Jon LaBadie wrote:
On Thu, Apr 21, 2005 at 10:49:00AM -0400, Vicki Stanfield wrote:Okay, I see that I asked my initial question badly. I know that you very seldom if ever get 2-1 compression. I just meant that I thought that 1.4 to 1 wasn't too much to ask. Perhaps that is incorrect.
If the dump is 28GB, should it not fit on a 20GB tape if it is compressed. Aren't the tapes basically 20/40GB?
Not in my mind. They are 20GB tapes. I.e. they can hold 20GB of ones and zero bits.
What those ones and zeros represent "could be" 40GB of data compressed ton 20GB of zeros and ones. I.e. 50% compression.
But the level of compression varies greatly with the type of data being compressed and the algorithm being used. In fact, the compressor used by your tape drive can actually expand some types of data rather than compressing it.
Using gzip as the software compressor I've tested data and gotten 0, 3, 10, 25, 50, 75, even 90+ percent compression. If the data in your dump compresses 50%, fine, it will fit on a 20GB tape. But if it only compresses 3%, I doubt it. And if it was already compressed by gzip (or has lots of gzipped files), it WILL EXPAND if the tapedrive compressor works on it.
Anyway, I am perfectly willing to stop using hardware compression altogether but haven't figured out how to make the setting permanent yet using GNU-mt 2.4.2.
Vicki