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What's the deal with tape compression?

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9iron

Technical User
Sep 27, 2007
47
US
The backup tape drive for my server is an HP Storageworks DAT 72, which of course uses those lil' 4mm tapes. All of the DAT 72 tapes indicate 36GB/72GB (compressed), yet I cannot seem to squeeze more than about 32GB of data onto the tape.

I checked the tape drive's device config, and compression is indeed set to "ON". I checked the properties of the folder I'm trying to backup on my server, and it tells me it's just a little over 32GB. (I dislike the way Windows reports this info. It gives you "size" and "size on disk". Annoying.)

This has something to do with the data on my server being uncompressed, right? Maybe?
 
If the amount of data written to tape is over 32gb then it is compressing, it is a very poor compression ratio but it is compressing.

the amount of compression is directly related to the type of data being backed up.

Compressed data does not compress. That means all zip, jpg, mp3, and any other such file will not compress, and in fact may even end up a little bit large.

Binary data compresses very little, such exe, dll, disc images.
 
I've got the same issue, am thinking of binning the USB Dat drives and going back to SCSI. The speel that HP give about the USB drive maximum being 50mb/sec transfer rate and near the 2:1 compression mark, is a load of rubbish. Although i can squeeze 50gb onto a tape, with Hardware compression enabled. 38gb with software.

Get Fat Up!
 
interface has nothing to do with compression ratio, that is up to the type of data being backed up. But USB will typically be slower than SCSI.
 
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