On 2022/9/1 10:30, Ming Lei wrote: > + > +- ``UBLK_IO_NEED_GET_DATA`` > + > + ublk server pre-allocates IO buffer for each IO by default. Any new projects > + should use this buffer to communicate with ublk driver. However, existing > + projects may break or not able to consume the new buffer interface; that's > + why this command is added for backwards compatibility so that existing > + projects can still consume existing buffers. Hi, Ming. Could you please add more information on UBLK_IO_NEED_GET_DATA. stefanha found it hard to understand. Myabe we should write like this: With UBLK_F_NEED_GET_DATA enabled, the WRITE request will be firstly issued to ublksrv without data copy. Then, IO backend receives the request and it can allocate data buffer and embed its addr inside a new ioucmd. After the kernel driver gets the ioucmd, the data copy happens(from biovecs to backend's buffer). Finally, the backend receives the request again with data to be written and it can truly handle the request. UBLK_IO_NEED_GET_DATA add one additional round-trip in ublk_drv and one io_uring_enter() syscall. Any user thinks that it may lower performance should not enable UBLK_F_NEED_GET_DATA. ublk server pre-allocates IO buffer for each IO by default. Any new projects should use this buffer to communicate with ublk driver. However, existing projects may break or not able to consume the new buffer interface; that's why this command is added for backwards compatibility so that existing projects can still consume existing buffers. Regards, Zhang.
