New ioctl - FSCTL_DUPLICATE_EXTENTS_TO_FILE

[David Disseldorp]

On Wed, 28 Jan 2015 19:22:03 +0100, David Disseldorp wrote:

> > "A new File System Control (FSCTL) has been added to support the new
> > FSCTL_DUPLICATE_EXTENTS_TO_FILE operation in [MS-FSA]."  
> 
> I haven't played with it yet, but from the [MS-FCC] 20150119 preview
> doc it looks very similar to copy-chunk, with a few small differences:
> - It's only supported on ReFS, which may indicate that it's implemented
>   using COW clones.

The [MS-FSA] draft can be found at:
http://download.microsoft.com/download/C/6/C/C6C3C6F1-E84A-44EF-82A9-49BD3AAD8F58/Windows/%5BMS-FSA-Diff%5D.pdf

It pretty much confirms that it's COW based:
"The purpose of this operation is to make it look like a copy of a
 region from the source stream to the target stream has occurred when
 in reality no data is actually copied. This operation modifies the
 target stream’s extent list such that, the same clusters are pointed
 to by both the source and target streams’ extent lists for the region
 being copied."

@Steve, it looks like the perfect candidate for cp --reflink mapping on
the client side.

Cheers, David