So, can you assign a folder or the same SSD (boot drive & apps) as the scratch disk?
As least with partioning an SSD, it is seen as a 'separate' disk and hence easier to assign/manage, copy and back up. What are the drawbacks of partitioning an SSD, if any?
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But having the OS and apps on an SSD makes start up and app loading MUCH quicker.
I have a iMac 27" 2.8 2009, model 11,1 – which has a sh*tty 1TB internal HDD. I'm upgrading the machine to get the most out of it for a little longer. Going to upgrade RAM by adding 16GB, plus the 2x 2GB it came with will = 20GB RAM.
I either leave the 1TB HDD in place, and take the optical drive out and stick a 1TB SSD in there - primarily to put the OS, apps, fonts and documents. The question is do I partition it into 2 partitions - one for the OS, apps and docs, and the other for photoshop scratch disk etc..
Or
I can also replace the 1TB HDD with an SSD as well, then RAID both SSDs as a scratch disk?
Would RAIDing these two SSD's and sticking the OS, Apps, documents on it give any speed advantages over a standard (unRAIDed) setup?
OP, there's a few bits you should be aware of in addition to a bit of input from me, all constructive… I owned that iMac, with the 2.8GHz i7 and 7200RPM drive - it's got a pretty significant limitation regarding the optical drive in that the drive bus is one of those setups that Apple installed that was never intended to run @ SATA III; if you install a drive that isn't specifically SATA II and is "automatic", the drive will run at SATA I speeds (see the note at the bottom of OWC's page for their relevant Data Doubler package -
https://eshop.macsales.com/item/OWC/DDIMCL0GB - and they'll back up what I'm alluding to here). OWC is one of the few places that I'm aware of that still sells SATA II-specific (3Gb/s) SSDs. Your "issue" here is that you could put a nice SATA III SSD in that drive bay but the bus controller will gimp it, and Apple put out a few other Macs with that "limitation".
I'll agree with you on the "faster" bit, however, once an OS or app is up and running the difference isn't hugely noticeable if you're up and running and using an application constantly - some of my employees live in AutoCAD all day, every day. Personally, I've read and heard gripes from friends that have to render 3D models - some of my modeling work involves dozens of acres of imported SHAPE files to be used for finite analysis, using the "door stop" computer and a small budget to update it. I always ended up with the highest billables wherever I worked, and now I run my own shop. IOW, I feel your pain…
Here's what I would do with your iMac - it's what I did with my iMac before I donated it - and it's still working fine at its new home.
Install a Mercury 3G or another SATA II-only SSD in the optical drive (referring to the bus controller limitation I alluded to earlier), using a suitably-sized drive for your needs, and install the OS and apps you use on this drive. Replace your HHD with a Samsung 850 EVO for your data/project storage; I use Samsung 850 Pro SSDs but install the OS and apps on them, they're in 2012 Mini Servers I own, and the EVO is used in several data storage-based arrays in my offices. Buy another, smaller SSD and install it in a TB1 external enclosure and use this device solely as a scratch disk.
OR, as an alternative:
Buy and install two identically-sized Mercury 3G or another SATA II-only SSDs and install them in both optical and HHD bays and RAID them as you see fit, and buy another, smaller SSD and install it in a TB1 external enclosure and use this device solely as a scratch disk. Consider purchasing a separate HHD external drive for backing up your setup.
The first setup is what I installed, and it transformed the iMac into a real screamer. On my work Mini Servers, we installed the OS and apps on a DAS SSD and installed two 1TB Samsung 850 Pro SSDs in RAID 0, and attached a fast SSD for a scratch disk - they're part of a video ingest workstation setup and they're pretty zippy! Good luck, and cheers!