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

[James Bottomley <James.Bottomley@HansenPartnership.com>]

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.
>