先看代码：src\backend\storage\buffer\bufmgr.c
/* * BgBufferSync -- Write out some dirty buffers in the pool. * * This is called periodically by the background writer process. * * Returns true if it's appropriate for the bgwriter process to go into * low-power hibernation mode. (This happens if the strategy clock sweep * has been "lapped" and no buffer allocations have occurred recently, * or if the bgwriter has been effectively disabled by setting * bgwriter_lru_maxpages to 0.) */ bool BgBufferSync(void) { …… /* * If we're not running the LRU scan, just stop after doing the stats * stuff. We mark the saved state invalid so that we can recover sanely * if LRU scan is turned back on later. */ if (bgwriter_lru_maxpages <= 0) { saved_info_valid = false; return true; } …… /* * Now write out dirty reusable buffers, working forward from the * next_to_clean point, until we have lapped the strategy scan, or cleaned * enough buffers to match our estimate of the next cycle's allocation * requirements, or hit the bgwriter_lru_maxpages limit. */ /* Make sure we can handle the pin inside SyncOneBuffer */ ResourceOwnerEnlargeBuffers(CurrentResourceOwner); num_to_scan = bufs_to_lap; num_written = 0; reusable_buffers = reusable_buffers_est; /* Execute the LRU scan */ while (num_to_scan > 0 && reusable_buffers < upcoming_alloc_est) { int buffer_state = SyncOneBuffer(next_to_clean, true); if (++next_to_clean >= NBuffers) { next_to_clean = 0; next_passes++; } num_to_scan--; if (buffer_state & BUF_WRITTEN) { reusable_buffers++; if (++num_written >= bgwriter_lru_maxpages) { BgWriterStats.m_maxwritten_clean++; break; } } else if (buffer_state & BUF_REUSABLE) reusable_buffers++; } BgWriterStats.m_buf_written_clean += num_written; …… /* Return true if OK to hibernate */ return (bufs_to_lap == 0 && recent_alloc == 0); } 从上述代码看出：
开宗明义，人家已经在注释中说了： This is called periodically by the background writer process.
而对于 bgwriter_lru_maxpages：
/*
* If we're not running the LRU scan, just stop after doing the stats * stuff. We mark the saved state invalid so that we can recover sanely * if LRU scan is turned back on later. */ if (bgwriter_lru_maxpages <= 0) { saved_info_valid = false; return true; }如果 bgwriter_lru_maxpages <=0，则立即返回。根本不进行 脏数据读写。
再看：
while (num_to_scan > 0 && reusable_buffers < upcoming_alloc_est)
{ …… if (buffer_state & BUF_WRITTEN) { reusable_buffers++; if (++num_written >= bgwriter_lru_maxpages) { BgWriterStats.m_maxwritten_clean++; break; } } …… }一旦超过 bgwriter_lru_maxpages，也将停止再写入。