This does not work out at all in practice. It sounds very simple (and is
if all you want is to dd if=/dev/hda of=/dev/hdb) but starts to fall apart
immediately. Consider all the different devices and how they provide the
byte stream abstraction and you start to see that the OS is frequently
involved in the process.
I've been involved, briefly, in the design of two such systems before
I realize what a mistake it was. After I pulled away from the last one,
I watched almost a million lines of code get writtten before they gave
up, realizing they were reimplementing what the OS already did.
Limiting your I/O device<->device throughput to 1/2 memory bandwidth is
not an unreasonable thing. And I don't say that lightly, I feel the need
for speed everyday, but not at the expense of the OS...
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