The number of times a APF-initiated buffer fetch attempt was
    successful in fetching the buffer. This is possible only if an EX
    lock can be obtained for the whole buffer. A successful APF fetch
    may actually issue an asynchronous read or simply ensure that a
    buffer that was already in the buffer pool now moves to the front
    of the LRU queue. In the latter case, the buffer then becomes
    least likely to be removed from the buffer pool.