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DAT72 tape fills at 26 GB


geohei

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Hi.

 

If off-topic, please advise which place suits better. Thanks!

 

Usually, DAT72 hold uncompressed 36 GB. Retrospect 7.0 manages to fill my DAT72 tapes (with hardware compression) at 26 GB, hence 10 GB below the uncompressed capacity. Only huge files (i.e. small amount of files) used.

 

Why is that?

 

Thanks.

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In order for DAT tapes to reach their rated capacity, you have to pump the drive very hard with data. Otherwise, the drives will "backhitch" (stop transferring, back up, get a running start, begin transferring again). Some drives, when they see that they are doing excessive backhitching, will increase the space between blocks of data so as to ensure that more time is given to collect data. You don't indicate what hardware you are using (CPU, memory, interface to tape drive, tape drive, etc.), or what the load on the machine is. Rated capacities are usually marketing numbers, anyway.

 

Russ

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Quote:

In order for DAT tapes to reach their rated capacity, you have to pump the drive very hard with data. Otherwise, the drives will "backhitch" (stop transferring, back up, get a running start, begin transferring again). Some drives, when they see that they are doing excessive backhitching, will increase the space between blocks of data so as to ensure that more time is given to collect data. You don't indicate what hardware you are using (CPU, memory, interface to tape drive, tape drive, etc.), or what the load on the machine is. Rated capacities are usually marketing numbers, anyway.

 

Russ

 


I use an Adaptec 19160 inside a Dell Core 2 Duo 2.66 GHz, 2 GB RAM. The computer is for sure not the bottleneck, nor is it the Adaptec. I run the SCSI bus with sync 20, which is way below the max. of the DAT72 (HP C7438A) of #125 MB/min.

 

As far as I understand, the backhitch only occurs when data is not transferred fast enough to the streamer. Do you think this might be possible with my configuration?

 

Thanks,

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... still no answers. The sprecialists should be reading here !?

 

I rephrase my main points:

 

# Can a DAT72 (36 GB native, uncompressed) tape be so bad, that 10 GB are "lost"?

 

I encountered some weird phenomena the past few days. 26.7 GB of data, which used to match without any problem onto a DAT72 tape (but only uncompressed), didn't fit on 5 tested DAT72 tapes I tested after the initial backup. In other words ... I did a backup of some 26.7 GB data, managed to get the stuff onto a DAT72 tape (uncompressed). After that, I did some further tests with the same data (!), ond could not backup this data any more to one single tape. It stopped at #25 GB and Retrospect requested another medium. I tested this with 5 more tapes; without any change.

 

# Any idea how this is possible?

 

Thanks,

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@rhwalker

 

Not really. I clearly pointed out that ...

 

1. my computer is very very fast, so that backhitching is certainly not the source of the problem (your idea).

2. all arguments mentioned in the link Robin Mayoff posted, don't explain the huge loss of 10 GB for DAT72!

 

But ... I found the solution!

 

I had a FRITZ!Card (ISDN fax PCI card) installed in my computer. For some reason (no clue why, never saw this before ...) removal of this card cured my 26 GB capacity problem. There seemed to be a PCI conflict between the Adaptec 19160 card and the FRITZ!Card.

 

Thanks,

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