On Thursday 23 October 2014 12:41:59 Debra S Baddorf did opine And Gene did reply: > On Oct 23, 2014, at 9:34 AM, Gene Heskett <[email protected]> wrote: [huge snip] > > If you are feeding the tape device compressed files, and the drives > > compressor is enabled too, this will quite often cause file > > expansions on the tape itself. The drives compressor, because it is > > intended to handle the compression on the fly, is generally not > > sophisticated enough to do any further compression and will add to > > the datasize, expanding what actually goes down the cable to the > > drives heads. > > > > For that reason, and because it also tends to cripple amanda's > > estimation ability, most of us long term amanda users turned off the > > drives compressors at least a decade ago. > > > > Most drives record that compressor on/off status in a short file > > ahead of the tape content you can see, which makes it difficult to > > turn the compression off, so a procedure like this must be done to > > every tape in the house. > > 1. Rewind the tape. > > 2. Read out to a scratch file, the first 32 kilobytes of the tape, > > this is the amanda header. > > 3. Rewind the tape again. > > 4. Use mt to turn off the compression. > > 5. Immediately write that scratch file back to the tape so that this > > hidden flag on the tape is turned off. > > > > If you eject the tape and then reload it after turning off the > > compression without doing the immediate rewrite, the drive will scan > > that beginning of the tape to establish what kind of a tape its > > dealing with, and that will re-enable the compression flag you just > > turned off. So do not do anything but rewind it, turn off the > > compression, and re-write the amanda identification header, all in > > one swell foop. > > > > Cheers, Gene Heskett > > If you do this relabeling just before you re-use each tape, then you > can also (rather than reading and re-writing the label) > do > amrmtape <config> tape-label > do the above steps 3 and 4 (rewind and turn of compression) > write the same label anew with amlabel -f ….. > > Not that this is conceptually any easier, really. > Deb
When I was forced to do that, I was screwing with a travan drive & tapes at the time, circa 1999 or so. I tried that, but amlabel was moving the drive before the write, probably to see what label it was over-writing and that was restoring the flag somehow. So I effectively went to the head of the line & made the drive an offer it could not refuse. 15 years later its possible it would now work, but someone with a tape drive needs to check it and see if it will now work, as I don't know & haven't used any tape in 6 or 7 years. All vdisk on a hard drive, and currently on the 2nd terrabyte drive in all those years. Considerably faster backups, and very fast restores since the hard drive really is random access. This is such a PITA when dealing with real tapes, that what I would like to see is amlabel reaching into the amanda.conf, and reading the DRIVE_COMPRESSION label, and applying that unconditionally as it labels or re-labels a tape. Set it once and forget it IOW. And at my age, 80, I am not about to go back to the tapes & drives I can afford (DDS-2's), those were crap. Cheers Deb, Gene Heskett -- "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Genes Web page <http://geneslinuxbox.net:6309/gene> US V Castleman, SCOTUS, Mar 2014 is grounds for Impeaching SCOTUS
