On Thursday 17 January 2002 09:10 am, Don Potter wrote:
>I ran the tapetype test to our tapedrive (ADIC DS9400D) using
> DLTTAPE IV. I frontpaneled the compression so I expected at
> least 40 GB when the tapetype was completed. But I only got
> about 17GB:
>
>Command: tapetype -d /dev/rmt/0n
>
>define tapetype unknown-tapetype {
> comment "just produced by tapetype program"
> length 17587 mbytes
> filemark 13 kbytes
> speed 1011 kps
>}
>
>Then I ran it with software compression (/dev/rmt/0cn) and I
> only got 20 GB:
>
>Command: tapetype -d /dev/rmt/0cn
>
>define tapetype unknown-tapetype {
> comment "just produced by tapetype program"
> length 19565 mbytes
> filemark 4 kbytes
> speed 1101 kps
>}
>
>Both ways I would of expected close to double the native writes.
> Any ideas why the compression would not of increased.
>
>Any suggestions would be appreciated.
>
>Don Potter
First Don, be aware that tapetype uses /dev/urandom as a data
source, and /dev/urandom prides itself on being as truely random
as it can be. It takes repeatable, predictable data to be able
to compress it by any great amount.. The output of urandom wil
typically drive a hardware compressor to make a file bigger, not
smaller.
What it boils down to is that the values you get from tapetype
will be truely the absolute worst case values. Typical hardware
compression will gain 2/1 on text and such sparse files, while a
really good software algorythm can easily double that again.
However, the hardware compression can be easily defeated by
preceeding it with a good software compressor so that the copy on
the tape might be 10 or more percent larger on tape than the
actual compressed file is.
If you have the cpu horspower, always use software only, with the
hardware compression in the drive disabled.
--
Cheers, gene
