Re: Tape compression fiasco
Hi Paul,
Thanks a lot for the info. BTW, I received a message from
Seagate about tape compression and some technical info on my DDS-2
tape drive, more especifically for CTD8000R-S. I think that is worthy
to post it here.
Luiz
----------Seagate Tape Compression Info by Michael S.-------------
From TapeSupport@seagate.com Mon Jul 23 08:27:16 2001
Date: Sun, 22 Jul 2001 20:38:06 -0500
From: TapeSupport@seagate.com
To: lgemedia@fnal.gov
Subject: Re: |Linux|SCSI||I am trying to backup my
Luiz:
Here is everything I could find on compression. I hope this helps you.
When using compression, if the drive looks for a series of data and
assigns it symbol or "token". Instead of writing the entire series of
data, only the token is written to tape, somewhat like taking shorthand.
When the same series of data is found repeatedly within a file,
substituting a token for each occurrance greatly reduces the amount of
data that must be written to tape. In order to restore the data later, a
legend of what each token represents is also written to tape.
If the drive encounters files that have already been compressed, new
tokens are assigned, causing the token legend to grow in size. At the same
time, files that are already compressed cannot be compressed further.
Since the legend continues to grow in size, it is possible that a
compressed backup could take up more space on tape than if no compression
were used.
To see this happen, run a backup without using compression. You should get
approximately the native capacity of the drive on the tape before it is
full. Then you can run a backup using only software compression. If you
are able to backup any amount above the native capacity of the drive, then
your data is being compressed.
According to Seagate Technology engineers hardware compression
is generally better, however, in some instances software
compression works better. This of course depends on by "better" you mean
faster, or save more space. certain types of files do not
compress because they are already compressed like .jpg, .wav, etc.
We feel that the only way to determine which is best for you
is to try both.
If by "better", you mean more reliable, both compression schemes
are equally reliable.
Hardware compression is at a hardware level. You can not view compression
results in the backup report. To verify hardware compression download and
run TDiag from Seagate's web site.
http://www.seagate.com/support/tape/utils/index.html
Another way to verify hardware compression is to select a backup greater
than the native capacity of the tape.
Click Tools, Media, and Identify. Note the capacity of the tape.
Create a backup Job with files selected greater than the identified
capacity of the tape.
If the backup completes without prompting the user for a second tape then
the drive is hardware compressing.
If you would prefer to use software compression, the backup report will
give you accurate compression results.
If this is, the external DAT, the model no., indicates it is, the
external case will need to be opened.
Turn off the PC:
With Tape Drives External Case Open:
1) On the jumper block, pins 9 and 10 will need to be jumpered.
This site has a picture of the jumper block.
http://www.seagate.com/support/tape/scsiide/gif/sc24_blk.gif
2) On the Dip Switches, Switch 6 needs to be set to ON.
This disables Data Compression. This site explains that.
3) Restart the PC.
While doing a backup the Progress Box will no longer say Hardware
Compression while backing up. When the backup is complete your report
will give you an accurate account of the backup before and after
compression.
Use our DOS diagnostic and do a 7 gb backup.
This program uses its own data to do a compression check.
Perform a read/write test with compression check.
use the -c switch after the sgtape21.exe file and it will generate its own
data, which is very compressible.
Here is the EXACT syntax to type:
a:\sgtape21.exe -c
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
TEST THE TAPE DRIVE IN DOS
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
You can test the drive functionality at a DOS level with our
diagnostic. If the drive cannot operate in DOS it will not
operate in Windows
sgtape21.exe
is our diagnostic and it is attached to this document
In Windows - TO SAVE DIAGNOSTIC FILE TO YOUR HARD DRIVE
1. Double Click the DIAGNOSTIC ICON in your email.
2. Click on DETACH.
3. Keep track of where you SAVE it to.
In Windows - TO UNPACK THE DIAGNOSTIC FILE TO A FLOPPY DISK
1. Insert a BLANK FLOPPY DISK in your FLOPPY DRIVE.
2. Double Click the diagnostic ICON
3. Type A: in the "unzip to folder box"
4. Click UNZIP
5. The ICON will unzip into files on the floppy disk.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++
DIAGNOSTIC MUST BE RUN IN 'TRUE' DOS
The best way to run this DIAGNOSTIC is to boot into DOS
with a START-UP DISK from Windows 98.
THIS DIAGNOSTIC WILL ERASE ANY DATA ON THE TAPE
do not use a tape that has data that you will need to
restore later.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++
To boot into DOS
1. Insert your Start-up Diskette in the Disk Drive.
2. RESTART your Computer.
3. Say YES to CD-rom support ... this loads the ASPI drivers
4. Your computer will end up BOOTED into DOS at the a:\>
5. Insert the floppy disk with the DIAGNOSTIC on it
===================================================================
SCSI TAPE DRIVES require an ASPI driver in DOS ...
The Windows 98 boot disk loads the "most common" ASPI drivers
If your SCSI devices are not detected, please check with your
SCSI controller manufacturer for the correct ASPI drivers.
==================================================================
*******************************************************************
TO RUN THE DIAGNOSTIC
Any data on this tape WILL BE ERASED during the test.
DO NOT use a tape which contains data you may need to retrieve later
*********************************************************************
type SGTAPE21.EXE and press ENTER
This will begin the DIAGNOSTIC
You must perform the Read & Write test.
Follow the prompts on the screen after selecting the Read & Write test.
If the test completes without errors, the SGTAPE utility will report
the drive is functioning correctly.
This points to your Computer of Operating System malfunctioning
If problems are encountered, note the text of the error message
and follow the utility's recommendations before repeating the test.
If drive passes the test at DOS go back to windows and test the drive
with your windows software.
After repeating the test,
THE DRIVE STILL FAILS
you can contact Seagate Customer Service at 1-800-468-3472
for Drive Replacement.
Seagate Customer Service will need your Model and Serial Number from the
back of your tape drive.
(See attached file: sgtape21.exe)
Michael S.
Seagate Technical Support
From: Paul Slootman <paul@murphy.nl>
To: debian-alpha@lists.debian.org
Subject: Re: Tape compression fiasco
Date: Mon, 23 Jul 2001 11:30:04 +0200
On Sun 22 Jul 2001, Luiz Emediato wrote:
>
> Companies say that if you have compressed files they won't be compressed
> again. This is obvious, but what about the tape density.
> As far as I know, compression would mean high density on tape.
Your assumption is wrong.
The density gives you 4GB, the compression doubles that to 8GB
(marketing speak, not my opinion!). To get more on a tape, you need
higher density, not higher compression. Two different things.
> It seems that the limit of 8GB stated by companies is a fiasco !
Well, er, DUH! :-) Same problem with 1GB = 1000"MB" = 1,000,000,000
bytes, not 1024 x 1024 x 1024 = 1,073,741,824 which means that the
GB used by marketing is more than 7% less than what a technical person
would expect.
> Has anyone backup more than 4GB of data on DDS-2 tapes successfully ?
Backup executables, text, data, ... anything that's not already
compressed (i.e. not mp3, jpg, gif, ...)
Paul Slootman
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