本节简单介绍了PostgreSQL 12 在日志记录上的改进：Don't log incomplete startup packet if it's empty。
>This will stop logging cases where, for example, a monitor opens a connection and immediately closes it. If the packet contains any data an incomplete packet will still be logged.
Author: Tom Lane

对于每一个客户端连接，PG都会请求Postmaster，然后fork一个后台进程来处理请求，Postmaster期望每一个客户端请求发送startup message给PG Server，startup packet中的信息用于配置fork的后台进程。对于端口扫描、HA解决方案中的心跳检测等等都会发请求给PG Server port，PG会启动进程来处理这些连接，但安全检测、HA脚本等不同于常规的客户端，对于这些请求，PG会产生一条日志条目，因此会造成日志文件的膨胀而导致不必要的IO开销。

**PG 11**
使用工具nc来访问数据库端口，日志中会产生无用的日志条目。
```
[xdb@localhost ~]$ psql -c 'select version();'
Timing is on.
Expanded display is used automatically.
                                                 version                                                 
---------------------------------------------------------------------------------------------------------
 PostgreSQL 11.2 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-16), 64-bit
(1 row)

Time: 56.253 ms
[xdb@localhost ~]$

[xdb@localhost ~]$ for i in {1..100}; do      nc -zv localhost 5110 ; done
Ncat: 0 bytes sent, 0 bytes received in 0.01 seconds.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to ::1:5110.
Ncat: 0 bytes sent, 0 bytes received in 0.01 seconds.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to ::1:5110.
Ncat: 0 bytes sent, 0 bytes received in 0.01 seconds.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to ::1:5110.
Ncat: 0 bytes sent, 0 bytes received in 0.01 seconds.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to ::1:5110.
Ncat: 0 bytes sent, 0 bytes received in 0.01 seconds.
...

```
数据库日志输出
```
2019-12-09 14:35:55.468 CST,,,16065,"::1:57554",5dedeb4b.3ec1,1,"",2019-12-09 14:35:55 CST,,0,LOG,08P01,"incomplete startup packet",,,,,,,,,""
2019-12-09 14:35:55.479 CST,,,16067,"::1:57556",5dedeb4b.3ec3,1,"",2019-12-09 14:35:55 CST,,0,LOG,08P01,"incomplete startup packet",,,,,,,,,""
2019-12-09 14:35:55.490 CST,,,16069,"::1:57558",5dedeb4b.3ec5,1,"",2019-12-09 14:35:55 CST,,0,LOG,08P01,"incomplete startup packet",,,,,,,,,""
2019-12-09 14:35:55.503 CST,,,16071,"::1:57560",5dedeb4b.3ec7,1,"",2019-12-09 14:35:55 CST,,0,LOG,08P01,"incomplete startup packet",,,,,,,,,""
2019-12-09 14:35:55.513 CST,,,16073,"::1:57562",5dedeb4b.3ec9,1,"",2019-12-09 14:35:55 CST,,0,LOG,08P01,"incomplete startup packet",,,,,,,,,""
...
```
这些日志其实是无用的日志信息，可以不作记录。

**PG 12**
```
[xdb@localhost ~]$ psql -h localhost -p 5120 -U pg12 -c 'select version();'
Timing is on.
Expanded display is used automatically.
                                                 version                                                 
---------------------------------------------------------------------------------------------------------
 PostgreSQL 12.1 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-16), 64-bit
(1 row)

Time: 60.207 ms
[xdb@localhost ~]$ 
[xdb@localhost ~]$ for i in {1..100}; do      nc -zv localhost 5120 ; done
Ncat: 0 bytes sent, 0 bytes received in 0.01 seconds.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to ::1:5120.
Ncat: 0 bytes sent, 0 bytes received in 0.01 seconds.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to ::1:5120.
Ncat: 0 bytes sent, 0 bytes received in 0.01 seconds.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to ::1:5120.
Ncat: 0 bytes sent, 0 bytes received in 0.01 seconds.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to ::1:5120.
Ncat: 0 bytes sent, 0 bytes received in 0.01 seconds.
Ncat: Version 7.50 ( https://nmap.org/ncat )
Ncat: Connected to ::1:5120.
Ncat: 0 bytes sent, 0 bytes received in 0.01 seconds.
[xdb@localhost ~]$ 
...
```
数据库日志信息，相对于PG 11，没有出现无用的日志信息
```
[pg12@localhost ~]$ tail -f $PGDATA/pg_log/postgresql-2019-12-09.log 
2019-12-09 14:18:59.317 CST [1813] LOG:  ending log output to stderr
2019-12-09 14:18:59.317 CST [1813] HINT:  Future log output will go to log destination "csvlog".

```

**参考资料**
[PostgreSQL 12 Improvement: Benign Log Entries “Incomplete Startup Packet”
](https://www.percona.com/blog/2019/12/03/postgresql-12-improvement-benign-log-entries-incomplete-startup-packet/)
[PATCH](https://git.postgresql.org/gitweb/?p=postgresql.git;a=patch;h=342cb650e)

ITPub博客

PostgreSQL DBA(140) - PG 12（Don't log incomplete startup packet if it's empty）

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