> > We've been using LTO3 tapes with bacula for a few years now.  Recently I've
> > noticed how variable our tape capacity it, ranging from 200-800 Gb.
> >   Is that strictly governed by the compressibility of the actual data being
> > backed up?

Hello,
  the lower bound 200 GB on 400 GB LTO-3 tapes is not possible due
to the drive compression, because it always compares, if compressed
data are shorter that original. In other case, it writes data uncompressed.
So, in all cases, you should see atleast 400 000 000 000 bytes written
on all tapes.

> > Or is there some chance that bacula isn't squeezing as much
> > onto my tapes as I would expect? 200Gb is not very much!

In bacula, look mainly for the reasons, why there is just 200 GB written.
If the tape is full, think about these:

- Weared tapes. Typical tape service life is written as 200 full cycles.
  However, read
  http://www.xma4govt.co.uk/Libraries/Manufacturer/ultriumwhitepaper_EEE.sflb
  where they experienced problems with some tapes just only after
  30 cycles! How many cycles could you have with your tapes?

- Do you use disk staging, so that tape writes are done at full speed?
  Do you have a good disk staging? Considering using SSDs for staging
  is very wise. If data rate is lower that 1/3 to 1/2 of native tape
  speed (based on drive vendor, HP or IBM), then drive has to perform
  tape repositions, which means another important excessive drive and
  tape wearing.

  My experiences are, that even HW RAID-0 with four 10k disks could not
  be sufficient and when there are data writes and reads in parallel,
  it could not put 80 MB/s to the drive, typically just 50-70 MB/s,
  which is still acceptable for LTO-3, but not good.

  Currently, I have 4 x 450 GB SSDs HW RAID-0 with over 1500 GB/s without
  problem running writes and reads in parallel and just after that I hope
  that it is really sufficient for >= LTO-3 staging and putting drives and
  tapes wearing to minimum.

- Dirty heads. You can enforce cleaning cycle, but then return to the
  two points above and other suggestiong, like using some monitoring
  like ltt on Linux (or I have some home made reporting tool using
  camcontrol on FreeBSD), where it would be possible to ensure, that
  your problem are weared tapes, or something else.

Best regards.

-- 
Rudolf Cejka <cejkar at fit.vutbr.cz> http://www.fit.vutbr.cz/~cejkar
Brno University of Technology, Faculty of Information Technology
Bozetechova 2, 612 66  Brno, Czech Republic

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