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Re: tapetype question
2006-08-02 05:11:59
On Tue, 1 Aug 2006 at 1:53pm, McGraw, Robert P. wrote
How is length calculated? I would think that the length should be closer to
400G if hardware compression is on.
Ah, the hardware manufacturers have brainwashed you well! ;) They'll be
so pleased.
In addition to the other comments posted, keep in mind that LTO is a
different animal when it comes to hardware compression than most other
hard drives. The hardware compressor is "smart", meaning that it won't
try to compress incompressible data. Most tape drives running in hardware
compression mode dumbly throw everything through the compressor. If the
data is already gzipped, e.g., or is otherwise incompressible (like
amtapetype's random data) this actually *expands* the data and you
end up with less data on tape than the native capacity. LTO doesn't do
that. That's why, despite amtapetype's warning that you were using
hardware compression, the tape size still came out right (200GB).
With my LTO3 drives, I leave them in hardware compressed mode, but still
use software compression on some DLEs. My users are pretty good (not very
good, mind you) at compressing their data on disk to save space, so the
tapelength I give amanda is only slightly bigger than the 400GB native.
Some nights I'll get 110% tape usage, though.
--
Joshua Baker-LePain
Department of Biomedical Engineering
Duke University
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