What are the real-world performance gains of having a photoshop scratch disk?

Question

I've used Photoshop for quite a while and often hear that providing it with a scratch disk will help with performance.

Most computers come with tons of RAM and hard drive space so does a scratch disk still provide any performance gains in today's computers? How does the type of media used affect the result such as an internal hard drive or a USB Key? At one point does it start providing providing results? (I suppose that's three questions)

Answer 1

I remember when it used to be very useful, typically when the files people were editing were the size of half their memory (or more) but now the memory has become more and more and the photos haven't become that much bigger, I see it as less and less important.

Not saying you won't see any speed improvements, but if you are just a casual user with a machine that has around 4gb of memory, and your output files are under 100Megs, I would say that you may not see any speed imporvements.

Answer 2

Okay first a few points:

1. The output file size does not reflect the amount of memory needed to process the image.
2. A scratch disk does not need to be a separate, empty or unused hard drive.

Photoshop uses a scratch disk to cache information about the open documents. It does this because it improves performance in most cases. Rather than regenerating data, it loads what it already processed off the scratch disk(s). It would use your RAM, but there simply isn't enough there, especially in 32-bit systems. A 10MP image from a point and shoot digital camera might save out as a 4-5MB JPEG file, but when you're toying with it in photoshop with several layers, running filters and adjustments (history states are stored), the amount of memory needed can approach 1GB.

In a sentence, yes having a proper scratch disk will improve performance, even in high end computers.

As far as the ideal conditions, Adobe has all sorts of recommendations. For casual photoshop usage, though, you can get away with any sufficiently fast hard drive. You don't need to go drop a bundle on an SSD or anything. I would avoid older hard drives, and just make sure you have 30-50% free on the drive for Photoshop to play with (even though it isn't likely to use it all, it means the drive spends more time reading and writing and less time seeking).

Answer 3

Check out this presentation written by one of the Adobe co-architects. Talks about many issues related to performance tuning, but as far as file IO goes, having a dedicated disk prevents other resources from competing with Photoshop.

Answer 4

It's not very useful anymore, but the other answers cover that.

As for the media questions: The whole idea is having a dedicated HDD, as to make it faster than reading from page file (as if something else is accessing your main HDD, photoshop can't, and so can't get it's stuff back from the disk), so a USB stick may work slightly better.

If you recall, Windows Vista shipped with a "Readyboost" feature, which was similar to this, as it used a USB drive to help applications get their stuff from the page file faster. This is because while the maximum throughput of a USB2 device is slower than your HDD, there's no disk lag, so for small parts as the PF tends to be, it's brilliant.

One place where you MAY see improvements these days is it you happen to own 2 SSDs (because you'd have your OS and applications on the primary, obviously), you could use one as a scratch disk, which would make messing about with stupidly large files many times faster.