How to determine the scratch disc size?

Question

hello,
once i was reading an adobe pdf "How to get better performance in photoshop cs5" - that was in 2009 or 2010, and may be outdated, but there was a calculation method
to determine the size of a scratch disc. (similar calculation see below, if i can remember right)
i am asking myself, how can i determine the correct size of an external SSD-scratch disc, only used by photoshop (completely empty):
should i buy a 128GB or 256GB or 512GB SSD which is only reserved for photoshop?
basic question 1 : i guess i should avoid to set the internal SSD as photoshop scratch disc, as it slows down everything?
basic question 2 : in sense of maximum performance: better buy an external USB3.0 or thunderbolt SSD? will photoshop really use the extra thunderbolt speed when swapping data?
secondary question:
can i calculate the size regarding my daily working habits?
i am mainly working like this:
- with my imac 27" late 2013 with 32GB RAM and 256 GB internal pci-e SSD (800 MB/sec), which will stay always half empty for performance reasons.
- OSX 10.8 mountain lion and 10.9 mavericks soon
- photoshop cs5, cs6 and cc (always without extended)
- 8bit and 16bit mode
- only RGB
- with latest phocus/Hasselblad and canon RAW Files which produce a basic .psb document at ...
- 10.000 x 7000 px at 300dpi
- with average 10 - 40 main image layers and 20-50 adjustement layers (try to reduce that in 16bit)
- .psb file is 2-20 GB big (file in finder)
- 16bit file compression is off, when saving .psb files (faster handling)
-  set photoshop to 70% ram usage (from 32GB RAM)
i wonder how to calculate ?
for example:
10.000 x 7000 px at 300dpi needs for one image layer at 16bit: 2GB RAM in photoshop cs6 or cc (just as a number), this may be wrong
so lets take 2GB RAM and multiply with 10 image layers in my .psb file (16bit) = 20 GB RAM, and multiply with 20 adjustment layers (guess they need less ram, for one lets say 500MB) = 20GB + 10GB = this 16bit .psb layer file would need 30GB RAM, so when i have 32GB in my imac, i set cs6 or cc to 70% ram usage, it misses at least round 8-10GB RAM > can i guess that photoshop would swap these 8GB onto my scratch disc? or do i miss something important in my thinking?
tricky thinking
thanks for help

Answer 1

station_two wrote:
The rule of thumb I follow says to figure on 50 to 100 times the size of your largest file ever multiplied by the number of files you have open.  I have seen the scratch file exceed 300 GB once, an admittedly rare occurrence, but it often exceeds 200 GB when stitching large panoramas and the like.
As an example—and stressing that I'm aware that others have even more scratch space than I do—I keep two dedicated, physically separate hard drives as my primary and secondary Photoshop scratch disks and a lot of GB free on my boot drive for the OS.  I also have 16 GB of RAM installed.
Additionally, if you only have a single HD, i.e. your boot drive, you'd need it to be large enough to accommodate both the swap files of the OS as well as Photoshop's scratch.
- i dont use HDD anymore only SSDs, both internal and external
- i set history state to only 5 or 6, to improve performance
- i set cache size to 4 and tiles to "big and flat" with 1028kb (there is no "big and much layers" option)
- is this still the rule of thumb? i read it in 2009 , too, guess it was outdated, as cs6 and cc have improved codes in terms of performance?
- if you say "50 to 100 times the size of your largest file ever multiplied by the number of files you have open.":
i will not open more than one document at same time to prevent performance lags, so lets calc like: dokument size in finder (you mean in finder or doc. size shown in photoshop?) = e.g. 5GB x 100 = 500GB, so my external scratch disc SSD, i would buy now, should be at least 500GB, USB 3.0 or thunderbolt ... maybe better thunderbolt, yes? with usb 3.0 i could gain 300MB/sec if thats enough for photoshop?
thanks