本節簡單介紹了PostgreSQL 12 在日誌記錄上的改進：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

對於每一個客戶端連線，PG都會請求Postmaster，然後fork一個後臺程式來處理請求，Postmaster期望每一個客戶端請求傳送startup message給PG Server，startup packet中的資訊用於配置fork的後臺程式。對於埠掃描、HA解決方案中的心跳檢測等等都會發請求給PG Server port，PG會啟動程式來處理這些連線，但安全檢測、HA指令碼等不同於常規的客戶端，對於這些請求，PG會產生一條日誌條目，因此會造成日誌檔案的膨脹而導致不必要的IO開銷。

**PG 11**
使用工具nc來訪問資料庫埠，日誌中會產生無用的日誌條目。
```
[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.
...

```
資料庫日誌輸出
```
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",,,,,,,,,""
...
```
這些日誌其實是無用的日誌資訊，可以不作記錄。

**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 ~]$ 
...
```
資料庫日誌資訊，相對於PG 11，沒有出現無用的日誌資訊
```
[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".

```

**參考資料**
[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)

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

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