How it works…

The pg_controldata utility is a PostgreSQL server application that shows the content of a server's control file. The control file is located within the data directory of a server, and it is created at database initialization time. Some of the information within it is updated regularly, and some when certain major events occur.

The full output of pg_controldata looks like the following (the bold values are those that may change over time as the server runs):

pg_control version number: 1002
Catalog version number: 201707211
Database system identifier: 6320137769675861859
Database cluster state: in production
pg_control last modified: Sun 31 Dec 2017 16:17:41 PM GMT
Latest checkpoint location: 0/19000728
Prior checkpoint location: 0/19000648
Latest checkpoint's REDO location: 0/190006F0
Latest checkpoint's REDO WAL file: 000000010000000000000019
Latest checkpoint's TimeLineID: 1
Latest checkpoint's PrevTimeLineID: 1
Latest checkpoint's full_page_writes: on
Latest checkpoint's NextXID: 0/703
Latest checkpoint's NextOID: 24978
Latest checkpoint's NextMultiXactId: 1
Latest checkpoint's NextMultiOffset: 0
Latest checkpoint's oldestXID: 616
Latest checkpoint's oldestXID's DB: 1
Latest checkpoint's oldestActiveXID: 703
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: Sun 31 Dec 2017 16:17:41 PM GMT
Fake LSN counter for unlogged rels: 0/1
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: off
max_connections setting: 100
max_worker_processes setting: 8
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
Float4 argument passing: by value
Float8 argument passing: by value
Data page checksum version: 0
Mock authentication nonce   a6b19bbbd477… (not shown in full

Never edit the PostgreSQL control file. If you do, the server probably won't start correctly, or you may mask other errors. And if you do that, people will be able to tell, so fess up as soon as possible!