You referred to the answer to your puzzlement yourself when you typed "I realise PS will still make use of the scratch disk despite the amount of RAM" [emphasis added].
Photoshop creates the scratch disk the instant you open an image file or create a new one. It sets its initial size based on its size, number of layers, history states, etc., regardless of installed RAM. It starts using the scratch disks from the get go, moving stuff off of the scratch disk into RAM and viceversa as Photoshop sees fit.
When setting up a scratch disk, figure on at least 100 times (or more) the size of your largest file, multiplied by the number of files you keep open at any given time.
The total amount of scratch disk space on your two drives (30 GB + 75 GB) is not that large, as per the above calculation.
Just for reference, my dedicated primary scratch disk is a 300 GB physically separate internal hard drive, and I also have other drives set up as secondary, etc., hard drives. Other users have a lot more scratch disk space than I do. I normally have no more than two files open in Photoshop at once, occasionally three or four.
Also keep in mind that the percentage of memory you allocate to Photoshop is not a percentage of total installed RAM but of available memory at any given time after the OS and other applications you have running have grabbed the RAM they need. Therefore the calculation you made is not necessarily accurate.
As to creating a RAM disk, I supposed there's no harm in trying, but the real question is whether you are seeing any real performance problems now, rather than just calculating RAM figures that may or may not be that relevant in actual use.
Conceivably, you could also get a much larger SSD as your primary scratch disk.
Thank you for you reply. Very useful.
I have been playing around and it's now clear, the amount of layers especially at that size will have a dramatic effect on the Scratch Disk....the P9x79 has SSD caching which I will explore as a solution as it looks like you can get 5GB/sec (youtube Asus) - looking to get 1TB HHD with 240GB SSD cache ......who need money anyway ....I will investigate the RAMDISK idea to see if it makes a difference in the mean time.
Thanks again for your help.
looking to get 1TB HHD with 240GB SSD cache ......who need money anyway
I have read many reports of people who have gotten into trouble using SSD caching, though I don't know about your particular hardware. The added complexity simply seems to increase the chance of faults and data loss.
A single SSD can only transfer up to something less than 600 megabytes / second through a SATA 3 link, simply because the link is 6 gigabits. Any benchmark showing higher must be influenced by a RAM cache, and the numbers don't represent true throughput.
But your idea is on the right track. If you're really looking to speed things up on a powerful system like that, build a RAID array of multiple modern SSDs and use it for drive C:. I have a 4 x 480 GB SSD array and it screams, with about 1.7 gigabytes / second throughput. I can hardly notice when Photoshop goes to its swap file.
Thanks Noel for the Heads Up on the SSD caching issue. I will look in to the details on a RAID configuration , I may need your help with the raid setup - it's new to me
Just so you know:
I can say with conviction that my RAID 0 array of SSDs works great, but I'm not sure I'm fully qualified to help build a system different than the one I chose. Some folks do find controller incompatibilities and whatnot, and I picked sizes, etc. based on my own needs. I researched the combination of hardware I got (OCZ Vertex 3 drives and Highpoint 2720SGL PCIe RAID controller), and I tested things pretty thoroughly before switching over to using the array as my system drive.
But I'll be happy to help with the information I do know. I also recommend the OCZ SSD forum, which is where I got a lot of good information.
So far in 4 months hard daily use I haven't had a single hiccup. It's been rock solid, and I wonder how I ever stood running my system from spinning drives before.
I actually have a 'Lacie Big' eSata x2 bay device....I can congfigure that to use Raid0. I will throw in two 120GB SSD sata3 drives and see what happens.