lightdb/postgresql pg_control文件的作用

　　记录了lightdb服务器内部的信息，如最新检查点时间、服务器状态、WAL文件位置，总之和数据库可能损坏相关的信息这里都有维护，还包括LC_CTYPE/LC_COLLATE等信息，和oracle控制文件差不多的。​​如果该文件被删除，实例将无法启动​​。运行时，会在共享内存中维护一份pg_control的镜像。

查看pg_controldata文件的内容

　　有两种方式，一种是通过SQL查询，如下：

select * from pg_catalog.pg_control_checkpoint();
select * from pg_catalog.pg_control_system();
select * from pg_catalog.pg_control_init();  -- 获取结果和pg_controldata相同
select * from pg_catalog.pg_control_recovery();

　　还有一种是通过命令行。如下：

[postgres@hs-10-20-30-194 global]$ lt_controldata ../../data
pg_control version number:            1300
Catalog version number:               202105121
Database system identifier:           6965242430856727465
Database cluster state:               shut down
pg_control last modified:             Tue 25 May 2021 01:10:07 AM CST
Latest checkpoint location:           17/7C6E96E0
Latest checkpoint's REDO location:    17/7C6E96E0
Latest checkpoint's REDO WAL file:    00000001000000170000007C
Latest checkpoint's TimeLineID:       1
Latest checkpoint's PrevTimeLineID:   1
Latest checkpoint's full_page_writes: on
Latest checkpoint's NextXID:          0:46350725
Latest checkpoint's NextOID:          16818
Latest checkpoint's NextMultiXactId:  1
Latest checkpoint's NextMultiOffset:  0
Latest checkpoint's oldestXID:        727
Latest checkpoint's oldestXID's DB:   1
Latest checkpoint's oldestActiveXID:  0
Latest checkpoint's oldestMultiXid:   1
Latest checkpoint's oldestMulti's DB: 1
Latest checkpoint's oldestCommitTsXid:0
Latest checkpoint's newestCommitTsXid:0
Time of latest checkpoint:            Tue 25 May 2021 01:09:54 AM CST
Fake LSN counter for unlogged rels:   0/3E8
Minimum recovery ending location:     0/0
Min recovery ending loc's timeline:   0
Backup start location:                0/0
Backup end location:                  0/0
End-of-backup record required:        no
wal_level setting:                    replica
wal_log_hints setting:                on
max_connections setting:              10000
max_worker_processes setting:         8
max_wal_senders setting:              10
max_prepared_xacts setting:           0
max_locks_per_xact setting:           64
track_commit_timestamp setting:       off
Maximum data alignment:               8
Database block size:                  8192
Blocks per segment of large relation: 131072
WAL block size:                       8192
Bytes per WAL segment:                16777216
Maximum length of identifiers:        64
Maximum columns in an index:          32
Maximum size of a TOAST chunk:        1996
Size of a large-object chunk:         2048
Date/time type storage:               64-bit integers
Float8 argument passing:              by value
Data page checksum version:           0
Mock authentication nonce:            11d5c8d76b72e3c68abe49250c775aa8a0a5e50ec275757b002b74d9547674d0

 　　pg_control控制文件中的内容由ControlFileData定义，在initdb/lt_initdb初始化时创建。