从源代码上理解 PostgreSQL 的 bgwriter_lru_maxpages

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。