Re: [Qemu-devel] [Qemu-block] [RFC PATCH 0/8] qemu-img convert with copy offloading

[Fam Zheng]

On Wed, 04/04 14:23, Stefan Hajnoczi wrote:
> On Thu, Mar 29, 2018 at 07:09:06PM +0800, Fam Zheng wrote:
> > [Posting a preview RFC for the general idea discussion and internal API 
> > review.
> > Libiscsi support is being worked on in the meantime.]
> > 
> > This series introduces block layer API for copy offloading and makes use of 
> > it
> > in qemu-img convert.
> > 
> > For now we implemented the operation in local file protocol with
> > copy_file_range(2).  Besides that it's possible to add similar to iscsi, nfs
> > and potentially more.
> > 
> > As far as its usage goes, in addition to qemu-img convert, we can emulate
> > offloading in scsi-disk (EXTENDED COPY), and do similar to drive-mirror.
> > 
> > The new bdrv_co_map_range can also be an alternative way to implement format
> > drivers in the future, once we make block/io.c use it in preadv/pwritev 
> > paths.
> 
> I posted concerns about the bdrv_co_map_range() interface.  It would be
> safer to only have a copy_range() interface without exposing how data is
> mapped outside the driver where race conditions can occur and the format
> driver no longer has full control over file layout.

It's a good point, but I couldn't think of a way to implement copy_range between
two format drivers: both of them need to recurse down to their bs->file and what
we eventually want is a copy_file_range() on two fds (or an iscsi equivalent):

    src[qcow2]           ->              dst[raw]
        |                                   |
        v                                   v
    src[file]            ->              dst[file]
        |                                   |
        v                                   v
       fd1               ->                 fd2
                    copy_file_range

Maybe we should add BlockDriver.bdrv_co_map_range_{prepare,commit,abort} and
call them from bdrv_co_copy_range(). This way the code path works pretty much
the same way to .bdrv_co_preadv/pwritev.

Fam