[rescue] mt erase

[Geoffrey S. Mendelson]

On Wed, May 09, 2007 at 04:25:32PM -0400, Kevin Foote wrote:
> i figured there is enough old *nix knowledge on this list to post this
> question.

> 
> im wondering what the mt erase option actually does? does it write zeros
> till the tape end ??

Only on reel to reel and QIC tape. 

On any tape that has end of tape indicator bits, it just writes a 
end of tape indicator. (4mm, 8mm, etc). 

On SCSI tape drives, it's a function of the drive and not the system.
mt just issues an erase command. it's up to the drive to implement it.

It's even more complicated because reel to reel tape does not stop
at the end of the reel. The drive senses an reflective marker and 
returns a sense bit. Hopefully the operating system noticed the sense
bit and returned an error condition to the program. The program was
supposed to write two tape marks (an end of tape indicator) and
rewind and unload the tape. 

Mainframe operating systems did that for you, and automaticly
wrote tape lables, mounted new tapes etc. UNIX systems did not,
and 3B2s used as telephone accounting systems were notorious for
running off the end of the reel. 

That's why if you look into several of the UNIX/Linux backup packages,
you find that you have a table of tape types and lengths. 

Geoff.

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