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From: jeq@laidbak.UUCP (Jonathan E. Quist)
Newsgroups: net.unix-wizards
Subject: Re: Re: dump(8) verification?
Message-ID: <267@laidbak.UUCP>
Date: Tue, 15-Oct-85 19:48:58 EDT
Article-I.D.: laidbak.267
Posted: Tue Oct 15 19:48:58 1985
Date-Received: Thu, 17-Oct-85 00:32:59 EDT
References: <2054@brl-tgr.ARPA> <324@aphasia.UUCP>
Reply-To: jeq@laidbak.UUCP (Jonathan E. Quist)
Organization: LAI Chicago
Lines: 23
Summary: 

In article <324@aphasia.UUCP> gww@aphasia.UUCP (George Williams) writes:
>Under VMS the magtape driver has a write with verify operation that its
>backup uses, this could easily be implemented in user code by doing a
>write, backing up, doing a read, and then comparing the written buffer
>with the read one. Simple, no need to worry about changing filesystems.
>
>Unfortunately slow too. It will at least double the tape IO time (and
>make a streaming tape drive stop streaming).

I'm sure I've seen specs on at least one tape controller that did
automatic write/verify operations.  If the drive read heads are
after the write heads in the tape path, this could conceivably be
no problem.  Even if the drive had to rewind to verify, the work
would still be done by the controller, not the cpu, so the
time penalty would not necessarily mean a great system performance
penalty.  (Excepting the problem with rewinding streaming drives.)

Now, the $68000 question.  Does anyone else out there know
of tape controllers that do write/verify operations automatically,
or is this just the product of a warped memory (faulty media?)?

Jonathan E. Quist
ihnp4!laidbak!jeq