On Thu, 17 Jan 2002 at 9:10am, 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:
tapetype writes random data, which compresses horribly. So horribly, in fact, that it expands. Thus your results. > > Then I ran it with software compression (/dev/rmt/0cn) and I only got 20 GB: Err, I don't think that does what you think it does. That's the compressing tape device, which should do the same thing as toggling the switch on the front panel. It's still hardware compression. > Both ways I would of expected close to double the native writes. Any > ideas why the compression would not of increased. Well, double is *always* optimistic. But, as I said above, random data doesn't compress. To use amanda with hardware compression, do the following: 1) Run tapetype on your tapes using *no* hardware compression. 2) Multiply that number by an estimate of how well your data compresses. 3) In your tapetype in amanda.conf, use the bigger number. 4) As amanda runs, you may need to tweak your tape length in amanda.conf. As an example, I've got a Sony AIT1 (35GB native) that I use with hardware compression. Depending on the data, I get between 45-55GB on there. I've set the length in my tapetype to 50000MB. -- Joshua Baker-LePain Department of Biomedical Engineering Duke University
