[dm-devel] [RFC PATCH 0/3] NVMe copy offload patches

[Nitesh Shetty]

On Wed, Feb 2, 2022 at 12:23 PM Mikulas Patocka <mpatocka at redhat.com> wrote:
>
> Hi
>
> Here I'm submitting the first version of NVMe copy offload patches as a
> request for comment. They use the token-based approach as we discussed on
> the phone call.
>
> The first patch adds generic copy offload support to the block layer - it
> adds two new bio types (REQ_OP_COPY_READ_TOKEN and
> REQ_OP_COPY_WRITE_TOKEN) and a new ioctl BLKCOPY and a kernel function
> blkdev_issue_copy.
>
> The second patch adds copy offload support to the NVMe subsystem.
>
> The third patch implements a "nvme-debug" driver - it is similar to
> "scsi-debug", it simulates a nvme host controller, it keeps data in memory
> and it supports copy offload according to NVMe Command Set Specification
> 1.0a. (there are no hardware or software implementations supporting copy
> offload so far, so I implemented it in nvme-debug)
>
> TODO:
> * implement copy offload in device mapper linear target
> * implement copy offload in software NVMe target driver

We had a series that adds these two elements
https://github.com/nitesh-shetty/linux_copy_offload/tree/main/v1

Overall series supports –
1.    Multi-source/destination interface(yes, it does add complexity
but GC use-case needs it)
2.    Copy-emulation at block-layer
3.    Dm-linear and dm-kcopyd support (for cases not requiring split)
4.    Nvmet support (for block and file backend)

These patches definitely need more feedback. If links are hard to read,
we can send another RFC instead. But before that it would be great to have your
inputs on the path forward.
But before that it would be great to have your inputs on the path forward.

PS: The payload-scheme in your series is particularly interesting and
simplifying plumbing, so you might notice that above patches borrow that

> * make it possible to complete REQ_OP_COPY_WRITE_TOKEN bios asynchronously
Patch[0] support asynchronous copy write,if multi dst/src payload is sent.

[0] https://github.com/nitesh-shetty/linux_copy_offload/blob/main/v1/0003-block-Add-copy-offload-support-infrastructure.patch

-- Nitesh