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

Javier González
2020-12-07T19:25:07Z

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?