Hi, I’m working on adding NEED_GET_DATA support to the SPDK ublk server, to avoid allocating I/O buffers until they are actually needed. It is very clear how this works for write commands with NEED_GET_DATA. We wait to allocate the buffer until we get UBLK_IO_RES_NEED_GET_DATA completion and submit again using UBLK_IO_NEED_GET_DATA. After we get the UBLK_IO_RES_OK completion from ublk, we submit the block request to the SPDK bdev layer. After it completes, we submit using UBLK_IO_COMMIT_AND_FETCH_REQ and can free the I/O buffer because the data has been committed. But how does this work for the read path? On a read, I can wait to allocate the buffer until I get the UBLK_IO_RES_OK completion. But after the read operation is completed and SPDK submits the UBLK_IO_COMMIT_AND_FETCH_REQ, how do I know when ublk has finished copying data out of the buffer so that I can reuse that buffer? I’m sure I’m missing something obvious, if anyone can provide a pointer I would appreciate it. Thanks, Jim Harris
