Should Persistent Disk Cache be on a separate drive from Media Cache?

Question

Hi guys =)   I'm building a system for editing with PP & AE and I'm trying to decide on my storage setup. Google, PPBM7, white papers, and helpful forum posters have mostly sorted out all my questions.
But one thing I'm still not clear on is Persistent Disk Cache. After reading this white paper I'm left a little ambiguous about how the different caches should be sorted.
What I have gathered (I think this is right):
Persistent Disk Cache is like a 'permanent' / stable copy of your RAM previews.
Persistent Disk Cache is totally different from the "Media Cache" / "Scratchdisk" that other guides about PP, PS, AI, etc. refer to.
PDC is also separate from the "Media Cache" used by After Effects
Media Caches should always have their own disk (if possible) since they're accessed constantly and benefit from I/O separation
What I'm not sure about is:
Should the PDC also be on its own separate disk from the Media Cache? The white paper, and other disk-setup guides, leave this ambiguous, often only mentioning "Media caches" in their recommended 'four drive configuration'.
I'm looking at this from 2 different directions:
Hypothetical. Whether it's practical or not, would performance increase by having PDC and Media Cache on separate drives?
Practical. In actual practice, if PDC is placed on the same drive as the other Caches/Scratchdisks, will this create noticeable slowdown/issues?
I understand that no answer can be absolute in the world of creative software because it will always be affected by exactly what you're doing and how you're using it. But a general guideline would be fine. =)

Answer 1

Persistent Disk Cache is like a 'permanent' / stable copy of your RAM previews.
It may be persistent in that even data garbage (=invalidated cahce files from crashed projects etc.) is sticky like chewing gum on your shoes, but I'm not too sure about "stable" or reliable. Some plug-ins like TrapcoParticular still struggle with this cache stuff even today... Also AE's caches are not necessarily a reflection of the RAM previews. Just like the conventional RAM caching they work on a per-layer/ per(pre)comp basis. The very specifics of how much data is actually pulled from the cache depend on the project and often enough you will see the program re-caching because something has changed.
Hypothetical. Whether it's practical or not, would performance increase by having PDC and Media Cache on separate drives?
Practical. In actual practice, if PDC is placed on the same drive as the other Caches/Scratchdisks, will this create noticeable slowdown/issues?
To quote from a Ferengi in an episode of one of the Star Trek TV series: "I don't know how to speak hypothetically." That is to say you won't know until you build the actual setup and start working. Of course separate drives are always better not just for speed but also data security/ safety, but the actual throughput will be determined by how you work and what makes up your project. With just a handful of simple graphics files you will never know the difference even on a slow system, but if you work with lots of 4k clips you could exhaust bandwidth on a fast system. You could be plugging all your devices to the same SATA buses and that might translate down to being on the same PCI bus and then none of this really makes a difference because you already get collisions on the PCI protocol level, if you get my meaning. So for better or worse, unless you have made up your mind about actual components it will and shall forever remain a very theoretical, abstract and academic debate. All the whitepapers in the world can be useless, if your workflow doesn't fit in one of the proposed standard models and example references...
Mylenium