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From: jc@sdcsvax.UUCP (John Cornelius)
Newsgroups: net.unix-wizards,net.periphs
Subject: Re: seek on raw magtape
Message-ID: <1277@sdcsvax.UUCP>
Date: Mon, 6-Jan-86 10:10:04 EST
Article-I.D.: sdcsvax.1277
Posted: Mon Jan  6 10:10:04 1986
Date-Received: Wed, 8-Jan-86 06:41:31 EST
References: <6267@utzoo.UUCP>
Reply-To: jc@sdcsvax.UUCP (John Cornelius)
Distribution: net
Organization: Western Scientific, San Diego
Lines: 14
Keywords: tape directory error gap rewrite erase
Xref: linus net.unix-wizards:13486 net.periphs:831
Summary: The medium makes it impractical

The reason you cannot randomly access tapes for writing is because if an error
occurs while writing, the procedure is to erase 3 inches of tape. Assume that
you had written 3 records of directory. If, while trying to rewrite the middle
one, an error occurs the system will back up over it, erase 3 inches of tape,
and attempt to rewrite the record. This would clobber the 3rd record and
perhaps several others as well since the standard gap between records is only
1/2".

A related issue is the location of the erase head on a tape drive. When writing
tape the drive erases tape ahead of the writing head. The erase head can be as
much as an inch ahead of the write head. This too can clobber the next record.

John Cornelius
Western Scientific