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

[Javier González]

Chaitanya,

Did you have a chance to look at the answers below?

I would like to start finding candidate dates throughout the next couple
of weeks.

Thanks,
Javier

On 06.10.2021 12:01, Javier González wrote:
>On 30.09.2021 09:43, Chaitanya Kulkarni wrote:
>>Javier,
>>
>>>
>>>Hi all,
>>>
>>>Since we are not going to be able to talk about this at LSF/MM, a few of
>>>us thought about holding a dedicated virtual discussion about Copy
>>>Offload. I believe we can use Chaitanya's thread as a start. Given the
>>>current state of the current patches, I would propose that we focus on
>>>the next step to get the minimal patchset that can go upstream so that
>>>we can build from there.
>>>
>>
>>I agree with having a call as it has been two years I'm trying to have
>>this discussion.
>>
>>Before we setup a call, please summarize following here :-
>>
>>1. Exactly what work has been done so far.
>
>
>We can categorize that into two sets. First one for XCopy (2014), and
>second one for NVMe Copy (2021).
>
>XCOPY set *********
>- block-generic copy command (single range, between one
>  source/destination device)
>- ioctl interface for the above
>- SCSI plumbing (block-generic to XCOPY conversion)
>- device-mapper support: offload copy whenever possible (if IO is not
>  split while traveling layers of virtual devices)
>
>NVMe-Copy set *************
>- block-generic copy command (multiple ranges, between one
>  source/destination device)
>- ioctl interface for the above
>- NVMe plumbing (block-generic to NVMe Copy conversion)
>- copy-emulation (read + write) in block-layer
>- device-mapper support: no offload, rather fall back to copy-emulation
>
>
>>2. What kind of feedback you got.
>
>For NVMe Copy, the major points are - a) add copy-emulation in
>block-layer and use that if copy-offload is not natively supported by
>device b) user-interface (ioctl) should be extendable for copy across
>two devices (one source, one destination) c) device-mapper targets
>should support copy-offload, whenever possible
>
>"whenever possible" cases get reduced compared to XCOPY because NVMe
>Copy is wit
>
>>3. What are the exact blockers/objections.
>
>I think it was device-mapper for XCOPY and remains the same for NVMe
>Copy as well.  Device-mapper support requires decomposing copy operation
>to read and write.  While that is not great for efficiency PoV, bigger
>concern is to check if we are taking the same route as XCOPY.
>
>From Martin's document (http://mkp.net/pubs/xcopy.pdf), if I got it
>right, one the major blocker is having more failure cases than
>successful ones. And that did not justify the effort/code to wire up
>device mapper.  Is that a factor to consider for NVMe Copy (which is
>narrower in scope than XCOPY).
>
>>4. Potential ways of moving forward.
>
>a) we defer attempt device-mapper support (until NVMe has
>support/usecase), and address everything else (reusable user-interface
>etc.)
>
>b) we attempt device-mapper support (by moving to composite read+write
>communication between block-layer and nvme)
>
>
>Is this enough in your mind to move forward with a specific agenda? If
>we can, I would like to target the meetup in the next 2 weeks.
>
>Thanks,
>Javier