checkpoint_flush_after参数,官方解释是: >Whenever more than checkpoint_flush_after bytes have been written while performing a checkpoint, attempt to force the OS to issue these writes to the underlying storage. Doing so will limit the amount of dirty data in the kernel's page cache, reducing the likelihood of stalls when an fsync is issued at the end of the checkpoint, or when the OS writes data back in larger batches in the background. Often that will result in greatly reduced transaction latency, but there also are some cases, especially with workloads that are bigger than shared_buffers, but smaller than the OS's page cache, where performance might degrade. This setting may have no effect on some platforms. The valid range is between 0, which disables forced writeback, and 2MB. The default is 256kB on Linux, 0 elsewhere. (If BLCKSZ is not 8kB, the default and maximum values scale proportionally to it.) This parameter can only be set in the postgresql.conf file or on the server command line. 在CentOS 7.x下使用sysbench测试,尝试0/256kB/2MB,只有可以忽略不计的细微区别. ``` ------------------------------------- checkpoint_flush_after = 2MB ------------------------------------- SQL statistics: queries performed: read: 107367 write: 111268 other: 16840 total: 235475 transactions: 8419 (84.16 per sec.) queries: 235475 (2353.96 per sec.) ignored errors: 43 (0.43 per sec.) reconnects: 0 (0.00 per sec.) Throughput: events/s (eps): 84.1616 time elapsed: 100.0337s total number of events: 8419 Latency (ms): min: 2.56 avg: 23.76 max: 311.66 95th percentile: 62.19 sum: 200002.13 Threads fairness: events (avg/stddev): 4209.5000/48.50 execution time (avg/stddev): 100.0011/0.00 ------------------------------------- checkpoint_flush_after = 256kB ------------------------------------- SQL statistics: queries performed: read: 110021 write: 114168 other: 17072 total: 241261 transactions: 8535 (85.31 per sec.) queries: 241261 (2411.50 per sec.) ignored errors: 29 (0.29 per sec.) reconnects: 0 (0.00 per sec.) Throughput: events/s (eps): 85.3106 time elapsed: 100.0462s total number of events: 8535 Latency (ms): min: 2.62 avg: 23.43 max: 260.37 95th percentile: 61.08 sum: 200017.57 Threads fairness: events (avg/stddev): 4267.5000/37.50 execution time (avg/stddev): 100.0088/0.02 ------------------------------------- checkpoint_flush_after = 0 ------------------------------------- SQL statistics: queries performed: read: 111431 write: 115279 other: 17380 total: 244090 transactions: 8689 (86.86 per sec.) queries: 244090 (2440.19 per sec.) ignored errors: 42 (0.42 per sec.) reconnects: 0 (0.00 per sec.) Throughput: events/s (eps): 86.8646 time elapsed: 100.0293s total number of events: 8689 Latency (ms): min: 2.60 avg: 23.02 max: 213.39 95th percentile: 59.99 sum: 199980.96 Threads fairness: events (avg/stddev): 4344.5000/16.50 execution time (avg/stddev): 99.9905/0.00 ``` **参考资料** [PG doc](https://www.postgresql.org/docs/12/runtime-config-wal.html)