On 11/27/24 12:14 PM, Martin K. Petersen wrote:
Once I had support for token-based copy offload working, it became clear to me that this approach is much simpler than pointer matching, bio pairs, etc. The REQ_OP_COPY_IN operation and the REQ_OP_COPY_OUT operation are never in flight at the same time. There are no synchronization hassles, no lifetimes, no lookup tables in the sd driver, no nonsense. Semantically, it's a read followed by a write.
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? Does this mean that a third operation is needed to cancel REQ_OP_COPY_IN operations if the REQ_OP_COPY_OUT operation fails? 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?
Aside from making things trivially simple, the COPY_IN/COPY_OUT semantic is a *requirement* for token-based offload devices.
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. 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?
Why would we even consider having two incompatible sets of copy offload semantics coexist in the block layer?
I am not aware of any proposal to implement two sets of copy operations in the block layer. All proposals I have seen so far involve adding a single set of copy operations to the block layer. Opinions differ however about whether to add a single copy operation primitive or separate IN and OUT primitives. Thanks, Bart.
