160Gb Dat-Tape Not Enough For 158Gb From Hd

[c7043dac-09a0-460e-a81b-c3b70d14f0c8]

hi there,

 

i've made a backup from 158GB of media (video). i collected all video files in a folder that did not exceed 158GB. I used a 160GB HP DAT-Tape (in a HP Storageworks 160GB/320GB USB DAT-drive / with MacPro - OSX 10.5.x - Retrospect 8.x). why did i need two tapes to overspan the complete back up?

 

thanks in advance.

 

ralph

[Lennart_T]

I explained that in your other thread.  

 

If you have the right sort of DAT tape and if you can provide data fast enough in a continous stream, you could get 160GB natively on one tape. Assuming 2:1 compression and fast enough data stream, you could get 320GB per tape.

Now, the catch is: Many types of files today are already compressed: Video, audio and pictures to name three types of files that can add up to large amounts of data. These can not be compressed further.

There's another catch: If data isn't available fast enough, the DAT device inserts gaps on the tape between data blocks. If data is really slow, the DAT stops the tape a while, rewinds a bit and starts again. This, of course, wastes tape capacity. Many small files are much slower to backup than a few large files, even if both types add up to 160GB.

[c7043dac-09a0-460e-a81b-c3b70d14f0c8]

 

If you have the right sort of DAT tape and if you can provide data fast enough in a continous stream, you could get 160GB natively on one tape.

 

lennart,

 

if i understand you well then i've exceed the maximum of the capacity of my configuration. i work with lots of files (small and large files), my device is connected by USB and since i work with video files there is no compression. i've used three 160/320GB-DAT tapes for 308GB

 

knowing this, i'm okay with my new tape device. but looking at the performance (at the details section) i see a high MB/M rate. for me it's hard to believe that USB is too slow for the device. or is it my lack of knowledge and do you consider the given speed performance it as a 'slow rate'( compared with the potential speed of USB)?

 

i could have bought a tape device with a SAS-interface. do you think it would have made a big difference? (i decided to buy a USB because of mobility which is worth the actual situation).

 

 

last question: how does retrospect know where each sole file is placed at the tape. especially when one tape is used several times for different backups.

 

 

best regards,

ralph

 

 

anyway, thanks for helping me out!

 

 

 

 

Edited July 23, 2011 by r@lph

[twickland]

i've used three 160/320GB-DAT tapes for 308GB

Not quite true. You filled the first two tape members and a bit of the third. Click on the "Members" tab in the Media Sets window to see how much data was actually written to each tape. Given that you say your files are basically incompressible, getting 150 GB per tape member isn't too bad.

 

how does retrospect know where each sole file is placed at the tape. especially when one tape is used several times for different backups.

The media set's catalog tells Retrospect where each file is located, regardless of how many backups are made or how many members are in the set.

 

The disadvantage to tape is that, because it's a sequential rather than random access medium, it can take a lot of time to restore data that spans many tape members. It's a good idea to perform a New Media backup (if you want to save the existing data on your old tapes) or a Recycle backup (if you don't want to save the existing data and prefer to reuse your tapes) whenever your current Media Set starts getting to be larger than is convenient for you to handle.