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From: grr@cbmvax.cbm.UUCP (George Robbins)
Newsgroups: net.unix,net.unix-wizards
Subject: Re: need help with multi-reel cpio(problem with the suggested solution)
Message-ID: <181@cbmvax.cbmvax.cbm.UUCP>
Date: Sat, 3-May-86 12:28:36 EDT
Article-I.D.: cbmvax.181
Posted: Sat May  3 12:28:36 1986
Date-Received: Sun, 4-May-86 06:26:34 EDT
References: <520@sdcc13.UUCP> <461@ncr-sd.UUCP> <1728@sdcsvax.UUCP>
Reply-To: grr@cbmvax.UUCP (George Robbins)
Organization: Commodore Technology, West Chester, PA
Lines: 41
Keywords: cpio tape backup
Xref: watmath net.unix:7773 net.unix-wizards:17937

In article <1728@sdcsvax.UUCP> coleman@sdcsvax.UUCP (Don coleman) writes:
>In article <461@ncr-sd.UUCP> greg@ncr-sd.UUCP (Greg Noel) writes:
>>In article <520@sdcc13.UUCP> bparent@sdcc13.UUCP (Brian Parent) writes:
>>>I'm having trouble with cpio going to multiple reels.  It seems to 
>...
>>The problem is this:  if the tape drive asserts EOT while writing or reading
>>a record, the driver returns an error.  That's it; that's the whole problem.
>...
>>No, the problem is with Unix's treatment of tape volumes.  Note that as it
>...
>>The mods are simple: when writing a tape, if you get an EOT, backspace the
>>record ("unwrite the record"), write a EOF marker (so you can't read it back),
>>backspace again (in front of the EOF, so a close (or a shorter write) will
>>work as expected), and return an error. 
>
>There are tape drives that cannot backspace(the qic-02 1/4" tape
>standard doesn't even contain any spacing commands)...
>
>don coleman@ucsd.edu

I don't know about these new fangled cartridge tapes, but from my IBM system
days, I seem to remember that the EOT indication from the drive was just an
advisory warning that the drive had sensed the EOT marker while writing the
last record.  The markers are on the back of the tape, and don't affect the
actual writing.  Thus you could just write out your logical end of tape
marker without any backup and erase foolishness.

Unfortunatly, the DC300 type cartridges give you about 3" of tape before those
little holes they shoot through the tape.  This means that for any reasonable
block size, there just isn't room to finish the block before the bits fall
into the holes...  Of course, if your drive does serpentine tricks, you get
the warning 3 inches after you've written over the holes?

I guess you are just supposed to know what size tape you are using and write
the number of bits with the cheap streaming drives that don't have spacing
commands.

-- 
George Robbins - now working with,	uucp: {ihnp4|seismo|caip}!cbmvax!grr
but no way officially representing	arpa: cbmvax!grr@seismo.css.GOV
Commodore, Engineering Department	fone: 215-431-9255 (only by moonlite)