If you have an SSD system drive, is a second dedicated scratch disk recommended?

Question

Looking to buy a new MacBook Pro which has one PCIe-based 512gb SSD system drive. Unlike earlier MacBook Pro models, there is no internal DVD drive to replace w. a second disk drive.
I know that with HDD's a dedicated scratch disk that run's at least as fast as your application drive enhances performance of Photoshop. Is this also true with SSD's?  Thanks.

Answer 1

Here's a suggestion (not aimed at anyone in particular, and I realize it may not be possible in all systems)...
If you're going to set up a system to access a certain amount of storage interactively - and what I mean is storage that's used all the time in the normal operation of the computer - consider making one big system partition out of a RAID array of SSDs.
With I/O operations averaging larger than the bare minimum (e.g., 4K bytes), RAID 0 effectively adds together the performance of the drives.  This is because there's effectively no seek time with an SSD.  It essentially randomly accesses stored data by address (it's quite complicated internally, but externally it works out that latency is almost negligible - literally measured as a few millionths of a second on a modern drive).
So...
Consider building your next system with multiple SSDs on task, making up an array of, say, a few terabytes.  This yields the following:
Everything done by the system is done at the speed that's the sum of the drive speeds.  With a modern system using SATA III, that's essentially about half a gigabyte per second per drive.  Imagine throughput of more than a gigabyte a second - that's real.  Forget about dedicating drives to tasks - EVERYTHING gets the benefit of the entire RAID array speed.
Since latency is virtually nonexistent, a great deal of multitasking can be done on the same volume without introducing any thrashing.  Photoshop can be writing to scratch, the system swapping like crazy, and you just keep working without notice.
Everything's consolidated on one volume, which simplifies a lot of things (e.g., backup is simpler, applications install where they like on the C: volume).
All the free space pools together, so what's available as transient storage for whatever you're doing at the time is maximized.  This tends to offset the extra cost of SSDs some.
SSDs themselves are fairly new tech, and it's not been widely known that they RAID together EXTREMELY well.  They really do.
People sometimes worry that using multiple drives to make up a single volume increases the chances for failure, but consider that SSDs are solid state, and so by nature have a good bit higher reliability (higher MTBF; often 2 million hours), and they don't generate nearly as much heat (they consume just a few watts).  Having SSDs in your system can actually increase the reliability of everything somewhat, because when everything is cooler it lasts longer.  Plus they don't make any vibration.
This is not fantasy, I've done it.
My current Dell workstation has a nearly 2 TB C: volume made up of four 480 GB SSDs.  My sustainable disk throughput is literally around 1.7 gigabytes per second (that's 15 times faster than a typical single hard drive).  I do also have some HDDs in the system, but they normally spend all day spun down.  They're for backup and very low access storage (e.g., downloads I've accumulated over time).
I wait for NOTHING on this system.
Oh, and it's been 100.0% reliable with this setup for 2-1/2 years now.  ZERO glitches.
-Noel