> A DDS-3 tape should hold 12GB, compressed, right? It looks like "taper"
> dies after 10GB. Does the TAPE-ERROR/short write mean the tape is bad? If
> the tape was more full, I wouldn't be suspicious; but it seems like I
> should be getting more space out of 125m.
DDS-3 holds 12 GB uncompressed. Period.
Don't count on what marketing people say. blah, 24 GB, blah... They have
no clue of technical details.
It depends on the compressibility of your data how much will fit with
_either_ hardware or software (gzip/bzip2) compression onto a tape.
_Do not_ use both hard and software compression. _It will_ enlarge your
data.
You get something around 20 GB of the usual data mix onto DDS-3 using
either hardware or software compression.
Lots of source code, executables, news or mails extend compressibility,
lots of mp3's or gzipped files reduce it. It's the amount of entropy in
a file being important for reducing it's size.
Do you really expect some algorithm being able to reduce an mp3 in size
with no loss of information which in turn already has only 1/10th of the
original PCM signal _with_ loss of information? - May this loss be
detectable with your ears or not. How do you want to compress _random_
data? What similarities do you expect in random data to be able to write
them in a short term?
I assume you're using hardware compression with gzipped images.
Something between 9.5 to 10 GB is here the usual limit on DDS-3.
Using software compression makes things clearer for Amanda. She knows
how much an image compresses in full and incrementals. This information
will be used in planner stage deciding what to backup in what level. The
usable tape size value can be very accurate.
Hardware compression saves your CPU times, but Amanda doesn't have a
clue about the compressibility of each image. So you use an average
value for tape size hoping for the best.
> How accurate is the size specification of the tape?
When I pushed Amanda to the limit by adding more and more hosts or
filesystems using software compression, I noticed tapetype's measure
being very accurate. Sometimes it didn't fit onto one tape (DDS-2, 4 GB)
because it failed to write 10-50 MB. This is a real good guess! You'll
have to pay lot's of money to get a measuring instrument working below 1
% error.
