Re: [RFC PATCH v2 0/2] add simple copy support

Hannes Reinecke
2020-12-07T14:56:26Z

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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