Hi I am working on a high throughput and low latency application which does not tolerate block layer overhead to send IO request directly to fiber channel lower layer SCSI driver. I used to work with libaio but currently I am looking for a way to by pass the block layer and send SCSI commands from the application layer directly to the SCSI driver using /dev/sgX device and ioctl() system call. I have noticed that sending IO request through sg device even with nonblocking and direct IO flags is quite slow and does not fill up lower layer SCSI driver TCQ queue. i.e IO depth or /sys/block/sdX/in_flight is always ZERO. Therefore the application throughput is even lower that sending IO request through block layer with libaio and io_submit() system call. In both cases I used only one IO context (or fd) and single threaded. I have noticed that some well known benchmarking tools like fio does not support IO depth for sg devices as well. Therefore, I was wondering if it is feasible to bypass block layer and achieve higher throughput and lower latency (for sending IO request only). Any comment on my issue is highly appreciated. Thanks Alireza -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
