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

[Damien Le Moal <dlemoal@kernel.org>]

On 11/28/24 11:09, Martin K. Petersen wrote:
> 
> Bart,
> 
>> What if the source LBA range does not require splitting but the
>> destination LBA range requires splitting, e.g. because it crosses a
>> chunk_sectors boundary? Will the REQ_OP_COPY_IN operation succeed in
>> this case and the REQ_OP_COPY_OUT operation fail?
> 
> Yes.
> 
> I experimented with approaching splitting in an iterative fashion. And
> thus, if there was a split halfway through the COPY_IN I/O, we'd issue a
> corresponding COPY_OUT up to the split point and hope that the write
> subsequently didn't need a split. And then deal with the next segment.
> 
> However, given that copy offload offers diminishing returns for small
> I/Os, it was not worth the hassle for the devices I used for
> development. It was cleaner and faster to just fall back to regular
> read/write when a split was required.
> 
>> Does this mean that a third operation is needed to cancel
>> REQ_OP_COPY_IN operations if the REQ_OP_COPY_OUT operation fails?
> 
> No. The device times out the token.
> 
>> Additionally, how to handle bugs in REQ_OP_COPY_* submitters where a
>> large number of REQ_OP_COPY_IN operations is submitted without
>> corresponding REQ_OP_COPY_OUT operation? Is perhaps a mechanism
>> required to discard unmatched REQ_OP_COPY_IN operations after a
>> certain time?
> 
> See above.
> 
> For your EXTENDED COPY use case there is no token and thus the COPY_IN
> completes immediately.
> 
> And for the token case, if you populate a million tokens and don't use
> them before they time out, it sounds like your submitting code is badly
> broken. But it doesn't matter because there are no I/Os in flight and
> thus nothing to discard.
> 
>> Hmm ... we may each have a different opinion about whether or not the
>> COPY_IN/COPY_OUT semantics are a requirement for token-based copy
>> offloading.
> 
> 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.

> 
>> Additionally, I'm not convinced that implementing COPY_IN/COPY_OUT for
>> ODX devices is that simple. The COPY_IN and COPY_OUT operations have
>> to be translated into three SCSI commands, isn't it? I'm referring to
>> the POPULATE TOKEN, RECEIVE ROD TOKEN INFORMATION and WRITE USING
>> TOKEN commands. What is your opinion about how to translate the two
>> block layer operations into these three SCSI commands?
> 
> COPY_IN is translated to a NOP for devices implementing EXTENDED COPY
> and a POPULATE TOKEN for devices using tokens.
> 
> COPY_OUT is translated to an EXTENDED COPY (or NVMe Copy) for devices
> using a single command approach and WRITE USING TOKEN for devices using
> tokens.

ATA WRITE GATHERED command is also a single copy command. That matches and while
I have not checked SAT, translation would likely work.

While I was initially worried that the 2 BIO based approach would be overly
complicated, it seems that I was wrong :)

> 
> There is no need for RECEIVE ROD TOKEN INFORMATION.
> 
> I am not aware of UFS devices using the token-based approach. And for
> EXTENDED COPY there is only a single command sent to the device. If you
> want to do power management while that command is being processed,
> please deal with that in UFS. The block layer doesn't deal with the
> async variants of any of the other SCSI commands either...
>


-- 
Damien Le Moal
Western Digital Research