Re: [PATCHv10 0/9] write hints with nvme fdp, scsi streams

[Christoph Hellwig <hch@lst.de>]

On Thu, Nov 28, 2024 at 05:51:52PM +0900, Damien Le Moal wrote:
> > Maybe. But you'll have a hard time convincing me to add any kind of
> > state machine or bio matching magic to the SCSI stack when the simplest
> > solution is to treat copying like a read followed by a write. There is
> > no concurrency, no kernel state, no dependency between two commands, nor
> > two scsi_disk/scsi_device object lifetimes to manage.
> 
> And that also would allow supporting a fake copy offload with regular
> read/write BIOs very easily, I think. So all block devices can be
> presented as supporting "copy offload". That is nice for FSes.

Just as when that showed up in one of the last copy offload series
I'm still very critical of a stateless copy offload emulation.  The
reason for that is that a host based copy helper needs scratch space
to read into, and doing these large allocation on every copy puts a
lot of pressure onto the allocator.  Allocating the buffer once at
mount time and the just cycling through it is generally a lot more
efficient.