Re: copy offload support in Linux - new system call needed?

["H. Peter Anvin" <hpa@zytor.com>]

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