[dm-devel] [RFC PATCH v2 0/2] add simple copy support

[Hannes Reinecke]

On 12/7/20 3:11 PM, Christoph Hellwig wrote:
> So, I'm really worried about:
> 
>   a) a good use case.  GC in f2fs or btrfs seem like good use cases, as
>      does accelating dm-kcopyd.  I agree with Damien that lifting dm-kcopyd
>      to common code would also be really nice.  I'm not 100% sure it should
>      be a requirement, but it sure would be nice to have
>      I don't think just adding an ioctl is enough of a use case for complex
>      kernel infrastructure.
>   b) We had a bunch of different attempts at SCSI XCOPY support form IIRC
>      Martin, Bart and Mikulas.  I think we need to pull them into this
>      discussion, and make sure whatever we do covers the SCSI needs.
> 
And we shouldn't forget that the main issue which killed all previous 
implementations was a missing QoS guarantee.
It's nice to have simply copy, but if the implementation is _slower_ 
than doing it by hand from the OS there is very little point in even 
attempting to do so.
I can't see any provisions for that in the TPAR, leading me to the 
assumption that NVMe simple copy will suffer from the same issue.

So if we can't address this I guess this attempt will fail, too.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                Kernel Storage Architect
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