• Shaohua Li's avatar
    raid5: add an option to avoid copy data from bio to stripe cache · d592a996
    Shaohua Li authored
    The stripe cache has two goals:
    1. cache data, so next time if data can be found in stripe cache, disk access
    can be avoided.
    2. stable data. data is copied from bio to stripe cache and calculated parity.
    data written to disk is from stripe cache, so if upper layer changes bio data,
    data written to disk isn't impacted.
    
    In my environment, I can guarantee 2 will not happen. And BDI_CAP_STABLE_WRITES
    can guarantee 2 too. For 1, it's not common too. block plug mechanism will
    dispatch a bunch of sequentail small requests together. And since I'm using
    SSD, I'm using small chunk size. It's rare case stripe cache is really useful.
    
    So I'd like to avoid the copy from bio to stripe cache and it's very helpful
    for performance. In my 1M randwrite tests, avoid the copy can increase the
    performance more than 30%.
    
    Of course, this shouldn't be enabled by default. It's reported enabling
    BDI_CAP_STABLE_WRITES can harm some workloads before, so I added an option to
    control it.
    
    Neilb:
      changed BUG_ON to WARN_ON
      Removed some assignments from raid5_build_block which are now not needed.
    Signed-off-by: default avatarShaohua Li <shli@fusionio.com>
    Signed-off-by: default avatarNeilBrown <neilb@suse.de>
    d592a996
raid5.h 21.4 KB