> Greetings.  I have a Sony SDX-500C AIT-2 tape drive (50 GB native).  I
> have been using the following entry for my tapetype, obtained from
> various places in the mailing list (I have no idea how this was
> originally generated):
> 
> # old - mailing list
> define tapetype AIT-2 {
>    comment "SDX-500C"
>    length 54538 mbytes
>    filemark 1541 kbytes
>    speed 2920 kps
> }
> 
> Its worked fine, but I haven't been pushing the capacity of the tapes
> (only writing about 40GB or so).  That's changing, however, and I'm
> concerned that the length might be too long, so I finally got around to
> (a) turning off hardware compression on the drive (DIP switch number 7
> is OFF), and (b) running the tapetype program on the drive.  Here's
> what it came up with (I edited the comment):
> 
> # new - obtained from the tapetype program
> define tapetype AIT-2 {
>    comment "SDX-500C - produced by the tapetype program - hw compression off"
>    length 48898 mbytes
>    filemark 2788 kbytes
>    speed 6074 kps
> }
> 
> My comments/questions:
> 
> 3) Neither 54538 mbytes nor 48898 mbytes corresponds to 50 GB, no
>    matter how I do the math (i.e. no product of 50, 1000s and 1024s
>    generates either of these numbers, and 54538/50 = 1090.76 MB/GB,
>    48898/50 = 977.96 MB/GB).  I'm (obviously..?) more inclined to
>    believe the smaller number, given the ways of marketing, but does
>    anyone have a quick explanation of how tapetype calculates this
>    number..?

Jon LaBadie has already mentioned how tapetype calculates its number.

As to the discrepancy between tapetype's number (48898) and the
advertised size of the tape, I do not think that there is much of a
discrepancy.  Actually, tapetype is coming up with a slightly larger
number than advertised.  Don't forget that amanda does everything in
power-of-two kb (1024) & mb (1048576) while disk and tape
manufacturers advertise in power-of-ten numbers.

48898 * 1048576 = 51273269248, which is 1273269248 bytes larger than
the advertised 50000000000 bytes.  Don't worry, be happy!

andrew