OK, here is my solution that allows full tape length for my backups.
Notice that I am using amtapetype only as an effective way to illustrate
both my problem and the solution.

This demonstrates using the OS and mt to turn tape drive hardware
compression ON:

* Michael D Schleif <mds AT helices DOT org> [2005:06:17:10:04:59-0500] scribed:
<snip />

>     # sudo /bin/mt -f /dev/nst0 compression 1
> 
>     # sudo /bin/mt -f /dev/nst0 defcompression 1
> 
>     # sudo -u backup amtapetype -c -f /dev/nst0 -o
>     Writing 1024 Mbyte   compresseable data:  384 sec
>     Writing 1024 Mbyte uncompresseable data:  1318 sec
>     WARNING: Tape drive has hardware compression enabled
>     Estimated time to write 2 * 1024 Mbyte: 2636 sec = 0 h 43 min


This demonstrates using the OS and mt to turn tape drive hardware
compression OFF:

    # sudo /bin/mt -f /dev/nst0 compression 0

    # sudo /bin/mt -f /dev/nst0 defcompression -1

* Michael D Schleif <mds AT helices DOT org> [2005:06:16:23:59:28-0500] scribed:
<snip />

>     # time sudo -u backup amtapetype -e 12g -f /dev/nst0 -o
>     Writing 32 Mbyte   compresseable data:  37 sec
>     Writing 32 Mbyte uncompresseable data:  35 sec
>     Estimated time to write 2 * 12288 Mbyte: 26880 sec = 7 h 28 min
>     wrote 298832 32Kb blocks in 76 files in 10632 seconds (short write)
>     wrote 310628 32Kb blocks in 158 files in 10840 seconds (short write)
>     define tapetype unknown-tapetype {
>         comment "just produced by tapetype prog (hardware compression off)"
>         length 9522 mbytes
>         filemark 0 kbytes
>         speed 908 kps
>     }
>
>     real    364m39.451s
>     user    0m2.724s
>     sys     0m28.719s


Next, the Compaq/Sony SDT-9000 a physical jumper labeled `disable dc'.
To this point, this jumper was OPEN.  Now I CLOSED this jumper.

Interestingly, now when I used the OS and mt to set both compression and
defcompression to 1, the following test _never_ returns an initial
estimate:

    # sudo -u backup amtapetype -c -f /dev/nst0 -o


Finally, with the jumper CLOSED, compression 0 and defcompression -1, I
get the following:

    # time sudo -u backup amtapetype -e 12g -f /dev/nst0 -o
    Writing 128 Mbyte   compresseable data:  114 sec
    Writing 128 Mbyte uncompresseable data:  114 sec
    Estimated time to write 2 * 12288 Mbyte: 21888 sec = 6 h 4 min
    wrote 365676 32Kb blocks in 93 files in 11010 seconds (short write)
    wrote 365676 32Kb blocks in 186 files in 10484 seconds (short write)
    define tapetype unknown-tapetype {
        comment "just produced by tapetype prog (hardware compression off)"
        length 11427 mbytes
        filemark 0 kbytes
        speed 1089 kps
    }

    real    373m59.100s
    user    0m2.907s
    sys     0m30.466s


Most importantly, last night's backup used nearly the whole 12 GB of tape!

Thank you, all of you, for your contributions to this challenge.

-- 
Best Regards,

mds
mds resource
877.596.8237
-
Dare to fix things before they break . . .
-
Our capacity for understanding is inversely proportional to how much
we think we know.  The more I know, the more I know I don't know . . .
--

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