On Wednesday 25 November 2015 17:35:59 Jon LaBadie wrote: > On Wed, Nov 25, 2015 at 01:22:24AM -0500, Gene Heskett wrote: > > On Tuesday 24 November 2015 21:41:44 Jon LaBadie wrote: > > ... > > > > You may wish to check your data. Create a huge file on that 2TB > > > holding disk. Make it 10-20% of the tape capacity, say 700GB-1TB. > > > Create it by "cat'ing" representative data together. Then tape > > > the file using a dd in a loop something like this (untested). > > > > > > SECONDS=0 > > > while dd if=<above_file> of=<your_tape_device> bs=<1M or 2M> > > > do > > > echo $SECONDS > > > done > > > echo $SECONDS > > > > > > The loop will terminate when dd reaches the end of the tape. > > > You can then count up the number of dd's run (plus a partial) > > > and figure out how much data was written and how long it took. > > > > > > Set your tape drive to use 1 or 2MB blocks (match above) and > > > actually, make several runs under these conditions: > > > > > > 1. hw compress off -- gives total capacity of tape and time to > > > tape 2. hw compress on -- should be more dd's run to see effect > > > of hw comp > > > > > > Use gzip to compress the file > > > > > > 3. hw compress off -- should give same tape capacity as 1, and you > > > can calculate if hw or sw compress is more effective 4. hw > > > compress on -- can hw compress shrink it more? Is the combo > > > better than the best of 2. or 3. > > > > This scenario is much easier said than done, Jon. The ONLY reliable > > way to disable hw compression once it has been enabled, is to read > > out the 32k tape label 1st block of the tape, rewind the tape, turn > > hw compression off, and without doing anything that will move the > > tape, re-write that label block back to the tape. Only that > > sequence seems to reset the hw compression flag in the tape header. > > > > Since I've been using v-tapes for around a decade now, they don't > > have hw compress ability. The recovery, should it be needed, is 10 > > to 100x faster from v-tapes as the hard drive is random access, > > whereas the tape is not. I am not setup to do offsite, but so far, > > knock on wood, smartctl has given me more than enough warning of an > > impending drive failure. > > With current tape formats and Linux kernel drivers, is anyone still > seeing the behavior to which Gene refers? Namely, that once a tape > has been used with hardware compression, it was very difficult for > amanda to use it in compression off mode. > > The problem stemed from two factors. Old drives (particularly DAT) > were in compress mode or not for both reading and writing. If the > drive was in compression off, and then read a compressed tape, the > mode was switched back to on, even for writing. > > The second factor was amanda's protective behavior. It never writes > to any tape until it reads it to check the tape is valid to use. > > For formats like AIT and LTO, I think the compression mode only > affects writes. For those formats, if compression does not shrink a > tape block, the block is written uncompressed. Thus when reading a > tape, each block has to be checked for compression status and that > status does not affect writing. > > Of course, not having those drives of those formats, I could be > totally wrong. > > jl
The above behavior would be far preferable to my experience with DAT's. They may have been good enough for the audio people, but lord help you if you wanted to use them as a long term backup medium. Tape life was maybe 20 rewrites, and by then the head drums were toast. I have all the tools needed to replace and recalibrate such a drum, but they are not for sale. To anyone, factory service only at outrageous prices. 4 years running, my drive spent the holidays from mid-November to mid-January in Oklahoma City, at nearly a thousand per trip. I finally got tired of voting with my wallet & bought a big hard drive. A long decade, 3, hundred dollar drives later its still the smartest thing I have ever done for a backup medium. Others have a different vision of backup. Financial records aren't a long term worry for me since my business name only exists to satisfy some sellers, quick, accurate recovery is more important. Cheers, 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>
