I respectfully disagree with the EMD folks that a userland approach is impossible, given all the failure scenarios.
I've never said that it was impossible, just unwise. I believe that a userland approach offers no benefit over allowing the kernel to perform all meta-data operations. The end result of such an approach (given feature and robustness parity with the EMD solution) is a larger resident side, code duplication, and more complicated configuration/management interfaces.
There is some code duplication, yes. But the right userspace solution does not have a larger RSS, and has _less_ complicated management interfaces. A key benefit of "do it in userland" is a clear gain in flexibility, simplicity, and debuggability (if that's a word).
But it's hard. It requires some deep thinking. It's a whole lot easier to do everything in the kernel -- but that doesn't offer you the protections of userland, particularly separate address spaces from the kernel, and having to try harder to crash the kernel. :)
Jeff
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