block: ignore underlying non-stack devices io_opt #229
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added 2 commits
August 17, 2025 15:56
This patch adds a new BLK_FLAG_STACK_IO_OPT for stack block device. If a stack block device like md raid5 declares its io_opt when don't want blk_stack_limits() to change it with io_opt of underlying non-stack block devices, BLK_FLAG_STACK_IO_OPT can be set on limits.flags. Then in blk_stack_limits(), lcm_not_zero(t->io_opt, b->io_opt) will be avoided. For md raid5, it is necessary to keep a proper io_opt size for better I/O thoughput. Signed-off-by: Coly Li <[email protected]>
Currently in md_submit_bio() the incoming request bio is split by bio_split_to_limits() which makes sure the bio won't exceed max_hw_sectors of a specific raid level before senting into its .make_request method. For raid level 4/5/6 such split method might be problematic and hurt large read/write perforamnce. Because limits.max_hw_sectors are not always aligned to limits.io_opt size, the split bio won't be full stripes covered on all data disks, and will introduce extra read-in I/O. Even the bio's bi_sector is aligned to limits.io_opt size and large enough, the resulted split bio is not size-friendly to corresponding raid456 level. This patch introduces bio_split_by_io_opt() to solve the above issue, 1, If the incoming bio is not limits.io_opt aligned, split the non- aligned head part. Then the next one will be aligned. 2, If the imcoming bio is limits.io_opt aligned, and split is necessary, then try to split a by multiple of limits.io_opt but not exceed limits.max_hw_sectors. Then for large bio, the sligned split part will be full-stripes covered to all data disks, no extra read-in I/Os when rmw_level is 0. And for rmw_level > 0 condistions, the limits.io_opt aligned bios are welcomed for performace as well. This patch only tests on 8 disks raid5 array with 64KiB chunk size. By this patch, 64KiB chunk size for a 8 disks raid5 array, sequential write performance increases from 900MiB/s to 1.1GiB/s by fio bs=10M. If fio bs=488K (exact limits.io_opt size) the peak sequential write throughput can reach 1.51GiB/s. Signed-off-by: Coly Li <[email protected]>
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Upstream branch: c17fb54 |
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Pull request for series with
subject: block: ignore underlying non-stack devices io_opt
version: 1
url: https://patchwork.kernel.org/project/linux-raid/list/?series=992286