Re: [TECH TOPIC] Settling Copy Offload via NVMe SCC

[Selva Jove <selvajove@gmail.com>]

Hi Bart and James,

Thanks for the heads up.
LSF/MM is a fine idea for the topic. It's just that we had a bit of uncertainty
about in-person event and December felt somewhat distant. So we thought
of making some progress by posting this to LPC.

Thanks,
Selva & Nitesh

On Fri, Jun 25, 2021 at 9:38 PM James Bottomley
<James.Bottomley@hansenpartnership.com> wrote:
>
> On Fri, 2021-06-25 at 09:02 -0700, Bart Van Assche wrote:
> > On 6/25/21 7:47 AM, Selva Jove wrote:
> > > The de-facto way of copying data in I/O stack has been pulling it
> > > from one location followed by pushing to another. The farther the
> > > application, requiring copy, is from storage, the longer it takes
> > > for the trip to be over. With copy-offload the trips get shorter as
> > > the storage device presents an interface to do internal data-
> > > copying.  This enables the host to optimise the pull-and-push
> > > method, freeing up the host CPU, RAM and the fabric elements.
> > >
> > > The copy-offload interface has existed in SCSI storage for at least
> > > a decade through XCOPY but faced insurmountable challenges in
> > > getting into the Linux I/O stack. As for NVMe storage, copy-offload
> > > made its way into the main specification with a new Simple Copy
> > > Command(SCC) recently. This has stimulated a renewed interest and
> > > efforts towards copy-offload in the Linux community.
> > >
> > > In this talk, we speak of the upstream efforts that we are doing
> > > around SCC -
> > > https://lore.kernel.org/linux-nvme/20210219124517.79359-1-selvakuma.s1@samsung.com/#r
> > >
> > > We'd extensively cover the design-decisions and seek the feedback
> > > on the plumbing aspects such as -
> > >
> > > 1. User-interface. Should it be a new ioctl/syscall, io_uring based
> > > opcode or must it fit into existing syscalls such as
> > > copy_file_range.
> > > 2. The transport mode between block-layer and NVMe. A chain of
> > > empty bios (like discard) vs bio with payload.
> > > 3. Must SCSI XCOPY compatibility be considered while we go about
> > > building interfaces around NVMe SCC?
> > > 4. Feasibility and challenges for in-kernel use cases, including
> > > the file-systems and device-mappers
> >
> > This topic seems closely related to "Storage: Copy Offload"
> > (
> > https://lore.kernel.org/linux-block/BYAPR04MB49652C4B75E38F3716F3C06386539@BYAPR04MB4965.namprd04.prod.outlook.com/
> > ).
>
> Not only that but it would seem to be better suited for LSF/MM on the
> IO track than the kernel summit track of Plumbers:
>
> https://lore.kernel.org/bpf/60ca126b.1c69fb81.90af4.93a7@mx.google.com/
>
> Since the plumbing aspects are more how would current practitioners use
> the API and how might we plumb it into filesystems.
>
> James
>
>
> > Notes about implementing copy offloading are available at
> > https://github.com/bvanassche/linux-kernel-copy-offload.
> >
> > Thanks,
> >
> > Bart.
> >
>
>