Question about mag tapes under Eunice

[Steve Williams]

> Maybe someone can enlighten me as to why this works the way it does:
> 
> My wife works on a Eunice system (version 3.2).  I gave her a mag
> tape with some stuff written on it in tar format (V7 tar, 1600 bpi).
> She was able to extract it using tar on her Eunice machine with no
> problems.  She then put some things on the tape to give me, again,
> using tar.  She had no problem putting things ON the tape, but when
> she tried to get a table of contents listing, tar complained about
> the block size being zero and no TOC was generated.  She recreated the
> tape with tar, only this time put a blocking factor (tar cvb 20 files).
> Again, there was no error, but when she tried a TOC, the same error
> happened.  At this point, she gave up and gave the tape back to me.
> I mounted the tape on my machine and managed to generate the TOC AND
> extract the tape with no problems (and yes, the files were the files
> that SHE put on the tape, not the ones I put on).  I don't understand..
> 
> Why is that VMS/Eunice can't read its own writing?


The default block size on a tape mounted with the VMS /FOREIGN switch is 512.
To read a blocked tar tape, you need to set the block size to a value greater
or equal to the block size on the tape.  For tar b 20, the block size is
20 x 512 or 10240.  Magic incantations which are known to work::

$ mount mtxx:/for/block=65534
	(te16)
$ mount mfxx:/for/block=65534
	(tu7x)

65534 is the maximum value for the /block parameter.  This allows you to read
the tape even if you don't know the blocksize.


					-Steve Williams
					{ihnp4,mot,ucscc,amdahl}!drivax!steve