Photoshop RAM versus scratch disk/cache use

I recently ugraded my Mac to a new (to me) Power Mac G5 Dual 2GHz with 4GB RAM and two HDDs, one with OSX at 500GB and the other with 250GB (currently empty).
I followed the Adobe recommendations on setting up Photoshop CS2 for performance. I have my cache and scratch disk as the second (250GB) disk and therefore a different volume to the operating system. This disk is currently empty. In the cache settings, Photoshop sees 3072MB of the 4GB memory as available to it, and I have set the amount that PSCS2 can use to 100% (as Adobe states this is OK if you have 4GB or more of RAM).
With nothing else running on the Mac, when I open a tiff image of around 70MB, I can see in the status line at the bottom of the image window that the Efficiency is sometimes 100% but often drops to below 90% and down to 75% when doing image manipulations, which means that the scratch disk is being used (Adobe states that the Efficiency should be 95-100% when all actions are being done in RAM). However, when I look at the RAM status, there is still 2.5GB free, so PSCS2 has obviously not used all the RAM available to it when it starts using the scratch disk. I can also see that activity on the 250GB drive occurs when I do these manipulations, so the scratch disk is being accessed rather than actions being done in RAM
The only time I see the available RAM figure reduce is when I open multiple images. If I open ten 70MB tiff files then the RAM usage goes up a lot.
So my question is, why does PSCS2 use the scratch disk almost immediately when I manipulate an image even when I have told it to use 100% of the available 3072MB of available RAM? Have I missed a setting somewhere?
Any help or guidance appreciated.

Hi Buko
Thank you for your response. I did work through the optimisation guidelines before posting my message. I guess it is more about understanding why PSCS2 used the scratch disk when there is over 2GB of memory available, but I accept that it does what it does for good reason.
Could you let me know what the partitioning of my scratch disk will achieve? I did not see that in the optimisation guidelines, but I will definitely give it a try. I am interested to now what is actually does.
Thanks again, much appreciated.
Simon

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Photoshop CS4 does NOT use RAM as scratch disk

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as far as I can tell.
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Hi PECourtejoie,
I really try to understand this, please.
I don't know if 'Hitting the hard disk brings unacceptable slowdowns in my workflow'.
Because I don't know what unacceptable slowdowns are.
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Hi,
I am very uncertain about this.
I have a PC with RAM 20GB, Win 7 64 bits. Photoshop CS4.
I have set a hard disk partition as a primary scratch disk.
In the Preferences I have available RAM: 18488 MB
I let Photoshop use 13000 MB (70%).
Every time that I do something on a picture in Photoshop the size of the Photoshop temporary file on the hard disk scratch partition grows.
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I thought that Photoshop automatically should use extra RAM as a scratch disk.
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LarryM01 wrote:
…Every time that I do something on a picture in Photoshop the size of the Photoshop temporary file on the hard disk scratch partition grows…
Perfectly normal, expected behavior.
Photoshop always uses a scratch disk, from the very instant you open an image file or create a new document. Always. No exceptions.
LarryM01 wrote:
…I have set a hard disk partition as a primary scratch disk…
If that's a partition of your boot drive, it's futile and you're much better off leaving the boot drive unpartitioned so as to not limit either the Photoshop scratch disk or the OS swap files. There's nothing to gain because the OS swap files and Photoshop's scratch disk are still competing for the use of the one set of read/write heads.
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Wo Tai Lao Le
我太老了

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Uaphil wrote:
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Good day!
What are your Photoshop > Preferences > Performance > Scratch Disks settings?
Maybe this page can provide some useful tips: (edited)
Optimize performance | Photoshop CS4, CS5, CS6, CC
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CanadianBear
KiltedTim's right of course. You seem to have adequate RAM.
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Happy Shopping
CCC

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Check your disk management. There may be a conflict with another drive or the partition be specifically used in some way e.g. for system restore.
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Photoshop unable to identify second scratch disk 'D'

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ScarCrow 28 wrote:
…Well yes, technically you've got me on that I will admit. I'll try to rephrase to be a bit more accurate. The best scratch disk is one you don't need to rely on performance wise, by having enough RAM to handle the data performance needy tasks that would otherwise be sent to the scratch disk, when enough RAM can't be accessed by Photoshop…
…Hows that?
Still wrong, alas.
The scratch disk is used always.
See this very short (two-message) thread:
http://forums.adobe.com/message/2847996#2847996
Excerpt:
[Chris Cox writes:]
…Photoshop needs to allocate scratch space for data, in case it needs to
write that data to disk later -- otherwise you would randomly fail with
"out of scratch space" in the MIDDLE of an operation…
Furthermore, Photoshop sets the size of the scratch disk ahead of time—the instant you open an image file or create a new document—basing it on assumptions the application makes taking into account your usual, past workflow practices, your settings and the pixel dimensions of the document.
Bottom line: the scratch disk always plays a part.
Do a forum search filtering the options with Chris Cox in the "Who" box, "scratch" as key word, and Photoshop Macintosh as the forum. You'll find some enlightening reading. Examples:
http://forums.adobe.com/search.jspa?q=scratch&resultTypes=MESSAGE&resultTypes=COMMUNITY&pe opleEnabled=true&dateRange=all&communityID=3341&username=Chris+Cox&numResults=15&rankBy=10 001
I'm not just insisting on arguing; I honestly want you and others to have a better Photoshop experience. Really.
Wo Tai Lao Le
我太老了

Photoshop RAM versus scratch disk/cache use

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