My system is set up as follows. One 750 Gb internal drive. This is partitioned into 250 GB and 500GB. The 250Gb holds the operating system. The 500Gb is the scratch disk & images in progress. A second internal 750 GB drive holds the Lightroom Library
Is this an optimal system set-up for the scratch disk or should I have the OS on a physically separate drive?
Macintosh G5, OSX 10.5.6, Photoshop CS4, 4 Gigs Ram
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Is this an optimal system set-up for the scratch disk
Far from it.
The scratch disk should be on a physically separate drive.
The way you have it now, the Photoshop scratch is competing with the OS swap files for the use of the one set of read/write heads on the one drive. The partitioning doesn’t change that fact one bit.
You one to keep the OS and all Adobe applications on the same volume.
I would say it depends upon typical file sizes you work on. If you work on very large files (say, 1 GB or larger), you probably want to have a couple hundred GB as scratch or more. If you just work on tiny Web graphics, then you can probably get away with a couple of GB.
Some of the other folks here can probably fine-tune those numbers for you.
It all depends on the size of your largest file not just the one you have open at any given time.
Photoshop creates the scratch the instant you open a file or create a new document, based on assumptions it makes based on the size of file itself, number of layers, history states, and your work habits.
35 to 50 times the size of your largest file or more is not unusual.
Is there a limit for how much Ps can benefit of the scratch disk (everything above X GB doesn’t improve anything)?
Yes, but it would be very difficult to describe. In general, smaller files require less scratch space. 30x to 60x your largest file size has been suggested, but hard drives are cheap so many folks just buy plenty of capacity and make available 100GB or 200GB for scratch on an underfilled drive for best speed.
I remember a time long ago when I told people they should have at least ten times their largest file size for scratch and it was true, but they pooh poohed it. Now you have to buy at least a 500gB separate drive.
Maybe not 500 GB for everyone…but, as you know, as computers have become more powerful and faster, and cheap large hard drives are the norm, it is far easier to create really large files on regular desktop and laptop systems. Hey, we used to be very comfortable having 44 MB SyQuest SCSI drives to back up our work from giant 200 MB internal hard drives. I recall Adobe saying 3x-to-5x maximum file size was the rule for scratch disk space then.
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