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

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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
hare@xxxxxxx                              +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
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