| T.R | Title | User | Personal
Name | Date | Lines |
|---|
| 6699.1 | Average 2:1 | netbk2.shr.dec.com::Peters | | Mon May 19 1997 09:14 | 10 |
|
The capasity of "compressed" backup data is the
average capasity. The base capasity ( 4 GB ) is still
guaranteed. The average compression is 2:1 ( 8 GB ). It
is possible to get 5:1 or even 10:1 compression on some
data.
Steve Peters
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| 6699.2 | | LEFTY::CWILLIAMS | CD or not CD, that's the question | Mon May 19 1997 10:03 | 5 |
| It's also possible to get almost no compression, or indeed data
expansion, with the wrong data set.
Chris
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| 6699.3 | | DECWET::ONO | Software doesn't break-it comes broken | Mon May 19 1997 11:06 | 7 |
| If you're backing up data that doesn't compress (random data,
JPEG images, etc.) the drive may run a little faster if you
disable compression instead of having the drive waste time trying
to compress.
Wes
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| 6699.4 | Thanks...its data compression ? | HGOV08::NANDAN | | Tue May 20 1997 06:01 | 20 |
|
Steve, Chris & Wes,
Thank you for your responses.
You have made it clear to me that be it the DAT or DLT, the data is
compressed (to whatever degree) before it is written to the tape.
I was given to understand that on the DLT, the compression is done
"physically" on tape, i.e the data is written "closer" together to
achieve compression. Which would mean that regardless of the type of
data (graphics, or otherwise), the compressed capacity would be
possible.
I would like to confirm that be it DAT or DLT, the compression is done
before the data is actually written to the tape.
Thanks in advance.
Nandan
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| 6699.5 | | LEFTY::CWILLIAMS | CD or not CD, that's the question | Tue May 20 1997 08:58 | 11 |
| The density of the data on the tape does not change - the data is
compressed before it is written, by HW in the tape drive.
If the data has been previously compressed (a ZIP file, compressed
image data such as a JPEG file, etc), then there will be little or no
additional compression, and may even be some expansion of the data.
This applies regardless of the tape drive.
Chris
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| 6699.6 | Which dirve? Which data? | DECWET::RWALKER | Roger Walker - Media Changers | Tue May 20 1997 10:45 | 26 |
| All hardware compression is done between the SCSI interface
and the tape head. Some older, less effective compresion
methods such as used on the IBM 3490 (DEC TKZ62) drives will
never expand the data but do not compress as well. This is
called IDRC and was developed when microprocessors were not so
common in hardware.
DLT drives are very sensitive to uncompressable data and may
loose 15% of the speed and capactiy. They use softare based
compression on the 68xxx processor.
Some newer drives use a new chip from IBM, this includes the
IBM 3590 (DEC TKZ90) and Sony AIT (DEC ?). This chip seem to be
able to adjust a little better.
Any statement of compression is assuming a "typical" set of
input data. IBM changed thier claim from 3/1 for the
3490 to 2.6/1 for the 3590 and also claim a much better
compression system!
Without knowing the input I usually assume 1.6 to 1 for safety.
DDS-2 specifies a common compression format the must be readable on
all DDS-2 drives. This is the only hardware compression that I know
of that can be shared by multiple vendors. I haven't tested these
to see how they react to uncompressable data.
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