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Size on Tape?

Posted on 2004-04-02
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Last Modified: 2010-04-03
When go to restore and look at my media, the column that is supposed to tell me the size of the backup set has a question mark?  Why is this?  What I am really wanting to know is how much room I have left on my tape.  Its DLT 4000 using DLT tape IV.  I want to know what kind of compression ratio I am getting with the hardware compression.  I there a utility I can use to look at the tape and find this out?
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Question by:donnatronious
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Expert Comment

by:sirbounty
ID: 10745805
What software are you using for backups?
Typically you would need to inventory and catalog the tape to determine its contents and what size it is...

On some backup software - you can have the jobs you run append to tape and then fail if enough room is not available...
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Author Comment

by:donnatronious
ID: 10745908
Whoops, I'm using NTBackup on win2k server.
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Accepted Solution

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sirbounty earned 200 total points
ID: 10745946
From my limited experience with NTBackup - I don't think you can gather this information properly from the software.
Consider it a trimmed down version of some of the more popular backup solutions provided by Veritas...
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Author Comment

by:donnatronious
ID: 10745960
I was hoping for maybe a small utility to just show me how much room and what type of compression rate I am gettting thanks
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LVL 67

Expert Comment

by:sirbounty
ID: 10746109
If I recall A DLT 4000 is a 20/40 drive, correct?
Also - I'm quite certain DLT IVs are 35/70.
But unfortunately, you won't get that with a 20/40GB drive...
The absolute maximum you will get is 40GB - but that's unlikely.
Depending on the type of files you're backing up, you should average about 1.4-1.6/1 compression ratio - meaning you're likely to get between 28-34GB of data on a single tape.  If you're backing up a lot of files that are already compressed, this ratio will decrease considerably.  

One more thing to add - and again, I don't know your situation, but this works in most environments...

If you're on a daily backup schedule - set the M-Th jobs to be differential only.  Then have the Friday job as a Full (normal).
So tonight's job would run and be a complete backup of everything.  Monday, you insert a new tape, different job definition (differential/append).
Monday only backs up the files that have changed since Friday's job completed.
Tuesday-Thursday - same thing - but still since Friday's job completed (so obviosly Thursday's job will back up more than Monday's).

In the event of a restore, you would need the full backup and then whatever the last day's backup was.
So, if you lose a file on Wednesday - you would need to restore it from Friday, then restore it from Tuesday's differential job...make sense?

Hope that helps..
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LVL 67

Expert Comment

by:sirbounty
ID: 10746111
Well, thanx for the grade.
I don't really feel I helped much here though...


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