RE: A question on Zero Copy

[Black_David@emc.com]

With my co-chair hat off:

> If you imagine even RAID controller devices have memory management that
> operate in a manner to resolve data discontinuities and header structure
> placement, then post mapping replaces look-ahead filtering.  Such post
> mapping has greater freedom. 

One can imagine what one likes, but that doesn't mean that all devices will
be built that way.  That sort of memory management has both hardware and
software implications that implementers may choose to trade off for other
things.  YP's description (allocate buffer, issue R2T to fill it) is a
perfectly
reasonable way to go about building a RAID device, as there are products
on the market today that operate in precisely that fashion.  As far as I can
tell, the issue of whether initiators and targets need to be able to put
inbound
data into pre-determined locations in memory is implementation dependent
and depends on a variety of factors that involve software and hardware
architecture/design issues.

--David 

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