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9536
September 15th, 2004 19:00
Seagate STD2401LW
I have a Seagate STD2401LW tape backup unit that came in a PowerEdge 4600 system. I am currently using NTBackup with Sony 20GB/40GB DDS4 tapes and have selected to use hardware compression. However, the backup job stops at 30GB. Is this just an NTBackup problem? I assume I should be able to get 40GB on this tape with compression is that incorrect?
I am planning on upgrading to better software in the near future, but I would like to do something if its possible in the mean time as we have not had a good backup in quite some time.
Thanks for you any help
matt
I am planning on upgrading to better software in the near future, but I would like to do something if its possible in the mean time as we have not had a good backup in quite some time.
Thanks for you any help
matt
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DELL-Bob D
899 Posts
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September 16th, 2004 20:00
Compression can only be achieved if the file is compressable.
Simple analogy:
If you have a 2 cubic foot piece of steel and a 2 cubic foot piece of sponge that you want to fit into a 1 cubic foot box. If you apply compression to the steel it will maintain the same size (maybe shrink a little depending on how good your compression tool is), on the other hand if you apply compression to the sponge you should be able to make it fit in the box.
Files on a computer follow this same model, some will shrink in size some will not depending on the structure of the file and the ability of the compression algorithm within the tape drive.
So in a perfect 2:1 compression world you would be able to backup 40GB on a 20/40 tape. In the real world all data is not equally compressable. It ranges from 1:1 (non-compressable) to somewhere greater than 1:1.
The fact that you are getting 30GB on a tape that only provides 20GB of native storage (1:1) tells me that your H/W compression is working. You just dont have a data set that is completely 2:1 compressable.
Hope this helps.
There are several good articles on the web that go into detail explaining h/w compression algorithms and how compression actually works. If you can't sleep one night give it a google! ;-)