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

[David Disseldorp]

On Thu, 27 Jan 2022 07:14:13 +0000, 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.
> 
> We have conducted a call with interested people late last year since 
> lack of LSFMMM and we would like to share the details with broader
> community members.
> 
> * 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.

I'd like to attend this discussion. I've worked on the LIO XCOPY
implementation in drivers/target/target_core_xcopy.c and added Samba's
FSCTL_SRV_COPYCHUNK/FSCTL_DUPLICATE_EXTENTS_TO_FILE support.

Cheers, David