No, I don't think it will all be in memory - but that's part of the performance calculation. If you need to check on the status of an XID and find that you need to read a page of data in from disk, that's going to be many orders of magnitude slower than anything we do with s snapshot now. Now, if you gain enough elsewhere, it could still be a win, but I'm not going to just assume that.
I was just suggesting this, because the memory costs have come down a lot(as you may know) and people can afford to buy more memory in enterprise scenario. We may not need to worry about MBs of memory, especially with the cloud computing being widely adopted, when we get scalability.
