[dm-devel] [LSF/MM/BFP ATTEND] [LSF/MM/BFP TOPIC] Storage: Copy Offload

[Javier González]

On 11.05.2021 00:15, Chaitanya Kulkarni wrote:
>Hi,
>
>* Background :-
>-----------------------------------------------------------------------
>
>Copy offload is a feature that allows file-systems or storage devices
>to be instructed to copy files/logical blocks without requiring
>involvement of the local CPU.
>
>With reference to the RISC-V summit keynote [1] single threaded
>performance is limiting due to Denard scaling and multi-threaded
>performance is slowing down due Moore's law limitations. With the rise
>of SNIA Computation Technical Storage Working Group (TWG) [2],
>offloading computations to the device or over the fabrics is becoming
>popular as there are several solutions available [2]. One of the common
>operation which is popular in the kernel and is not merged yet is Copy
>offload over the fabrics or on to the device.
>
>* Problem :-
>-----------------------------------------------------------------------
>
>The original work which is done by Martin is present here [3]. The
>latest work which is posted by Mikulas [4] is not merged yet. These two
>approaches are totally different from each other. Several storage
>vendors discourage mixing copy offload requests with regular READ/WRITE
>I/O. Also, the fact that the operation fails if a copy request ever
>needs to be split as it traverses the stack it has the unfortunate
>side-effect of preventing copy offload from working in pretty much
>every common deployment configuration out there.
>
>* Current state of the work :-
>-----------------------------------------------------------------------
>
>With [3] being hard to handle arbitrary DM/MD stacking without
>splitting the command in two, one for copying IN and one for copying
>OUT. Which is then demonstrated by the [4] why [3] it is not a suitable
>candidate. Also, with [4] there is an unresolved problem with the
>two-command approach about how to handle changes to the DM layout
>between an IN and OUT operations.
>
>* Why Linux Kernel Storage System needs Copy Offload support now ?
>-----------------------------------------------------------------------
>
>With the rise of the SNIA Computational Storage TWG and solutions [2],
>existing SCSI XCopy support in the protocol, recent advancement in the
>Linux Kernel File System for Zoned devices (Zonefs [5]), Peer to Peer
>DMA support in the Linux Kernel mainly for NVMe devices [7] and
>eventually NVMe Devices and subsystem (NVMe PCIe/NVMeOF) will benefit
>from Copy offload operation.
>
>With this background we have significant number of use-cases which are
>strong candidates waiting for outstanding Linux Kernel Block Layer Copy
>Offload support, so that Linux Kernel Storage subsystem can to address
>previously mentioned problems [1] and allow efficient offloading of the
>data related operations. (Such as move/copy etc.)
>
>For reference following is the list of the use-cases/candidates waiting
>for Copy Offload support :-
>
>1. SCSI-attached storage arrays.
>2. Stacking drivers supporting XCopy DM/MD.
>3. Computational Storage solutions.
>7. File systems :- Local, NFS and Zonefs.
>4. Block devices :- Distributed, local, and Zoned devices.
>5. Peer to Peer DMA support solutions.
>6. Potentially NVMe subsystem both NVMe PCIe and NVMeOF.
>
>* What we will discuss in the proposed session ?
>-----------------------------------------------------------------------
>
>I'd like to propose a session to go over this topic to understand :-
>
>1. What are the blockers for Copy Offload implementation ?
>2. Discussion about having a file system interface.
>3. Discussion about having right system call for user-space.
>4. What is the right way to move this work forward ?
>5. How can we help to contribute and move this work forward ?
>
>* Required Participants :-
>-----------------------------------------------------------------------
>
>I'd like to invite file system, block layer, and device drivers
>developers to:-
>
>1. Share their opinion on the topic.
>2. Share their experience and any other issues with [4].
>3. Uncover additional details that are missing from this proposal.
>
>Required attendees :-
>
>Martin K. Petersen
>Jens Axboe
>Christoph Hellwig
>Bart Van Assche
>Zach Brown
>Roland Dreier
>Ric Wheeler
>Trond Myklebust
>Mike Snitzer
>Keith Busch
>Sagi Grimberg
>Hannes Reinecke
>Frederick Knight
>Mikulas Patocka
>Keith Busch
>
>Regards,
>Chaitanya
>
>[1]https://content.riscv.org/wp-content/uploads/2018/12/A-New-Golden-Age-for-Computer-Architecture-History-Challenges-and-Opportunities-David-Patterson-.pdf
>[2] https://www.snia.org/computational
>https://www.napatech.com/support/resources/solution-descriptions/napatech-smartnic-solution-for-hardware-offload/
>      https://www.eideticom.com/products.html
>https://www.xilinx.com/applications/data-center/computational-storage.html
>[3] git://git.kernel.org/pub/scm/linux/kernel/git/mkp/linux.git xcopy
>[4] https://www.spinics.net/lists/linux-block/msg00599.html
>[5] https://lwn.net/Articles/793585/
>[6] https://nvmexpress.org/new-nvmetm-specification-defines-zoned-
>namespaces-zns-as-go-to-industry-technology/
>[7] https://github.com/sbates130272/linux-p2pmem
>[8] https://kernel.dk/io_uring.pdf


I would like to participate in this discussion too.

Cc'in Selva and Kanchan, who have been posting several series for NVMe
Simple Copy (SCC). Even though SCC is a very narrow use-case of
copy-offload, it seems like a good start to start getting generic code
in the block layer.

Javier