• shaohua.li@intel.com's avatar
    block: hold queue if flush is running for non-queueable flush drive · 3ac0cc45
    shaohua.li@intel.com authored
    In some drives, flush requests are non-queueable. When flush request is
    running, normal read/write requests can't run. If block layer dispatches
    such request, driver can't handle it and requeue it.  Tejun suggested we
    can hold the queue when flush is running. This can avoid unnecessary
    requeue.  Also this can improve performance. For example, we have
    request flush1, write1, flush 2. flush1 is dispatched, then queue is
    hold, write1 isn't inserted to queue. After flush1 is finished, flush2
    will be dispatched. Since disk cache is already clean, flush2 will be
    finished very soon, so looks like flush2 is folded to flush1.
    In my test, the queue holding completely solves a regression introduced by
    commit 53d63e6b
        block: make the flush insertion use the tail of the dispatch list
        It's not a preempt type request, in fact we have to insert it
        behind requests that do specify INSERT_FRONT.
    which causes about 20% regression running a sysbench fileio
    Stable: 2.6.39 only
    Cc: stable@kernel.org
    Signed-off-by: default avatarShaohua Li <shaohua.li@intel.com>
    Acked-by: default avatarTejun Heo <tj@kernel.org>
    Signed-off-by: default avatarJens Axboe <jaxboe@fusionio.com>