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From: Greg Lehey <grog@lemis.de>
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Subject: Re: Tape drive problems? maybe?
In-Reply-To: <199610041714.TAA00465@ghost.mep.ruhr-uni-bochum.de> from Robert Eckardt at "Oct 4, 96 07:14:46 pm"
To: roberte@mep.ruhr-uni-bochum.de (Robert Eckardt)
Date: Mon, 4 Nov 1996 12:27:04 +0100 (MET)
Cc: questions@FreeBSD.org (FreeBSD Questions)
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Robert Eckardt writes:
>
>> I am running FreeBSD 2.1.5-RELEASE and have just bought a shiny new
>> Exabyte 8505XLI tape drive. I am attempting to back up several filesystems
>> as different tape files. e.g.
>>
>> cd /somewhere ; tar cf /dev/nrst0 *
>> cd /somewhere/else ; tar cf /dev/nrst0 *
>>
>> I have done this before with other UNIXes.
>>
>> Now I am attempting to restore these directories (actually just verify
>> that this is happening) with:
>> mt rewind
>> tar tf /dev/nrst0 > filelist.1
>> tar tf /dev/nrst0 > filelist.2
>
> Try:
> tar tf /dev/nrst0 > filelist.1
> tar tf /dev/nrst0 > /dev/null
> tar tf /dev/nrst0 > filelist.2
>
> I found with my Wangtek that the tape driver has problems with EOF marks.
> mt -f /dev/nrst0 fsf <num>  doesn't work at all (for me)
> -- usually the SCSI bus locks.
>
>> Does anybody know what is going on?

Robert's on the right track.  Streaming tapes, especially helical-scan
versions, aren't very good about positioning on a tape mark, and
*sometimes* you won't find the next file first time: you'll be
positioned somewhere in the tape mark, which on Exabytes are quite
large.  Someone once told me that they're the equivalent of a megabyte
or two.

I do regular backups to DDS and Exabyte drives every night, and then
read in the files again with this script fragment:

for i in $filesystems; do
  echo +++ Backup of file $file, file system $i >$backuplist/$tapeid.$file
  try=1
  while [ `wc -l $backuplist/$tapeid.$file | awk '{print $1}'` -lt 2 -a $try -lt 4 ]; do
    try=`expr $try + 1`
    /opt/bin/tar tvflb $TAPE 128 >>$backuplist/$tapeid.$file
  done    
  /opt/bin/gzip $backuplist/$tapeid.$file&
  file=`expr $file + 1`
done

Greg