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bool                BgBufferSync(void)                {                    ……                /*                 * Information saved between calls so we can determine the strategy                 * point's advance rate and avoid scanning already-cleaned buffers.                 */                static           bool          saved_info_valid = false;                static           int        prev_strategy_buf_id;        static          uint32         prev_strategy_passes;        static          int        next_to_clean;        static     uint32     next_passes;                      /* Moving averages of allocation rate and clean-buffer density */                static     float     smoothed_alloc = 0;        static     float     smoothed_density = 10.0;                        ……                             if (saved_info_valid)                {                    ……                strategy_delta = strategy_buf_id - prev_strategy_buf_id;                strategy_delta += (long) passes_delta *NBuffers;                ……         }                else                {                    ……            }                fprintf(stderr,"before saving, prev_strategy_buf_id is %d \n", prev_strategy_buf_id);                /* Update saved info for next time */                prev_strategy_buf_id = strategy_buf_id;                prev_strategy_passes = strategy_passes;                saved_info_valid = true;                                ……                                fprintf(stderr,"prev_strategy_buf_id is %d \n", prev_strategy_buf_id);                /* Return true if OK to hibernate */                return (bufs_to_lap == 0 && recent_alloc == 0);            }

执行的结果显示，即使我用psql 变更了某行的值，直到最后，prev_strategy_buf_id 的值也没有发生根本性的变化。

[postgres@localhost bin]$ ./postgres -D /usr/local/pgsql/data        LOG:  database system was shut down at 2012-11-02 15:30:38 CST        saved_info_valid false.        before saving,prev_strategy_buf_id is: 0        prev_strategy_buf_id is 0         LOG:  autovacuum launcher started        LOG:  database system is ready to accept connections        before saving,prev_strategy_buf_id is: 0        prev_strategy_buf_id is 0         before saving,prev_strategy_buf_id is: 0        prev_strategy_buf_id is 0         before saving,prev_strategy_buf_id is: 0        prev_strategy_buf_id is 0         before saving,prev_strategy_buf_id is: 0        prev_strategy_buf_id is 0         before saving,prev_strategy_buf_id is: 0        prev_strategy_buf_id is 0                 ……                        before saving,prev_strategy_buf_id is: 0        prev_strategy_buf_id is 0         LOG:  received fast shutdown request        LOG:  aborting any active transactions        LOG:  autovacuum launcher shutting down        LOG:  shutting down        LOG:  database system is shut down        [postgres@localhost bin]$

[作者：技术者高健@博客园  mail:  ]

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