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From: Elric@tsc3b21.UUCP (Frank )
Newsgroups: comp.sys.att
Subject: Re: 3B2 tape to tape copy
Message-ID: <205@tsc3b21.UUCP>
Date: Tue, 27-Oct-87 07:32:08 EST
Article-I.D.: tsc3b21.205
Posted: Tue Oct 27 07:32:08 1987
Date-Received: Sat, 31-Oct-87 05:34:52 EST
References: <1590@netsys.UUCP>
Organization: Transportation Systems Consulting, FL
Lines: 51

in article <1590@netsys.UUCP>, len@netsys.UUCP (Len Rose) says:
>  Something I've been trying to do for some time has me
>  quite frustrated..I need to be able to copy tape to tape
>  on our 3B2 system here (yes,we have two CTC's) , and have
>  not been able to come up with a fast method of accomplishing
>  this.
>
>  [ ... ]
>
>  dd < /dev/rSA/ctape1 > /dev/rSA/ctape2
>  cp /dev/rSA/ctape1 /dev/rSA/ctape2
> 
>  This works fine,does not waste system resources,but is slow as hell.
> -- 
> Len Rose -* Netsys Public Access Network *- The East Coast Machine
> 301-540-3656,3657,3658,3659    3B2/Unix SV3.0

Well Len, while I don't have ANY tape drives, I used to work quite heavily
with them.  I found that the best method of tape to tape copy, albeit not
on a 3B2, was using "dd" as you did, but setting the buffer size as well.
We were using 300 XL/P's which are 450 ft and I found a buffer size of
128K worked well enough and didn't leave a large gap at the end of the tape.

If you have the time to waste, try using "dd if=large_disk > ctape2 bs=4096".
Substitute the device name of a large disk drive in place of "large_disk".
When dd hits the end of the tape you'll get a message indicating how many of
the 4K blocks were written (something like "##+0 records out").  This is
the maximum storage on that *particular* tape (other tapes may be +/- 3% in
length and so your mileage may vary).

Once you have this value you can decide on a block size for use in the copy
that will allow the use of most of the tape.  Hint:  pick a multiple of 64K
if possible since most tape drives I have used (not controllers, drivers,
etc) prefer it.  Note that I said, "most I have used".

The command line would be similar to:
	dd < /dev/rSA/ctape1 > /dev/rSA/ctape2 bs=128k

The characters 'k' and 'b' (for blocks) are recognized by the dd command
and the argument is multiplied out appropriately.

Good Luck!  It may take a lot of playing around to get the *best* buffer size.

-----
"Use the Source, Luke.  Feel it flow through your fingertips..."
-----
Frank (crash) Edwards
...!codas!usfvax2!{pdn,jc3b21}!tsc3b21!crash
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