Hi NeilBrown, Thanks for your kindly reply. Originally, I doubt that would it affect the performance or not, but I agree with your point. Thanks. 2016-06-02 12:38 GMT+08:00 NeilBrown <nfbrown@xxxxxxxxxx>: > On Mon, May 30 2016, Joey Liao wrote: > >> Hi, >> >> I have no idea why does raid0_run() in raid0.c use >> blk_queue_max_hw_sectors() to set max_hw_sectors as the chunk size, >> but the other raid types doesn't? > > git is your friend.... admittedly you need the 'history' git tree to go > back before 2.6.12, but it is available. > > http://git.kernel.org/cgit/linux/kernel/git/history/history.git/commit/?id=f556ef000efc90a45a285f4f0b4fd70bb70f > > > >> >> What's the purpose to limit the max_hw_sectors in raid0? > > unfortunately the commit doesn't answer that question. I think it was > to ensure requests larger than one chunk were not created. If they were > they would just have to be split, so there is no much to gain. > >> >> Is it related to the source code logic issue or the performance issue? >> >> Besides, I have an interesting observation. If I remove all the >> following queue limitation codes in raid0_run() of raid0.c, the block >> size in iostat is still the same as chunk size even the input block >> size is much larger than the chunk size. Why??? > > Because when you write to a RAID0 you *must* divide each request up into > chunk-sizes sub-requests, and send them to different devices. > > NeilBrown > > >> >> - blk_queue_max_hw_sectors(mddev->queue, mddev->chunk_sectors); >> - blk_queue_max_write_same_sectors(mddev->queue, mddev->chunk_sectors); >> + //blk_queue_max_hw_sectors(mddev->queue, mddev->chunk_sectors); >> + //blk_queue_max_write_same_sectors(mddev->queue, mddev->chunk_sectors); >> >> - blk_queue_io_min(mddev->queue, mddev->chunk_sectors << 9); >> - blk_queue_io_opt(mddev->queue, >> - (mddev->chunk_sectors << 9) * mddev->raid_disks); >> + //blk_queue_io_min(mddev->queue, mddev->chunk_sectors << 9); >> + /*blk_queue_io_opt(mddev->queue, >> + (mddev->chunk_sectors << 9) * mddev->raid_disks);*/ >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-raid" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
