Thank You all for all your input...you have enlightened me immensely
.....became so used to using commercial applications that due the
thinking for you.
Don
Gene Heskett wrote:
>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.
>
