Aerospike SSD模式下，刷写磁盘和写入数据swb的关系

void
ssd_flush_current_swb(drv_ssd *ssd, uint64_t *p_prev_n_writes,
    uint32_t *p_prev_size)
{
  uint64_t n_writes = cf_atomic64_get(ssd->n_wblock_writes);
  //ssd->n_wblock_writes表示ssd->swb_write_q队列中有多少个swb，该swb已写满脏数据
  // 如果swb_write_q队列中有脏数据的swb，则需要将该swb先刷写完成，这里不能先刷写current_swb
  if (n_writes != *p_prev_n_writes) {
    *p_prev_n_writes = n_writes;
    *p_prev_size = 0;
    return;
  }
  //因为current_swb刷写时，其他线程可能正在向里面写入，所以需要加锁；
  //swb_write_q中的swb不需要，因为已写满，其他的线程不能向里写了
  pthread_mutex_lock(&ssd->write_lock);
  n_writes = cf_atomic64_get(ssd->n_wblock_writes);
  // 还需要在锁里面再检查一次。因为可能正好有一个swb放到了swb_write_q里了
  if (n_writes != *p_prev_n_writes) {
    pthread_mutex_unlock(&ssd->write_lock);
    *p_prev_n_writes = n_writes;
    *p_prev_size = 0;
    return;
  }
  //如果current_swb不为空，那么可以刷。刷前需要将剩下的部分全部置成0清空
  ssd_write_buf *swb = ssd->current_swb;
  if (swb && swb->pos != *p_prev_size) {
    *p_prev_size = swb->pos;
    // Clean the end of the buffer before flushing.
    if (ssd->write_block_size != swb->pos) {
      memset(&swb->buf[swb->pos], 0, ssd->write_block_size - swb->pos);
    }
    //刷写到磁盘：swb->wblock_id*ssd->write_block_size为lseek的偏移，
    //写入大小是ssd->write_block_size
    ssd_flush_swb(ssd, swb);
  }
  pthread_mutex_unlock(&ssd->write_lock);
}

int
ssd_write_bins(as_storage_rd *rd)
{
  ...
  //在ssd->write_lock锁内进行操作，向里写入脏数据
  //如果current_swb为NULL，则从ssd->swb_free_q拿一个
  pthread_mutex_lock(&ssd->write_lock);
  ssd_write_buf *swb = ssd->current_swb;
  if (! swb) {
    swb = swb_get(ssd);
    ssd->current_swb = swb;
  }
  //如果current_swb空间不够了：（将剩下的清0），将swb放到swb_write_q队列
  //ssd->n_wblock_writes加一，表示队列里多了一个成员
  //从swb_free_q中拿一个当做current_swb
  if (write_size > ssd->write_block_size - swb->pos) {
    if (ssd->write_block_size != swb->pos) {
      // Clean the end of the buffer before pushing to write queue.
      memset(&swb->buf[swb->pos], 0, ssd->write_block_size - swb->pos);
    }
    cf_queue_push(ssd->swb_write_q, &swb);
    cf_atomic64_incr(&ssd->n_wblock_writes);
    swb = swb_get(ssd);
    ssd->current_swb = swb;
    ...
  }
  uint32_t swb_pos = swb->pos;
  swb->pos += write_size;
  cf_atomic32_incr(&swb->n_writers);
  pthread_mutex_unlock(&ssd->write_lock);
  ...
  //向swb写入
}

void *
run_ssd_maintenance(void *udata)
{
  ...
  uint64_t prev_n_writes_flush = 0;
  uint32_t prev_size_flush = 0;
  while (true) {
    ...
    ssd_flush_current_swb(ssd, &prev_n_writes_flush, &prev_size_flush);
    ...
  }
}

结论：

        该函数是刷写current_swb的后台线程。prev_n_writes_flush的入参是0，结合上面2个函数的介绍如果ssd_write_bins函数在写入时，current_swb满了，将他放到swb_write_q队列，那么这里current_swb就不能刷写了，需要等待ssd_write_worker后台线程将swb_write_q队列的脏数据刷写后再刷。从而保证数据一致性。