of course I have more questions
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Hehehe.. surprise!
Talking about games... you were saying that basically if you want to get good performance (with or without having streaming implemented in your game) you will have your vm scheme customized to assure optimal CPU load and integrate well with your processing and content pipeline so this OS provided scheme might not be used by those titles pushing for peak efficiency above all.
Well, you'll have your streaming scheme customized, but again I don't really consider this to be the same thing as an HDD-based VM system. For one thing, VM typically works on large-ish page sizes (the size depends on the underlying OS page support), and the load of a page (of 'n' Kbytes) is usually triggered by an access to the memory associated with that page via hardware page protection mechanisms. Sure, you can hit pages such that they're loaded before they're needed, but the granularity of a page may be very different to a sector-based loading scheme (with sectors being ~2Kbytes).
Actually while thinking about this, it comes to mind that it's worth considering the size of the streamable data if you have thoughts about pushing it onto a VM system. Both PS3 and 360 OS's use a 32-bit ABI (it's not like we have GB's of memory on a console.), so the maximum addressable space is 4GB. Taking into account reserved space (hardware mappings for SPUs, RSX, and other reservations), you've likely got a much smaller space available for to map your VM into than the size of your entire streamable data set. If you see what I mean.
Basically, if you've got a BD-ROM with a single streamable data set of 25GB, then you're not going to be able to fit it into your addressable space, like it or not. So having it all mapped via VM.. well, it wouldn't work. FWIW I can't remember how much you're able to map with the OS VM system - and even if I could remember, I doubt I'd be able to go into any more detail without a lawyer coming up and *smashing me in the face with a hammer*.
My summary still stands that VM is not really anything to do with streaming data systems. With regards to your PSN/smaller title point, these tend to know they're running from HDD, so I doubt that streaming performance (such as you'd get from BD-ROM) is of primary concern. So normal file access may suffice (with a bit of WAD-style file packing where needed).
And using the OS doesn't reduce the testing.. if you feed chuff to the OS, it's going to bail just as if you'd written it yourself. The testing guys won't even be aware of whether you were using OS routines, or your own anyway..
Cheers,
Dean