Can you tell me about Photoshop, scratch disks, and iMacs?

[MarieMeyer]

How important is it to have a "scratch disk" for Photoshop? I have the impression that it is only really needed if working on gigantic files. I'm in the process of speccing out a new iMac to replace a late-2015 model. (3.1 GHz Intel Core i5, 8GB RAM, 1TB Fusion Drive). I need more RAM for LR and this model is not upgradeable, that is what is motivating the new (reluctant) new purchase; I figure I should try to think ahead as much as possible in hopes of "future proofing" this purchase.

FWIW, I often have multiple Adobe CC apps open at once (Bridge, Lightroom, Photoshop, InDesign and Illustrator). I think I recall hearing that a scratch disk is beneficial in this situation, as it stops PS and AI from starving the others of RAM. Is that the case?

Since I'm planning on getting another iMac, my scratch disk would have to be external. What is a good one, and which type of port do I want to run it off?

[D Fosse · Accepted solution]

The scratch disk is always used. Contrary to what people think, it's not a simple "overflow" situation, it's a dynamic process where data are constantly moved back and forth. Think of the scratch disk as Photoshop's main memory, and RAM as a fast access cache.

You should always set Photoshop's RAM usage low enough so that the OS and other applications have a fair amount to work with, 4-8 GB or so.