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

[Keith Busch <kbusch@kernel.org>]

On Wed, Nov 27, 2024 at 03:14:09PM -0500, Martin K. Petersen wrote:
> 
> Bart,
> 
> > Submitting a copy operation as two bios or two requests means that
> > there is a risk that one of the two operations never reaches the block
> > driver at the bottom of the storage stack and hence that a deadlock
> > occurs. I prefer not to introduce any mechanisms that can cause a
> > deadlock.
> 
> How do you copy a block range without offload? You perform a READ to
> read the data into memory. And once the READ completes, you do a WRITE
> of the data to the new location.
> 
> Token-based copy offload works exactly the same way. You do a POPULATE
> TOKEN which is identical to a READ except you get a cookie instead of
> the actual data. And then once you have the cookie, you perform a WRITE
> USING TOKEN to perform the write operation. Semantically, it's exactly
> the same as a normal copy except for the lack of data movement. That's
> the whole point!

I think of copy a little differently. When you do a normal write
command, the host provides the controller a vector of sources and
lengths. A copy command is like a write command, but the sources are
just logical block addresses instead of memory addresses.

Whatever solution happens, it would be a real shame if it doesn't allow
vectored LBAs. The token based source bio doesn't seem to extend to
that.