using hardware compression on tape device

[Voja]

Hello all.
I'm having troubles making big tar backup (68GB) on 40/80GB tape, on Red Hat 9.
From what I noticed so far, no matter is it HP, IBM or any other tape manufacturer, I have about 20% less of noted capacity.
So, on 35/70 I can put only about 28GB.
Same with 40/80 GB
"Hardware compression" is lit when I do "mt -f /dev/st1 compression 1", but no use (I use "tar -czvf" for backup).
When I put that tape after unsuccessfull backing up files, the drive says that it is compressed 40GB tape, but it holds only as much data as it could be written on a 40GB uncompressed DLT cartridge.
Where and what am I doing wrong?

 

[mrregan]

I think that using the z option in tar is already
compressing the data. The tape drive cannot optimize the
data any further. So you are probably getting 40 gb or more of data into your 28 gb tape. Try using the b option to see if you can get bigger blocks of data (fewer interblock gaps)

 

[Voja]

Yes, you are right - z option is compressing the data, but on software level. "mt -f /dev/stX compression 1" should set the hardware compression with ratio 1:2, so no matter what "-z" does, it should be twice smaller than regular backup.
To put it simple:
I have 68 gb of movies, installations etc. Most of it is binary data and "-z" has very small effect on it (but lets do everything possible).
I was hoping to put those 68 GB of data on 40/80GB tape by turning hardware compression on and by getting 80GB instead of 40 (or, lets say around 68-69 gb instead of 34-35, because those tapes I "tried" can't hold more than 80% of their nominated capacity, no matter is it HP, IBM, Sony or any other manufacturer).

 

[mrregan]

I am not a hardware compression expert, but I believe that
hardware compression is virtually identical to software compression. The reason you use hardware compression is to eliminate system overhead required to compress a file. By letting the hardware compress a file you "speed up" the process of tape backup. Since the tape drive does not see patterns in the data which would allow it to compress the data any further, you are not getting additional compensation. You can test this by eliminating the z option and see how many blocks are copied to the tape drive. If the number of blocks is virtually identical, then the hardware compression algorithms are pretty much the same as the software. Did you try the 'b' option for blocking? (tar -czbf)

 

[zeland]

DAT x/y tape are actually only x-GB in capacity but can get y-GB if your data can be compressed to a 2:1 ratio. Your problem lies in the data you are trying to store. Movies (mpg, avi, wma, etc) are already compressed. Compressing a compressed file may actually make it bigger. Regardless of either software or hardware compression, movie files will never compress to a 2:1 ratio, thus you can only store up to 40GB of movies in your DAT 40/80 tape.


--== Anything can go wrong. It's just a matter of how far wrong it will go till people think its right. ==--

 

[Voja]

So, you want to say that "/80" is a "lie"? Yes, I understand that for example txt file can be compressed to 20% of its original size etc etc, bit that's what "-z" does... Well, thank you both, I was hoping that I have to change some parameter about densities to get 2:1 compression... :-( In my dreams...