• Shaohua Li's avatar
    md/raid1: prevent merging too large request · 12cee5a8
    Shaohua Li authored
    For SSD, if request size exceeds specific value (optimal io size), request size
    isn't important for bandwidth. In such condition, if making request size bigger
    will cause some disks idle, the total throughput will actually drop. A good
    example is doing a readahead in a two-disk raid1 setup.
    So when should we split big requests? We absolutly don't want to split big
    request to very small requests. Even in SSD, big request transfer is more
    efficient. This patch only considers request with size above optimal io size.
    If all disks are busy, is it worth doing a split? Say optimal io size is 16k,
    two requests 32k and two disks. We can let each disk run one 32k request, or
    split the requests to 4 16k requests and each disk runs two. It's hard to say
    which case is better, depending on hardware.
    So only consider case where there are idle disks. For readahead, split is
    always better in this case. And in my test, below patch can improve > 30%
    thoughput. Hmm, not 100%, because disk isn't 100% busy.
    Such case can happen not just in readahead, for example, in directio. But I
    suppose directio usually will have bigger IO depth and make all disks busy, so
    I ignored it.
    Note: if the raid uses any hard disk, we don't prevent merging. That will make
    performace worse.
    Signed-off-by: default avatarShaohua Li <shli@fusionio.com>
    Signed-off-by: default avatarNeilBrown <neilb@suse.de>
raid1.h 4.44 KB