using backup with 1/4 inch tape media

using backup with 1/4 inch tape media

Post by Tom Merri » Wed, 02 Jan 1991 03:36:01



I found a couple of unexplained things while using backup at the 3002 level
and 1/4" low density cartridges (rmt0.4 and rmt0.5).

First, I wrote a tape label using dd, forward spaced over it, then did a
backup to the cartridge.  I got a write error stating that I had defective
media.

Second, I did several backups to the same tape cartridge (w/o a tape label).
Everything finished correctly, so I did a tcopy on the tape and found out that
my 60 MB cartridge had almost 112 MB on it!

Is backup ignoring the hardware and always writing high density?  This would
explain both problems above.

Tom Merrick
Ohio State University

 
 
 

using backup with 1/4 inch tape media

Post by Patrick Tor » Thu, 03 Jan 1991 00:03:15



>Second, I did several backups to the same tape cartridge (w/o a tape label).
>Everything finished correctly, so I did a tcopy on the tape and found out that
>my 60 MB cartridge had almost 112 MB on it!
>Is backup ignoring the hardware and always writing high density?  This would
>explain both problems above.
>Tom Merrick
>Ohio State University

I was having the same trouble, still am actually,
thing is , I found that low density is 120meg and high is 150 meg.

I came to this conclusion looking through the doc from the
IBM AIX v3 System Administators class.

you need dc6150 tapes to get 150 meg on a tape, and you'll pay extra $$
for the extra 30 meg.

--

{emory,gatech}!nanovx!msa3b!torre

 
 
 

using backup with 1/4 inch tape media

Post by Lanfranco Alba » Thu, 03 Jan 1991 18:30:59


Quote:>Second, I did several backups to the same tape cartridge (w/o a tape
>label).  Everything finished correctly, so I did a tcopy on the tape
>and found out that my 60 MB cartridge had almost 112 MB on it!

>Is backup ignoring the hardware and always writing high density?  This
>would explain both problems above.

I believe there is something more to it... the AIX filesystem structure
is tricky, but what I _think_ is happening is that full 8-Kbytes blocks
are being used, thus with an average of 4 Kbytes wasted per file; df is
reporting on a full-block basis; and tape backup is NOT copying the
extra per-file 'slack', only the "meat" of each file (I'm not sure what
both df and tape backup are doing with holey files, but I would guess
them to be more in-sync in this respect).

I had a filesystem occupying 190 megabytes, and its backup fitted inside
a single DC6150 150-Mbytes tape; and I don't even know if that tape was
actually full... no problems re-reading it, anyway!

Well, at least this is saving us tapes!

--
Lanfranco Albani - CAD.LAB s.p.a., v. Stalingrado 45, Bologna, Italia

Phone: (work:) ++39 (51) 371099, (home:) ++39 (51) 727372;
Fax: ++39 (51) 366964 (work only), Fidonet: 2:332/401.4 (home only).

 
 
 

using backup with 1/4 inch tape media

Post by Ken Lerm » Sun, 06 Jan 1991 08:05:24



.I found a couple of unexplained things while using backup at the 3002 level
.and 1/4" low density cartridges (rmt0.4 and rmt0.5).
.
.First, I wrote a tape label using dd, forward spaced over it, then did a
.backup to the cartridge.  I got a write error stating that I had defective
.media.
.
.Second, I did several backups to the same tape cartridge (w/o a tape label).
.Everything finished correctly, so I did a tcopy on the tape and found out that
.my 60 MB cartridge had almost 112 MB on it!
.
.Is backup ignoring the hardware and always writing high density?  This would
.explain both problems above.
.
.Tom Merrick
.Ohio State University

My understanding of these devices is that while the high capacity
devices can read both high and low capacity tapes, they can only write
high capacity tapes.  The capacity is determined by the number of
tracks which the drive writes.  Since the high capacity drives have a
narrower head, there is no way for them to write the wide tracks
required to write in low capacity mode.

Ken

 
 
 

1. using rdump with 1/4 inch tape media

 DH> I can easily backup the 75 Meg onto one DC600 cartridge (in QIC 120
 DH> format).  The same 75 Meg requires 4 cartridges when I use rdump to
 DH> backup across the network.

If you backup on a local tape, the tape will most likely be operated in the
"streaming mode", where there is little or no stop-and-go. When operated via
network, the tape data rate might be about as high as the network data rate
(measured with ftp on an idle 4Mbit token ring). So the tape operates in
stop-and-go mode, significantly reducing its capacity. In the manual of the
tape, there is a picture saying the capacity is reduced from 150 MB to 30 MB
when operated in non-streaming mode. I personally have trouble believing this
relation, especially at large block sizes, but its given in IBM's docs...

        Andreas

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