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a=6eWqkTHjU83fiwn7nKZWdM+Sl24=:19 a=z/mQ4Ysz8XfWz/Q5cLBRGdckG28=:19 a=lCpzRmAYbLLaTzLvsPZ7Mbvzbb8=:19 a=wKuvFiaSGQ0qltdbU6+NXLB8nM8=:19 a=Ol13hO9ccFRV9qXi2t6ftBPywas=:19 a=xqWC_Br6kY4A:10 a=Wb1JkmetP80A:10 a=GoEa3M9JfhUA:10 a=b9we5gFtQRnb6FQepoYA:9 cc=ntf awl=host:12070 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwNzI5MDAyNyBTYWx0ZWRfX8Je+y0nzphNS CYbFS0l+jfEyBmQ+4EOyn4JlILRk/abaRbfWc8A9v1rYl2EGGF7/qUFJZiy4/xSHUvTAtLlG8w/ insYqyK/AqvAHo3GvLk3JuBA/mzd/CtFLkjy/jiXGr9PPcMl8tBrlvmgFs8tqjWHJ1C66keT/ui s8+sOiBhoJjFdMsjEC2wprbV2J0qVxgnOtK4VOivIEByqt6Lh8ffY4uaNG6ARroonel+MESOCIr G3YiD+aRYhP7RCoT/j5d8UFQ3iATCCOXL0ysaFaY3MxndQe3ih44EHOb+fHkTJuONb8u5eMKgKW JbasOv+caeo9mTCtcoFIA4KhKbHtQC+ntLxlWHNHqrbOwZb7s50MRX1x54fSYm7CpvMF3ifjACj yCf4Ot4r5QKsdG5y5Tkt6l9MxiL2aCG3haxpooJSzu7VulRYY7J8hYj3OC651jU/Yjplo6ET X-Proofpoint-GUID: F1kuRFGfaNLyGX5eGve85trF2OM4O19k Damien, > My bad, yes, that is the definition in sysfs. So io_min is the stride > size, where: Depends on the RAID type. For RAID0 and 1 there is no inherent penalty wrt. writing less than the stride size. But for RAID 5/6 there clearly is. > stride size x number of data disks == stripe_size. > > Note that chunk_sectors limit is the *stripe* size, not per drive stride. > Beware of the wording here to avoid confusion (this is all already super > confusing !). The choice of "chunk" to describe the LBA boundary queue limit is unfortunate since MD uses chunk_sectors as the term for what you call stride. > Well, at least, that is how I interpret the io_min definition of > minimum_io_size in Documentation/ABI/stable/sysfs-block. But the wording "For > RAID arrays it is often the stripe chunk size." is super confusing. Not > entirely sure if stride or stripe was meant here... The stripe chunk or stripe unit is what you call stride. Stripe width is the full stripe across all drives. > As for read_ahead_kb, it should be bounded by io_opt (upper bound) but > should be initialized to a smaller value aligned to io_min (if io_opt > is unreasonably large). In retrospect I am not really a fan of using io_opt for read_ahead_kb since, to my knowledge, there is no guarantee that the readahead I/O will be naturally aligned. That said, I don't really know of devices where this matters much for reads. With writes, this would be much more of an issue. -- Martin K. Petersen