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Storage Capacity

OK, a real easy one here.  We run a HP DLT40 DAT drive and use HPDLT IV 40-80GB data cartridges.  My question is that why does the tape only store appx. 28-30 GB and then ask to be replaced, like it is full?  I was told that the storage capacity depends on the actual drive but how can this be?
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godleman
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godleman
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pratiganCommented:
Thsi could be based upon your compression ratio.  Refer to this link which outlines the capacity tables for those drives aswell as the tapes.  The root link is also a very good source of information for you.
:)
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SunBowCommented:
> does the tape only store appx. 28-30 GB and then ask to be replaced

This is up to the choices you make with the backup program, not the drive or the media.  The capacity is three different numbers, which get progressively lower. One is the old "unformatted". The ideas involve how the bits for overhead are counted or not. To advertise large number, the vendor counts every bit, including overhead for defining tracks etc.

Think of HD or FD with boot sector. It is not a file. But it uses bits. So capacity of total bits for drive is different, larger than capacity for files in bits remaning available.

The tape also has (lower) # for formatted capacity. This is what is left after the tape is formatted, divided into different areas as best the drive can do. Then tape can be formatted again for the SW for backup, which deals with filing systems. Its overhead also is not part of your realized capacity for your own files.

The indexing system, for finding locations on the tape, as you've noticed, has a rather substantial overhead.
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Handy HolderSaggar makers bottom knockerCommented:
pratigan is right, that drive has a native capacity of 20GB.

You can see HP use the estimated double capacity of the drive in the name of the device from http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=lpg50338 and also same for AIT capacity (which makes their part book look silly since Compaq who they bought quote the native capacity in their names in the same book).

>I was told that the storage capacity depends on the actual drive but how can this be?

Because of the design of the head. The tapes are the same length and width but the smaller the magnetic footprint of the head the closer the tracks of data can be to each and the more data you can get on each track. The media is secondary, it has to be better quality to reduce the blurring of magnetic propagation and impurities after it has been written to. You are wasting money buying DLT4 tapes.
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godlemanAuthor Commented:
Sorry for taking so long to get back to you chaps.  So if I understand you correctly the particular drive we have only has a native capacity of 20GB.  This is because of the design of the head, etc.  So the tapes are labelled as 40-80GB as they will store up to that amount depending on the capacity of the drive that is used.  I guess what andyalder is saying is that there are cheaper tapes around that will store up to our drives capacity, and we dont really need up to 80GB tapes as there's no way the drive will put that much data on to them.  Just let me know if I have understood all this correctly.  

Not dure who gets the points here as pratigan and andyalder were correct.  Can I share them out?
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Handy HolderSaggar makers bottom knockerCommented:
It's hardly worth bothering CS for a points split with only 40 points, award it to pratigan.
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godlemanAuthor Commented:
OK thanks Pratigan.
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pratiganCommented:
Thank you and Thank you Andy.... Have A Great Day !!
<:-)
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