[rescue] mt erase

[Charles Shannon Hendrix]

Geoffrey S. Mendelson wrote:
> 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. 

I've always wished UNIX had more sensible control over mountable volumes.

It's just kind of dumb that anyone who can mount a volume also has complete
access to it.

Not just that, but you can't reserve it.

For example, if I mounted a tape on an IBM mainframe, the Prime, or the DEC
minis that I used in college, no one else could access that drive.  In some
cases even the operator would have to kick me off first.

But with UNIX, I put a tape in a drive, and anyone with tape drive access can
get to it.

It's more flexible I guess, but it seems like it should have been done better.

Kind of like cron and print spooling... useful and they work, but older
systems have far more useful and interesting job and spooling systems.

When you look at the latest and greatest UNIX print spooling system, CUPS, it
does have some old mainframe/mini era features, but it's really messy and big.

It used to be really neat to look at the spooler on a Data General MV20000 and
fire off a print job you'd done months back, and even be able to reformat it
for the line printer instead of the console, etc.



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shannon          | I want this Perl software checked for viruses.  Use Norton
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