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

[Javier González]

On 07.12.2020 15:56, Hannes Reinecke wrote:
> 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.

Good point. We can share some performance data on how Simple Copy scales
in terms of bw / latency and the CPU usage. Do you have anything else in
mind?