[stomachmonkey]

Dave, the thing with PS is at some point it's simply not practical to have enough RAM to hold an image without resorting to scratch space.

PS keeps a copy of the original, a separate copy of each channel, current state, undo states, clipboard etc...

To prevent swap with a 20 GB file you'd need 80-100 GB of RAM in the box.

Dedicated scratch drives make a big difference. 10k RPM discs, RAID5 or SSD is the way to go.

gchappel, Efficiency tells you how much of the RAM allocated/available to/for PS is being utilized.

You can run at 100% efficiency and still have it hit scratch.

Click the tab in the bottom left where efficiency shows up and select scratch instead. If the first number is larger than the second you are using scratch.

This is easy to validate in OS X, simply open a finder window and watch the available drive space, as you work you can see PS chewing disc space while efficiency remains at 100%.