On 1/7/20 8:14 PM, Chaitanya Kulkarni wrote:
Hi all, * 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 block layer, device drivers and file system 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 Stephen Bates Zach Brown Roland Dreier Ric Wheeler Trond Myklebust Mike Snitzer Keith Busch Sagi Grimberg Hannes Reinecke Frederick Knight Mikulas Patocka Matias Bjørling [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 Regards, Chaitanya
This is a very interesting topic and I would like to participate in the discussion too. The dm-clone target would also benefit from copy offload, as it heavily employs dm-kcopyd. I have been exploring redesigning kcopyd in order to achieve increased IOPS in dm-clone and dm-snapshot for small copies over NVMe devices, but copy offload sounds even more promising, especially for larger copies happening in the background (as is the case with dm-clone's background hydration). Thanks, Nikos
