On Tue March 11 2003 12:05, Eric Sproul wrote:
>On Tue, 2003-03-11 at 10:37, Jon LaBadie wrote:
>> My point being, not that your drive is like mine, just that
>> nothing you have written/discovered rules out the existance of
>> switch settings.
>>
>> OTOH, you know it comes up in HW compression mode and that you
>> can change it under SW control and that is suitable for you. 
>> Maybe further exploration is non-productive.
>
>John,
>
>That's an interesting point about the drive's built-in switches
> that may not be visible in its eventual casing.  My drives came
> already mounted in a library, so they do have a shell that makes
> them hot-swappable, which could be removed to get access to the
> drive itself.  But as you said, I've already got a workable SW
> solution to override the default setting every time the st module
> is unloaded/loaded, so it's moot. ;)
>
>Thanks for the info.
>Eric

Two other points come up here, Eric.

1. When a tape is inserted in most modern drives, the "tape 
recognition cycle" will discover that this tape has been compressed 
previously, and will turn the compression back on regardless of 
your wishes.

So, at risk of boreing the list yet again:

To get around this, one would assume that amcheck has already been 
run, and that the correct tape for tonights session is indeed 
loaded into the drive, then you need to wrap amdump up in a script 
that goes something like this:

a: rewind tape
b: dd tape label to scratch file, don't forget the 'bs=32k'
c: rewind tape
d: turn compression off by whatever method
e: dd 10 megs or more worth of /dev/zero to the drive, causing it to
        flush its buffers.  This will cause that compression flag to be 
        reset permanently on this tape.  Using bs=32k of course
f: rewind tape
g: dd the scratch file back to the tape to restore the proper label.
h: amdump

I'll leave the details of a real shell script up to you, but you 
should have the idea's format to work from now.  You would need to 
do this once per tape that has been previously written with the 
compression enabled.  Once thats done it can be skipped, at least 
until you add more tapes, which may themselves need to be 
'adjusted' if they come from the maker with that flag on in the 
hidden headers normal mortals cannot otherwise see.

2: Jon's name isn't John. :-)

-- 
Cheers, Gene
AMD K6-III@500mhz 320M
Athlon1600XP@1400mhz  512M
99.24% setiathome rank, not too shabby for a WV hillbilly