Gene, Thanks for your information.
I tried the gzip way but it took too long on my backup server. With my old LTO2 drive I took 90% of the compressed value and used that as the length. With "device-property "LEOM" "true"" and part_size 40GB, I was getting close to filling my tapes to 100%. If I ever get a faster backup server I will retry the gizp way again. Thanks again, Robert > -----Original Message----- > From: owner-amanda-us...@amanda.org [mailto:owner-amanda-us...@amanda.org] > On Behalf Of gene heskett > Sent: Wednesday, August 24, 2011 10:43 AM > To: amanda-users@amanda.org > Subject: Re: Do you use this library > > > On Wednesday, August 24, 2011 10:17:33 AM McGraw, Robert P did opine: > > > JF, > > > > > > Recently you sent me your tapetype for an lto4 drive. I have a question > > about the length parameter. > > > > An lto-4 tape has a non-compress length of ~800GB and in compress about > > double. I know these are max values. > > > > In you answer below you have length set to ~772GB. If I am going to run > > in compress mode so should the length value be double or ~1.4TB. > > You cannot double compress in normal practice. Feeding a highly > compressed > file to the lightning fast compression methods used in tape drives will > often result in the file growing larger by many percentage points. > > Gzip can generally beat the tape drives at their own game, albeit at the > expense of cpu time to do it right. > > There are also pretty valid arguments against using the drives compression > as it isolates the backup program, preventing it, regardless of the name > of > the program in question, from keeping track of how much of the tape has > been used since the best the programs can do is count the bytes sent down > the cable to the drive. If the drive is massaging the data, shrinking or > expending it, then the program has no real clue when the tape is full till > the drive reports EOT. > > Because of these considerations, it is pretty universal here that the > recommendation is to turn off the drives compressor and let software > methods do the data smunching. Then amanda, or any another other program > that does track the tape capacity, will then know how much of the tape has > been used. > > This can be very difficult to actually do for those tape formats, most of > them, that record the compressor's state in a hidden header file we never > see. DDS is one such format where you will have to write a script that: > > dd reads the first block of the tape out to a file, then > mtx re-winds the tape, then > mtx turns off the drives compressor, then > dd re-writes the first block of the tape with the saved file > > All without ejecting the tape or going through a tape recognition cycle as > most drives do for a freshly inserted tape, and which would turn the > compression back on despite your wishes. Only a similar operation will > actually turn it of for most drives today if it has ever been turned on > and > the tape written to. > > You can of course ignore this advice Robert. > > > Thanks > > > > Robert > [...] > > Cheers, gene > -- > "There are four boxes to be used in defense of liberty: > soap, ballot, jury, and ammo. Please use in that order." > -Ed Howdershelt (Author) > > He's dead, Jim.
