Hello,
We are begining to implement NetApp VTL now.
Regarding Tape sizing, if you put Pshical Lib behind VTL then you can use 
Netapp Feature called Tape Smart Sizing, which allows Phsical Tapes to be 
correctly utilized.
I prefer inline VTL and Phisical for couple of reasons.
1. Ability to use Phsical Lib what ever way you want when you want to.
2. Utilizing Networker Cloning ( need scripts and processes in place)
3. Networker is aware of everything being done via cloning etc.

There are many best practices for VTL's , i.e create distintion between 
phisical devices\Tapes\pools etc vs VTL's. Reducing the Tape size could 
also help others-Test a lot before setteling into final solution.
I belive SUN and other also have good products in VTL area, if you have 
scripting help then inline may work better, if not the first solution may 
appeal you.
Thanks

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