On Friday 30 September 2005 08:10, uwe.kaufmann AT infoconsult DOT nu wrote:
>Hi Listers,
>
>Just to give my 0.02EUR
>
>> > mt -f /dev/nst0 defcompression off (or 0 depending on your mt)
>> > mt -f /dev/nst0 compression off (or 0 depending on your mt)
>
>I am running SuSE 9.3 and the only available mt operation is
>"datcompression"
>
>See:
>athene:~ # mt --version
>GNU mt version 2.5
>athene:~ # man mt
>[...]
> datcompression
>              (some  SCSI-2  DAT  tapes) Inquire or set the
compression
>status
>              (on/off). If the count is one the compression status is
>printed.
>              If  the  count is zero, compression is disabled.
> Otherwise, com-
>              pression is enabled. The command uses the SCSI ioctl to
> read and
>              write  the Data Compression Characteristics mode page
> (15). ONLY
>              ROOT CAN USE THIS COMMAND.
>
>
>And in fact I do have the problem that I use at one site the Tandberg
> SLR7 without knowing how to switch off hw-compression. The mt
> datcompression {off|0} operation fails. That leads to my
amanda.conf:
>
>define tapetype SLR7 {
>    comment "just produced by tapetype prog (hardware compression
on)"
>    length 16500 mbytes        # Manually decreased again
>    filemark 186 kbytes
>    speed 1300 kps
>}
>
>athene:~ # dmesg | grep SLR
>  Vendor: TANDBERG  Model: SLR7              Rev: 0483
>  Type:   Sequential-Access                  ANSI SCSI revision: 02
>Attached scsi tape st0 at scsi0, channel 0, id 3, lun 0
>st0: try direct i/o: yes (alignment 512 B), max page reachable by HBA
>1048575
>
>
>BTW: Is it true that a tape formerly written with hw-compression will
>"override" the 'drive compression off setting' because the
compression
>on/off status has been written on the header of the tape? If yes, I
> should "reformat" a lot of tapes as described by Gene :-/
>
>
>Best regards
>Uwe

I believe thats pretty much the universal case, Uwe.  Think about
it... Otherwise, how would the drive ascertain if the tapes data
needed to be run thru the hardware decompression to make it valid
data?  The only other 'by the tape' method of defining it would be a
slider button on the tape.  But that could be moved by accident, so
if I were designing such a device, I'd 'simplicate' that option
out of the design before it ever hit the production line.  But thats
just the Old Farts thinking, based on 57 years of steering electrons
for a living.  I'll be 71 next week.

-- 
Cheers, Gene
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
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Copyright 2005 by Maurice Eugene Heskett, all rights reserved.