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
