Ideally the scratch disk should be a different drive than the drive that Photoshop is on, which is likely the OS drive. If you have two physical SSD drives, then put the PS program on one, and the scratch disk on another. That way the machine isn't trying to access the program and the scratch on the same drive. At least this is how I've always understood it.
You say that _both_ drives are M.2? We live in interesting times regards storage performance, and M.2 drives are such a game changer, I am not sure that the old guide lines still apply. With that sort of drive performance I think there is no reason why you shouldn't leave the C:\ drive as your primary Scratch volume.
I checked out the Dell 7710 specs, and that is a serious workstation, but also a serious investment with start price of nearly $9000. You could build one heck of a desktop for that sort of money.
Trevor, the 7710 starts at under $2000 (Precision 7710 Workstation Laptop | Dell ). My configuration was just over $4500, which is A LOT of $$$ for me. But I tend to keep a machine for a number of years. It doesn't have to be the latest and greatest as long as it does the job.
He is your first problem.
You need to see how much room your pagefile.sys OS configuration is using.
I would take that configuration and manually configure it to span both your drives. Take what ever suggest as a proper size pagefile and split over both drives.
I Would then determine how much memory you are allowing PS to use and set that to 70% of your memory.
I would then take your 50% of the remaining free space on your C:\ and 50% remaining free space on your D:\ that is left after allowing for installed apps and pagefile usage and set that as available for use by PS for a scratch drive configuration.
Jim, the guys on the Premiere Pro Hardware forum, (who are my go to people for this sort of thing) say not to mess with your pagefile.sys. Least ways, not to move it off the boot drive. I think Windows sets it to match however much RAM you have installed. I wish Noel still posted here, as he would dot ever 'i' and cross every 't' and send us to sleep with the level of detail (Bless ).
Doc_Pit OK about the price of the workstation. I looked at the price options in the link I posted, and assumed it was definitive. I use an MSI GT70 ONE which I bought in 2013, and it still feels sharp and runs all the CC apps smoothly. I paid just short or NZ$4000 for it, but it was money well spent IMO.
Am I incorrect in assuming that as long as there is enough free space on the two designated scratch drives (and, as long as the drives themselves perform well) there should be no problem? I was puzzled as to whether there was any special reason that people sometimes counsel against offering PS space on the OS drive as scratch space vs. having a completely separate drive. Is the objective simply to insure that there is ENOUGH space on a high performing drive? I would think that as long as there is enough space, PS will just use what it needs. (Of course, I'm no expert, and what I "would think" is not necessarily right.)
Regarding the Dell 7710, here's the kicker. I had been using a Dell XPS, which I liked a lot. But it kept giving me more and more problems. Finally, and reluctantly, I decided to spring for a new computer, and I bought the 7710. I still use the XPS now and then. After I blew the money on the 7710, I never again had any problem with the XPS!
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I was puzzled as to whether there was any special reason that people sometimes counsel against offering PS space on the OS drive as scratch space vs. having a completely separate drive. Is the objective simply to insure that there is ENOUGH space on a high performing drive?
Traditionally, the advice for hard drives was not purely about space, but also very much about speed. When you have a single hard drive mechanism, it can only be retrieving data from wherever the heads happen to be. If you have Photoshop asking the drive to read/write for the document, and Photoshop also asking the drive to read/write the scratch file, and the OS also requesting read/writes for other things, that can often be too many places for a mechanical hard drive mechanism to try and be at the same time, so somebody's going to wait while the heads get moved around the platters. But if you put the scratch file on a separate disk, that spreads out the demand across two hard disks, for more parallel read/writes and less waiting.
But I think Trevor.Dennis has a good point about the latest SSDs. They're many times faster than hard drives to start with, partly because they can read and write many locations simultaneously instead of being limited by where mechanical heads are, and because the number of read/write "lanes" has increased (especially with m.2), resulting in data transfer rates no hard drive can match. In addition, because bigger SSDs have more chips, they generally have more channels they have to read/write and transfer even more data at once. If a superfast m.2 SSD is large enough, it's probably worth testing to see if it can be used as both boot and scratch drive without much or any performance penalty. And any "penalty" might still result in net performance that's faster than a SATA SSD, and still many times faster than any combination of hard disks.
Adding to what Conway said, we have to remember that the guidelines date back to when state of the art was a mechanical HDD with IDE interface and maximum throughput of 130Mbs. My early HDDs used to do about 80Mb/s continuous, and sharing that bandwidth between programe data, Windows pagefile, and Photoshop Scratch files (that can be many Gb in size) was obviously going to present a bottleneck and affect overall performance. Then along came SSDs with 250Mb/s soon heading on up to 500Mbs on the same cable/interface, and the situation was not so clear cut.
Now we have the M.2 drives. Mike Irish shared his PC build with us on a thread in this forum, and even in the time he was planning and putting his build together, things kept changing. The most recent M.2 drives improved and reached 3Gb/s.
My MSI gaming laptop has a pair of SSDs in a raid0, and I have measured about 850Mb/s continuous. Photoshop loads in about four seconds, and trivial apps like the MS Office programs, essentially load instantaneously. With a drive performance more than three times faster than my laptop, I just can't see using the boot drive for Scratch space as having a negative impact on performance.
If you have Windows 8.1 or 10, open up Resource Monitor, select the drive you are interested in, and run some filters on a _really_ big image file. You'll soon see if the drive is topping out. (I have a lot of drives in case you were wondering)
Thanks very much for the information. That was really helpful.