• Shaohua Li's avatar
    raid5: only dispatch IO from raid5d for harddisk raid · 765d704d
    Shaohua Li authored
    We made raid5 stripe handling multi-thread before. It works well for
    SSD. But for harddisk, the multi-threading creates more disk seek, so
    not always improve performance. For several hard disks based raid5,
    multi-threading is required as raid5d becames a bottleneck especially
    for sequential write.
    
    To overcome the disk seek issue, we only dispatch IO from raid5d if the
    array is harddisk based. Other threads can still handle stripes, but
    can't dispatch IO.
    
    Idealy, we should control IO dispatching order according to IO position
    interrnally. Right now we still depend on block layer, which isn't very
    efficient sometimes though.
    
    My setup has 9 harddisks, each disk can do around 180M/s sequential
    write. So in theory, the raid5 can do 180 * 8 = 1440M/s sequential
    write. The test machine uses an ATOM CPU. I measure sequential write
    with large iodepth bandwidth to raid array:
    
    without patch: ~600M/s
    without patch and group_thread_cnt=4: 750M/s
    with patch and group_thread_cnt=4: 950M/s
    with patch, group_thread_cnt=4, skip_copy=1: 1150M/s
    
    We are pretty close to the maximum bandwidth in the large iodepth
    iodepth case. The performance gap of small iodepth sequential write
    between software raid and theory value is still very big though, because
    we don't have an efficient pipeline.
    
    Cc: NeilBrown <neilb@suse.com>
    Cc: Song Liu <songliubraving@fb.com>
    Signed-off-by: default avatarShaohua Li <shli@fb.com>
    765d704d
raid5.h 28.9 KB