On 12/19/2011 02:57 PM, Dave Chinner wrote:

That was my immediate thought, but sendfile has plumbing that is
page cache based and we require completely different infrastructure
and semantics for an array offload.

The plumbing is internal to the kernel and doesn't mean we have to use
the same VFS methods.

e.g. for an array offload, we have to flush the source file page
cache first so that the data being copied is known to be on disk,
then invalidate the destination page cache if overwriting or extend
and pre-allocate blocks if not. Then we have to map both files and
hand that off to the array.

Then there's a whole bunch of tricky questions about what the state
of the destination file should look like while the copy is in
progress, whether the source file should be allowed to change (e.g.
it can't be truncated and have blocks freed and then reused by other
files half way through the copy offload operation), and so on.

sendfile() has well known, fixed semantics that we can't change to
suit what is needed for an offload operation that could potentially
take hours to complete. Hence I think an new syscall is the way to
go....

Perhaps what we need first in an explicit enumeration of the semantics
you're looking for.

	-hpa

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