Path: utzoo!attcan!seachg!jalsop
From: jalsop@seachg.uucp (John Alsop)
Newsgroups: comp.sys.ncr
Subject: Re: The 8mm Gigabyte Gap
Message-ID: <1991Feb26.151004.13192@seachg.uucp>
Date: 26 Feb 91 15:10:04 GMT
References: <343@gandp> <1991Feb23.175824.537@nncrcae.Columbia.NCR.COM>
Reply-To: jalsop@seachg.UUCP (John Alsop)
Organization: Sea Change Corporation, Mississauga, Ontario, Canada
Lines: 50

In article <1991Feb23.175824.537@nncrcae.Columbia.NCR.COM> wescott@Columbia.NCR.COM (Mike Wescott) writes:
>In article <343@gandp> rg@gandp.UUCP (Dick Gill) writes:
>> The backup is run from crontab and uses cpio in a script that
>> has worked for me for years. (I did have to remove blocking
>> through dd to get it to run with the drive)
>
>> To get a better handle on the problem, I said:
>>
>>                   dd if=/dev/rmt/1yy of=/dev/null
>> 
>> and, when I got the results 6 hours later, guess what?  EOF
>> occurs at about 1.5 GB; got the same result with another new
>> tape! Is this right? Where is the missing 0.6 GB of capacity?
>
>Possibly, the blocking factor is too small and to keep streaming
>the exabyte pads with extra blocks (actually rewrites blocks).
>The same thing happens with streaming tapes.
>--

The Exabyte drive automatically switches between streaming and start/stop
mode, depending on the rate at which data is supplied to it.  It needs to
get around 250K per second in order to stream.  This is around 15MB per
minute, and your Tower is unlikely to be supplying data that fast,
especially using cpio.  2-4 MB/minute is more likely. When the tape drive
stops writing data, it will write a "gap track", which wastes about 10K of
tape capacity.

However, the drive does have an on-board 240K buffer, which means that data
will be written to the tape in 240K chunks. So you will probably lose about
10k capacity for each 240k block written to tape.  This is only about 4% of
the tape capacity, and doesn't explain the loss you are seeing.  (Note: the
onboard buffer can be disabled by the device driver, but I can't think why
anyone would want to do this).

My guess is that the cause of your capacity loss is poor tape quality,
resulting in rewritten data.  We have seen significant variations in tape
capacity from one cartridge to the next.  Exabyte branded tapes are no
better than Sony tapes in this respect.

The exabyte drive maintains statistics on read/write error counts - I don't
know whether NCR's driver can display them or not.

-- 
John Alsop

Sea Change Corporation
6695 Millcreek Drive, Unit 8
Mississauga, Ontario, Canada L5N 5R8
Tel: 416-542-9484 Fax: 416-542-9479
UUCP: ...!uunet!attcan!seachg!jalsop


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