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Tuesday, February 25, 2014

DPP Solutions: Scratch Disk Fever

How a Thunderbolt drive cured a most debilitating condition

LaCie 5big

Things seemed a little sluggish after my upgrade to OS X Mavericks, especially when I was running several apps at once. Launching Aperture with Photoshop already open was just asking for the dreaded spinning rainbow. Alright, time for a RAM upgrade.

That story has a happy ending—I wasn't aware how far RAM prices had fallen. For under $100, I added 8 GB of RAM to my system for a total of 12 GB.

Surely, things would be screaming-fast now. And they were, at first, but with multiple JPEGs and PSDs open while working on a website design, Photoshop was bogging down, and taking the whole OS with it. I was distraught. This shouldn't be happening.

Turns out, the problem wasn't RAM (although I'm still very happy that I added more). The problem was that I had forgotten about Photoshop Scratch Disks.

Shying away from attention under Performance in Photoshop's preferences are the Scratch Disk settings. The concept of a scratch disk is a good one: As RAM maxes out, Photoshop starts using hard drive space as "virtual" memory. It's a really smart feature that lets you work on larger files and keep a deeper log of undoable history states than your system memory would otherwise permit. The downside is that it's terribly slow compared to RAM.

You don't usually notice it, though. My big oversight was that I never set up Scratch Disks correctly on this computer and installation of Photoshop. See, the one thing you really don't want to do is use your system hard drive—the one that runs your OS and apps—as a scratch disk. If you do, all of the reading and writing that Photoshop is doing to supplement RAM is competing with the reading and writing your OS does just to exist and run Photoshop in the first place. It's like trying to have two separate telephone conversations at once: Someone is eventually getting placed on hold, or neither conversation will go well.

I could hear my hard drive wheezing as spinning rainbows had apparently permanently replaced my cursor, and then I remembered, "Scratch disk!" Sure enough, an inspection of the disk usage in the OS X Activity Monitor showed Photoshop furiously reading from and writing to my system drive.

This story also has a happy ending because I fortunately have an 8 TB Thunderbolt drive that I use primarily for backups. Thunderbolt is a relatively new hardware interface introduced in 2011 with the MacBook Pro, and now ubiquitous across Apple's lineup. Thunderbolt is fast: With transfer speeds of up to roughly 1 GB per second, it's almost twice as fast as USB 3.0 (600 MB/s) and more than 17 times faster than USB 2.0. That speed makes Thunderbolt drives a great choice for scratch disk use. (It's important to note that those speeds are the maximum "on paper" speeds, and that in actual use, speeds are slower than the maximum and depend on other factors besides the interface.)


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