problem with using 'dd' to copy 'tar' tapes

[Joe Smith]

In article <1990Oct7.230945.3387 at rice.edu> sdm7g at dale.acc.Virginia.EDU (Steve D. Majewski) writes:
>In trying to make a copy of a tar tape with dd, the copies don't seem to
>be exactly the same as the originals. 
>
>The original, I can loop thru via: "dd if=/dev/nrst0" or "tar -tvf
>/dev/nrst0"  and at the end, the tape is properly positioned at the start
>of the next file.
>
>Reading the copy, the tape sometimes ends up positioned at the end of the
>current file, rather than the beginning of the next. 

That's strange.  Every time I've tried reading an original tape with
multiple tar saves, it always stops before the start of next file.  For
instance:

    tar cvf /dev/nrst8 dir1
    tar cvf /dev/nrst8 dir2
    mt -f /dev/nrst8 rew
    tar xvf /dev/nrst8	# restores dir1
    tar xvf /dev/nrst8	# always says "unexpected EOF"
    tar xvf /dev/nrst8	# restores dir2

This matches the man page for tar:

     If there are multiple archive  files  on  a  tape,  each  is
     separated from the following one by an EOF marker.  tar does
     not read the EOF mark on the tape after it finishes  reading
     an  archive  file  because tar looks for a special header to
     decide when it has reached the end of the archive.   Now  if
     you  try  to  use tar to read the next archive file from the
     tape, tar does not know enough to skip over the EOF mark and
     tries  to  read  the  EOF  mark  as an archive instead.  The
     result of this is an error message from tar to the effect:
          tar: blocksize=0
     This means that to read another archive from the  tape,  you
     must  skip  over  the EOF marker before starting another tar
     command.  You can accomplish this using the  mt(1)  command,
     as in "mt -f /dev/nrst8 fsf 1"

So, I can't explain why you luck out on your originals.

Joe Smith (408)922-6220 | SMTP: jms at tardis.tymnet.com or jms at gemini.tymnet.com
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