On Tuesday 16 November 2004 11:54, Michael J. Pawlowsky wrote:
>A long time ago I ran tapetype.
>I left it with the results I received and everything worked fine for
> the past couple of years until today.
>
>The problem is that today I hit the 20Gb limit for data that I want
> to back up.
>tapetype did not take into account  the HW compression of the drive.
>
>I'm suppose to be able to get 40Gb on a tape. They are DLT IV tapes.
>So I need to know what do I have to do, to get amanda to recognize
> that the tape drive uses hardware compression.
>
>Any links to the docs that might explain this would greatly be
> appreciated.
>
>Thanks,
>Mike

Generally speaking Mike, the majority of us recommend that any 
compressors in the drives be turned off.  The reasons? 3 of them.

1. If you have the horsepower in the cpu, gzip can beat the hardware 
compressors rather handily.  I get notes in my dump reports that such 
and such a partition, being mostly sparse data like text, was 
compressed to less than 10% of its original size.  OTOH, if I get a 
report that so and so was still 94% of its original size after gzip 
smunched it as best it could, then compressing that partition full of 
already compressed data, such as rpm and tar.gz's or .bz2's is a 
waste of time and kilowatt hours, turn gzip of by changing to a 
different dumptype that doesn't use it.

2.  With the compressor turned off, run amtapetype again, and put that 
result in your tapetype section of amanda.conf.  It will probably be 
a couple of percent less than the brochure claimed by the time the 
filemarks are compensated for.  In your case, the 20GB is the 
marketing depts claim for the uncompressed tapes capacity, but I've 
had gzip put 12GB worth of real data on a 3.9GB tape several times.
Under similar conditions, you could put maybe 60GB on that 20GB tape 
by using gzip.  My 'average' compression is to about 38% of the 
original size on a typical run.

3.  With compression turned off, amanda, who counts bytes sent down 
the cable to the drive when checking the totals against the tapetype 
entry in your amanda.conf, will very rarely ever actually hit the 
tapes EOT, and will probably never hit it if you set the tapetype 
size down one percent in the tapetype entry.

-- 
Cheers, Gene
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