modelling physical writes

How do we get solid numbers to determine how many physical writes a specific sql or pl/sql package will perform? When we use 10046 I believe it only gets physical reads because the dbwriter is asynchronous? I have some concerns about using AWR for this.

Also, how do we get reliable redo numbers? When I use set autotrace on and run the same insert multiple times the redo seems to vary by 20% or so.

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