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Hardware compression - to dip switch or not


benher

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My AIT-2 drive has two compression related dip switch options.

 

The manual claims if I turn switch 7 ON, then data compression is "enabled"

Another switch (8) is available to "disable host control of compression".

 

So...how do I interpret this...and how does it affect retrospect 7.5?

 

a. sw7 "enabled" means that all data written to drive will be compressed.

- or -

b. sw7 "enabled" means that software on the computer can control compression, but that the default for the drive is to NOT compress (unless specifically told to by software).

 

If I select [x] Software compression - in a retrospect backup, will it claims that if hardware compression is available it will use that instead...so to cause retrospect to use compression on the tape I should check this box?

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My understanding of hardware compression was...

 

Backup software sends blocks of data to tape drive. Before the blocks it sends a command "write blocks". Part of this command is a flag to either compress or not compress the following blocks.

 

If compression is disabled on the device, then any compression flag is ignored.

If compression is forced (like with swith #8 above), then again compression flag is ignored but always compressed.

 

So the only question remains...does the Retrospect software turn on or off this compression command flag. And if so..under what conditions.

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One reason you might want to disable hardware compression is if you want to be sure you'll be able to read the tapes on another manufacturer's brand of tape drive. I assume hardware compression is not standard across brands, so if in the future you decide to switch brands, you won't be able to read your old hardware-compressed tapes in the new drive. You'll have to keep the old tape drive around for that. To get around this, you can disable hardware compression in the drive and let Retrospect do software compression. It won't be as fast, but you'll have the security of knowing your tapes will be readable by Retrospect no matter what manufacturer's drive you're using.

 

But I believe the original question is if he wants to do hardware compression, should he check the "software compression" box or not. The answer is yes, because that tells Retrospect to let the tape drive do hardware compression if it supports it, which in his case it does if he turns on switch 7.

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Quote:

One reason you might want to disable hardware compression is if you want to be sure you'll be able to read the tapes on another manufacturer's brand of tape drive. I assume hardware compression is not standard across brands, so if in the future you decide to switch brands, you won't be able to read your old hardware-compressed tapes in the new drive. You'll have to keep the old tape drive around for that. To get around this, you can disable hardware compression in the drive and let Retrospect do software compression. It won't be as fast, but you'll have the security of knowing your tapes will be readable by Retrospect no matter what manufacturer's drive you're using.

 

But I believe the original question is if he wants to do hardware compression, should he check the "software compression" box or not. The answer is yes, because that tells Retrospect to let the tape drive do hardware compression if it supports it, which in his case it does if he turns on switch 7.

 


No, no, no.

The drive is the same, regardless of the enclosure. If you have, say, an AIT-3 drive or a DLT drive, another AIT-3 drive or DLT drive will be able to read the tapes, regardless of enclosure brand.

 

As for software compression, that should be "off". Software compression is just that: Compression in software (Retrospect), not hardware. It has NOTHING to do with the hardware compression in the drive. Software compression eat a LOT of CPU power and should be used only for devices without hardware compression, such as DVD burners.

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