Hello Changheun! > I want to discuss using max_xfer_blocks instead of opt_xfer_blocks as > a optional. For example, device reports opt_xfer_blocks is 512KB and > 1MB as a max_xfer_blocks too. Currently rw_max is set with 512KB only. Because that's what the device asks for. If a device explicitly requests us to use 512 KB I/Os we should not be sending it 1 MB requests. The spec is very clear. It says that if you send a command *larger* than opt_xfer_blocks, you should expect *slower* performance. That makes max_xfer_blocks a particularly poor choice for setting the default I/O size. In addition to being slower, max_xfer_blocks could potentially also be much, much larger than opt_xfer_blocks. I understand your 512 KB vs. 1 MB example. But if the max_xfer_blocks limit is reported as 1 GB, is that then the right value to use instead of 512 KB? Probably not. If a device does not report an opt_xfer_blocks value that suits your workload, just override the resulting max_sectors_kb in sysfs. This is intentionally a soft limit so it can be adjusted by the user without having to change the kernel. -- Martin K. Petersen Oracle Linux Engineering
