On Thursday 12 June 2003 09:41, Bill Nolf wrote:
>First time - setup.
>
>I have a HP-DAT 40 that should get 40GB with hardware compression.
>
>I ran amtape type and got the following.
>
>length 16534 mbytes
>filemark 0 bytes
>speed 2570 kbytes
>
>Does this look right? The first run didn't come close to 40gb or
> even 20gb.
>
>thanks, Bill
With hardware compression on, the output of /dev/urandom overwhelms
the compressor, and the data will expand such that what you see above
as 16534 megabytes will be about 20,000 megabytes by the time its
actually *on the media*.
This is the main reason most of us are so adament about turning off
the hardware compression and using the software compression where its
appropriate. gzip can beat the hardware quite handily when run at
its 'best' setting against those disklist entries that contain
compressible data. OTOH, a directory full of tar.gz's and rpm's is
not compressible, so use a non-compressing dumptype for those and
save lots of time. Its another good reason to break your disklist
entries up into subdirs such as /usr/local, /usr/share, etc etc.
Each of those directories will contain a different style of data.
Compress them all at start time and look at the email, if a level 0 of
that entry didn't compress more than 10%, or maybe even expands
(compression reported as 107% for instance), don't waste the time
trying.
With hardware compression on, amanda has no idea how much tape is left
because of this internal processing the drive does, so you have to
make your tapetype smaller than 40Gb to compensate, by about the
ratio you saw above. With it off, and the tapetype set to the tapes
uncompressed capacity, then amanda can keep track of how many bytes
has been written to the tape quite well, and make use of all the tape
if she can and needs to.
Following the above outline, I have occasionally seen amanda put more
than 10 gigabytes of "from the disk" data on a single DDS2 tape,
nominally 4 Gb. I would expect similar ratios from a 20Gb DDS4 tape
under the same conditions which could theoreticly hit 50Gb of raw
data on that 20Gb tape. Under admittedly ideal conditions... YMMV of
course :-)
--
Cheers, Gene
AMD K6-III@500mhz 320M
Athlon1600XP@1400mhz 512M
99.26% setiathome rank, not too shabby for a WV hillbilly
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Copyright 2003 by Maurice Eugene Heskett, all rights reserved.
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