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    raid1: Rewrite the implementation of iobarrier. · 79ef3a8a
    majianpeng 提交于
    There is an iobarrier in raid1 because of contention between normal IO and
    resync IO.  It suspends all normal IO when resync/recovery happens.
    
    However if normal IO is out side the resync window, there is no contention.
    So this patch changes the barrier mechanism to only block IO that
    could contend with the resync that is currently happening.
    
    We partition the whole space into five parts.
    |---------|-----------|------------|----------------|-------|
            start   next_resync   start_next_window    end_window
    
    start + RESYNC_WINDOW = next_resync
    next_resync + NEXT_NORMALIO_DISTANCE = start_next_window
    start_next_window + NEXT_NORMALIO_DISTANCE = end_window
    
    Firstly we introduce some concepts:
    
    1 - RESYNC_WINDOW: For resync, there are 32 resync requests at most at the
          same time. A sync request is RESYNC_BLOCK_SIZE(64*1024).
          So the RESYNC_WINDOW is 32 * RESYNC_BLOCK_SIZE, that is 2MB.
    2 - NEXT_NORMALIO_DISTANCE: the distance between next_resync
          and start_next_window.  It also indicates the distance between
          start_next_window and end_window.
          It is currently 3 * RESYNC_WINDOW_SIZE but could be tuned if
          this turned out not to be optimal.
    3 - next_resync: the next sector at which we will do sync IO.
    4 - start: a position which is at most RESYNC_WINDOW before
          next_resync.
    5 - start_next_window:  a position which is NEXT_NORMALIO_DISTANCE
          beyond next_resync.  Normal-io after this position doesn't need to
          wait for resync-io to complete.
    6 - end_window:  a position which is 2 * NEXT_NORMALIO_DISTANCE beyond
          next_resync.  This also doesn't need to wait, but is counted
          differently.
    7 - current_window_requests:  the count of normalIO between
          start_next_window and end_window.
    8 - next_window_requests: the count of normalIO after end_window.
    
    NormalIO will be partitioned into four types:
    
    NormIO1:  the end sector of bio is smaller or equal the start
    NormIO2:  the start sector of bio larger or equal to end_window
    NormIO3:  the start sector of bio larger or equal to
              start_next_window.
    NormIO4:  the location between start_next_window and end_window
    
    |--------|-----------|--------------------|----------------|-------------|
        | start   |   next_resync   |  start_next_window   |  end_window |
     NormIO1   NormIO4            NormIO4                NormIO3      NormIO2
    
    For NormIO1, we don't need any io barrier.
    For NormIO4, we used a similar approach to the original iobarrier
        mechanism.  The normalIO and resyncIO must be kept separate.
    For NormIO2/3, we add two fields to struct r1conf: "current_window_requests"
        and "next_window_requests". They indicate the count of active
        requests in the two window.
        For these, we don't wait for resync io to complete.
    
    For resync action, if there are NormIO4s, we must wait for it.
    If not, we can proceed.
    But if resync action reaches start_next_window and
    current_window_requests > 0 (that is there are NormIO3s), we must
    wait until the current_window_requests becomes zero.
    When current_window_requests becomes zero,  start_next_window also
    moves forward. Then current_window_requests will replaced by
    next_window_requests.
    
    There is a problem which when and how to change from NormIO2 to
    NormIO3.  Only then can sync action progress.
    
    We add a field in struct r1conf "start_next_window".
    
    A: if start_next_window == MaxSector, it means there are no NormIO2/3.
       So start_next_window = next_resync + NEXT_NORMALIO_DISTANCE
    B: if current_window_requests == 0 && next_window_requests != 0, it
       means start_next_window move to end_window
    
    There is another problem which how to differentiate between
    old NormIO2(now it is NormIO3) and NormIO2.
    For example, there are many bios which are NormIO2 and a bio which is
    NormIO3. NormIO3 firstly completed, so the bios of NormIO2 became NormIO3.
    
    We add a field in struct r1bio "start_next_window".
    This is used to record the position conf->start_next_window when the call
    to wait_barrier() is made in make_request().
    
    In allow_barrier(), we check the conf->start_next_window.
    If r1bio->stat_next_window == conf->start_next_window, it means
    there is no transition between NormIO2 and NormIO3.
    If r1bio->start_next_window != conf->start_next_window, it mean
    there was a transition between NormIO2 and NormIO3.  There can only
    have been one transition.  So it only means the bio is old NormIO2.
    
    For one bio, there may be many r1bio's. So we make sure
    all the r1bio->start_next_window are the same value.
    If we met blocked_dev in make_request(), it must call allow_barrier
    and wait_barrier. So the former and the later value of
    conf->start_next_window will be change.
    If there are many r1bio's with differnet start_next_window,
    for the relevant bio, it depend on the last value of r1bio.
    It will cause error. To avoid this, we must wait for previous r1bios
    to complete.
    Signed-off-by: NJianpeng Ma <majianpeng@gmail.com>
    Signed-off-by: NNeilBrown <neilb@suse.de>
    79ef3a8a
raid1.h 5.1 KB