I was thinking along the lines of continuous heavy load of writing (some read) rather large files (5G+ would be average - multiple!) - does that warrant caching or is it only lots of smaller files? That and lots of ~0.5G files (read mostly) is what defines the main load on the system.

I ask because I'm not 100% sure of what the caching is for. I had thought it was like the journal log for fast writing to be later written to the filesystem itself, but now I think I may be wrong in my judgement. It now sounds like a fast access for usual suspects.

Now you see how a terabyte and a half disk space can be used in a matter of hours :)