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I have 16GB of RAM and I do not believe that I can saturate all that RAM. I am only dealing with 16MP+ photographs and I will not be opening 20+ images at a time.

From my understanding scratch disks are a must in the pre-64bit architecture days where only 2 gb of RAM can be used for PS, so if everything can be fitted into RAM then essentially the scratch disk is useless correct?

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Welcome to Graphic Design. This question would have been better asked over at Super User, but since it has already been answered and accepted, I will leave it here. – Philip Regan Aug 17 '11 at 10:05
up vote 2 down vote accepted

You are correct. Photoshop will not touch your scratch disk if it has enough memory.

Optimal memory settings are usually allocating around 70% of total ram to Photoshop.

Photoshop Memory Usage setting Using 70% is the smart move until and unless tests show that more is faster.

The operating system version, Photoshop version, other programs, etc, all influence whether too high a setting improves performance, or instead causes virtual memory paging and/or degrades the ability of the system to have enough spare memory for drive caching. It is a performance see-saw effect under some conditions. -- Mac Performance Guide

Also to note is that Photoshop does not pre-allocate the memory. It only uses it if it needs it.

Enough is enough

Remember too that enough is enough— Photoshop will use what it needs, up to the limit set with this preference.

Using a setting of 90% instead of 70% is meaningless when Photoshop really only needs and uses 40%. Photoshop will use what it needs up to the limit; it does not pre-allocate the memory, it’s simply a future limit when Photoshop is launched.

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thanks, the links are awesome – bash- Aug 17 '11 at 4:59

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