Re: FAST VP
"Therefore, you will have visibility to the allocated capacity (which already includes the overhead) and can stop at 90% of the total capacity of the pool. Does this make sense?"
This is where the confusion is. When you say allocate storage at 90% of total capacity of pool and the 90% already includes overheads, that means the remaining 10% is still not accounting for overheads as stated by you earlier.
So we indeed have to consider the overheads up front, isn't it? Although it may vary as explained by you.
However, if 10% is still accounting for overheads and we are keeping the capacity to 90% (including the overhead), aren't we wasting space?
It is more simpler to just keep the pool capacity to 90% and I think it would be very hard to keep a tab on overheads and calculate capacity based on that.