a nontransparent
way, that mode of operation is not compatible with VMS.
I believe Unix/Linux may try to make tapes appear to
be byte stream oriented, this being a somewhat different
concept from so-called streaming hardware. But under
Unix the programming interface may be hiding the detailed
information about the blocking through its use of buffers.
My experience is that tar is quite unhappy if the block
size is different from the default and you do not
specify, or incorrectly specify, the blocksize via
the command line.
George
> -- glen
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