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Hardware Compression


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Hi All,

 

I am using Retrospect 6 and using a 5 day backup set. I am backing up to tapes that are 20/40 GB. The size of the volumes in My Computer Container is approx 36GB all up. Now when i run an easy script backup, it works fine, though when the tape gets to about 22GB capacity, it asks for a new tape. I am using Hardware Compression so i thought 40 GB would be my capacity on the tape, though it only seems to be writing about half of that. I have also tried Software Compression from within Retrospect though that did not make a difference. Any Ideas???

 

Todd.

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I can think of two things off the top of my head that could cause this. First, you may be backing up files that are already compressed (MP3s, videos, zip files, etc.). Your tape drive can't compress them any further, so you fill your tapes sooner than you might expect.

 

The second possibility is that Retrospect is reading the data very slowly, or your tape drive can write data faster than the data source can supply it, causing a lot of underruns. (Underruns happen when a tape drive can't get enough data to fill an entire tape block; the tape drive has to write empty "filler" data to finish off a tape block and then wait for enough data to come in to fill the next data block.) This can happen especially over a slow network link, or if your computer is trying to do something else (like a virus scan) at the same time a backup is happening.

 

In my experience I get maybe 25% compression on data from typical Mac and Windows desktops.

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