Why do I get only 4.5 GB out of a 5 GB Exabyte 8 mm tape?

[philx509]

I use Retrospect 6.0 for Windows Professional. My question concerns the amount of "overhead" that Retrospect uses on a backup job.

 

Ive been backing up to tape a LOT of zip files, mp3s, etc., and other files that can't be compressed further. My drive is an Exabyte 8720 (internal version) on SCSI-160. No problems there.

 

Doing all these backups, I consistently get only 4.4 - 4.6 GB of files on one 112m tape. Never 4.9 or 5.0. If I use a 160 m tape, I get about 6.5 - 6.7 GB. This is a local backup, streaming data at 50+ MB/second, so I don't think I'm getting buffer underruns.

 

What is happening here?

[Mayoff]

Please see the media capacity FAQ

 

http://www.dantz.com/index.php3?SCREEN=common_questions_detail&ACTION=COMMON_QUESTIONS_DETAIL&id=49

[philx509]

Quote:

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Mayoff said:

Please see the media capacity FAQ

 

http://www.dantz.com/index.php3?SCREEN=common_questions_detail&ACTION=COMMON_QUESTIONS_DETAIL&id=49

 

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Thanks. I've gone through the FAQ, but that doesn't tell me why Retrospect identifies at 112 m Exa tape as 4.3 GB, when the rated capacity is 5.0 GB. Is this based on my historical usage, or does this include a "fudge factor" to account for lost capacity?

 

Also, is there a lot of overhead for each file stored on tape, in effect a file "header" or "trailer."

 

philx509

[Mayoff]

Actually the FAQ does say:

 

http://www.dantz.com/index.php3?SCREEN=kbase&ACTION=KBASE&id=27409

 

http://www.dantz.com/index.php3?SCREEN=kbase&ACTION=KBASE&id=27410

 

http://www.dantz.com/index.php3?SCREEN=kbase&ACTION=KBASE&id=27411

 

http://www.dantz.com/index.php3?SCREEN=kbase&ACTION=KBASE&id=27418

 

http://www.dantz.com/index.php3?SCREEN=kbase&ACTION=KBASE&id=27365