Is there some tool that can let me understand which volume is under stress and in case move it on the other head?

I wrote a tool called "topvol" a while ago that does just this. You can find it here:

http://communities.netapp.com/docs/DOC-1262

I also have an updated version that allows you to filter on aggregate too, if you'd be interested.

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Romeo Theriault
System Administrator
Information Technology Services