News Group Posts

dd to and from tape; using diff to compare

See dd also.

From: Tony Lawrence <tony@pcunix.com>
Subject: Re: Edge backup with Seagate Travan tape.
Date: Tue, 19 Nov 2002 12:08:20 -0500

Ulven wrote:
> ahh...
> 
> Here's the correct version:
> 
> # dd if=/etc/termcap of=/dev/rStp0
> 288+1 records in
> 288+1 records out
> # dd if=/dev/rStp+ of=/termcapcopy
> dd: cannot open /dev/rStp+: No such file or directory (error 2)
> # dd if=/dev/rStp0 of=termcapcopy
> 289+0 records in
> 289+0 records out
> # diff /etc/termcap termcapcopy
> Warning: missing newline at end of file termcapcopy
> 3477a3478
> 
> something wrong here... does this tells you anything?
> 

It tells me you don't understand dd :-)

If you want to test something like that, do it like this:

cp /etc/termcap /tmp/this
tar cvf /dev/rStp0 /tmp/this
rm /tmp/this
tar xvf /dev/rStp0
diff /tmp/this /etc/termcap

The problem with using dd for this is the differing blocks- notice you 
get 288 full blocks plus a PARTIAL block going out to the tape, but the 
tape can only give you full blocks back. - therefore there is null data 
in the read, and that naturally won't pass diff.


> 
> 
> 
> "Bill Vermillion" <bv@wjv.comREMOVE> wrote in message
> news:H5t2B3.1I62@wjv.com...
> 
>>In article <arbp8v$2720$1@news.dataguard.no>,
>>Ulven <freddy@REMOVESPAMulven.com> wrote:
>>
>>>Hi Bill.
>>>Here's the resault:
>>
>>># dd if=/var/log/backup.txt of=/dev/rStp0
>>>1+0 records in
>>>1+0 records out
>>>#_
>>
>>Who knows how bitg backup.txt is.  I just suggested /etc/termcap
>>as it is decent size.
>>
>>The concept is to use dd to put a file to tape. 'if' is the name of
>>the input file and in this case  'of' is the name of the tape.
>>
>>The we reverse this and use 'if' for the tape devcie and 'of' for
>>the name of the file we will create as data is taken from the tape.
>>
>>Then we compare, using diff, to see if the contents of the file
>>we put on tape match what we just took off the tape.
>>
>>
>>>Then:
>>># dd if=/dev/rStp0 of=/var/log/temp
>>># pwd
>>>/var/log
>>># ls
>>>backup.txt
>>># _
>>># diff /etc/termcap termcapcopy
>>>diff: cannot stat termcapcopy: No such file or directory (error 2)
>>
>>You have to diff the files you put to tape and take from tape.
>>
>>I said  eg  dd if=/etc/termcap   eg meaning 'for example.  It's a
>>standard reference but perhaps I should have said   e.g. - but
>>everyone seem to skip that now.
>>
>>--
>>Bill Vermillion - bv @ wjv . com
> 
> 
> 

-- 


Tony Lawrence
Unix/Linux Support Tips, How-To's, Tests and more: http://aplawrence.com
Free Unix/Linux Consultants list: http://aplawrence.com/consultants.html

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