프로세스 구조

$ pg_ctl -D $PGDATA -l /home/postgres/server.log start
waiting for server to start.... done
server started
$ PM=$(head -1 $PGDATA/postmaster.pid); ps -o pid,ppid,cmd --forest -p $PM --ppid $PM
    PID    PPID CMD
     33       1 /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
     34      33  \_ postgres: io worker 0
     35      33  \_ postgres: io worker 1
     36      33  \_ postgres: io worker 2
     37      33  \_ postgres: checkpointer 
     38      33  \_ postgres: background writer 
     40      33  \_ postgres: walwriter 
     41      33  \_ postgres: autovacuum launcher 
     42      33  \_ postgres: logical replication launcher 
$ head -1 $PGDATA/postmaster.pid
33
$ cat /home/postgres/server.log
2026-09-26 10:24:03.341 UTC [33] LOG:  starting PostgreSQL 18.6 on aarch64-unknown-linux-gnu, compiled by gcc (GCC) 11.5.0 20240719 (Red Hat 11.5.0-14), 64-bit
2026-09-26 10:24:03.341 UTC [33] LOG:  listening on IPv6 address "::1", port 5432
2026-09-26 10:24:03.341 UTC [33] LOG:  listening on IPv4 address "127.0.0.1", port 5432
2026-09-26 10:24:03.342 UTC [33] LOG:  listening on Unix socket "/tmp/.s.PGSQL.5432"
2026-09-26 10:24:03.346 UTC [39] LOG:  database system was shut down at 2026-09-26 10:24:02 UTC
2026-09-26 10:24:03.347 UTC [33] LOG:  database system is ready to accept connections

postgres=# SELECT pid, backend_type FROM pg_stat_activity ORDER BY pid;
 pid |         backend_type         
-----+------------------------------
  34 | io worker
  35 | io worker
  36 | io worker
  37 | checkpointer
  38 | background writer
  40 | walwriter
  41 | autovacuum launcher
  42 | logical replication launcher
  66 | client backend
(9 rows)

/* src/backend/postmaster/postmaster.c, ServerLoop() (일부 생략) */
for (;;)
{
    /* 소켓에 새 접속이 오거나, 시그널 핸들러가 latch를 세울 때까지 잠든다 */
    nevents = WaitEventSetWait(pm_wait_set, DetermineSleepTime(), events, ...);

    for (int i = 0; i < nevents; i++)
    {
        if (pending_pm_shutdown_request) process_pm_shutdown_request(); /* 종료 요청 */
        if (pending_pm_reload_request)   process_pm_reload_request();   /* 설정 reload */
        if (pending_pm_child_exit)       process_pm_child_exit();       /* 자식 종료 */
        if (pending_pm_pmsignal)         process_pm_pmsignal();         /* 자식의 요청 */

        if (events[i].events & WL_SOCKET_ACCEPT)
        {
            if (AcceptConnection(events[i].fd, &s) == STATUS_OK)
                BackendStartup(&s);                 /* 접속이 오면 fork */
            closesocket(s.sock);                    /* postmaster 쪽 소켓은 바로 닫는다 */
        }
    }

    LaunchMissingBackgroundProcesses();             /* 빠진 백그라운드 프로세스를 다시 띄운다 */
    ...
}

postgres=# ALTER SYSTEM SET log_line_prefix = '%m [%p] %b ';
postgres=# ALTER SYSTEM SET log_connections = 'receipt,authentication,authorization,setup_durations';
postgres=# ALTER SYSTEM SET log_disconnections = on;
postgres=# SELECT pg_reload_conf();
 pg_reload_conf 
----------------
 t
(1 row)

$ psql -X -q -h 127.0.0.1 &
$ psql -X -q &
$ psql -X -c "SELECT pg_sleep(3600)" &
$ PM=$(head -1 $PGDATA/postmaster.pid); ps -o pid,ppid,cmd --forest -p $PM --ppid $PM
    PID    PPID CMD
     33       1 /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
...  (앞과 같은 백그라운드 프로세스 8줄)
     83      33  \_ postgres: postgres postgres 127.0.0.1(45092) idle
     92      33  \_ postgres: postgres postgres [local] idle
     99      33  \_ postgres: postgres postgres [local] SELECT

postgres=# SELECT pid, backend_type, client_addr, state, query FROM pg_stat_activity WHERE backend_type = 'client backend' ORDER BY pid;
 pid |  backend_type  | client_addr | state  |                                                            query                                                             
-----+----------------+-------------+--------+------------------------------------------------------------------------------------------------------------------------------
  83 | client backend | 127.0.0.1   | idle   | 
  92 | client backend |             | idle   | 
  99 | client backend |             | active | SELECT pg_sleep(3600)
 115 | client backend |             | active | SELECT pid, backend_type, client_addr, state, query FROM pg_stat_activity WHERE backend_type = 'client backend' ORDER BY pid
(4 rows)

$ grep -E 'connection (received|authenticated|authorized|ready)' /home/postgres/server.log | head -8
2026-09-26 10:24:03.750 UTC [83] not initialized LOG:  connection received: host=127.0.0.1 port=45092
2026-09-26 10:24:03.751 UTC [83] client backend LOG:  connection authenticated: user="postgres" method=trust (/var/lib/postgresql/data/pg_hba.conf:119)
2026-09-26 10:24:03.751 UTC [83] client backend LOG:  connection authorized: user=postgres database=postgres application_name=psql
2026-09-26 10:24:03.752 UTC [83] client backend LOG:  connection ready: setup total=1.821 ms, fork=0.140 ms, authentication=0.154 ms
2026-09-26 10:24:03.792 UTC [92] not initialized LOG:  connection received: host=[local]
2026-09-26 10:24:03.792 UTC [92] client backend LOG:  connection authenticated: user="postgres" method=trust (/var/lib/postgresql/data/pg_hba.conf:117)
2026-09-26 10:24:03.792 UTC [92] client backend LOG:  connection authorized: user=postgres database=postgres application_name=psql
2026-09-26 10:24:03.792 UTC [92] client backend LOG:  connection ready: setup total=1.050 ms, fork=0.170 ms, authentication=0.089 ms

postgres=# ALTER SYSTEM SET max_connections = 5;
postgres=# ALTER SYSTEM SET superuser_reserved_connections = 0;

$ pg_ctl -D $PGDATA -l /home/postgres/server.log restart -m fast
waiting for server to shut down.... done
server stopped
waiting for server to start.... done
server started
$ for i in 1 2 3 4 5; do psql -X -q & done
$ psql -X -c "SELECT 1"
psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: FATAL:  sorry, too many clients already
$ grep -B1 'too many clients' /home/postgres/server.log
2026-09-26 10:24:14.697 UTC [389] not initialized LOG:  connection received: host=[local]
2026-09-26 10:24:14.697 UTC [389] client backend FATAL:  sorry, too many clients already

$ mkdir -p /home/postgres/wal
$ pg_receivewal -D /home/postgres/wal &
$ PM=$(head -1 $PGDATA/postmaster.pid); ps -o pid,ppid,cmd --forest -p $PM --ppid $PM
    PID    PPID CMD
    412       1 /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
...  (백그라운드 프로세스 8줄)
    530     412  \_ postgres: walsender postgres [local] streaming 0/3FCE280
$ ps -o pid,ppid,cmd -C pg_receivewal
    PID    PPID CMD
    523       0 pg_receivewal -D /home/postgres/wal
$ WS=$(pgrep -f 'postgres: walsender')
$ grep "\[$WS\]" /home/postgres/server.log
2026-09-26 10:24:20.908 UTC [530] not initialized LOG:  connection received: host=[local]
2026-09-26 10:24:20.909 UTC [530] walsender LOG:  connection authenticated: user="postgres" method=trust (/var/lib/postgresql/data/pg_hba.conf:124)
2026-09-26 10:24:20.909 UTC [530] walsender LOG:  replication connection authorized: user=postgres application_name=pg_receivewal
2026-09-26 10:24:20.909 UTC [530] walsender LOG:  connection ready: setup total=1.240 ms, fork=0.149 ms, authentication=0.218 ms

postgres=# SELECT pid, backend_type, application_name, state FROM pg_stat_activity WHERE backend_type = 'walsender';
 pid | backend_type | application_name | state  
-----+--------------+------------------+--------
 530 | walsender    | pg_receivewal    | active
(1 row)

$ mkdir -p /home/postgres/archive

postgres=# ALTER SYSTEM SET logging_collector = on;
postgres=# ALTER SYSTEM SET archive_mode = on;
postgres=# ALTER SYSTEM SET archive_command = 'cp %p /home/postgres/archive/%f';

$ pg_ctl -D $PGDATA -l /home/postgres/server.log restart -m fast
waiting for server to shut down.... done
server stopped
waiting for server to start.... done
server started
$ sleep 1
$ PM=$(head -1 $PGDATA/postmaster.pid); ps -o pid,ppid,cmd --forest -p $PM --ppid $PM
    PID    PPID CMD
    641       1 /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
    642     641  \_ postgres: logger 
    643     641  \_ postgres: io worker 0
    644     641  \_ postgres: io worker 1
    645     641  \_ postgres: io worker 2
    646     641  \_ postgres: checkpointer 
    647     641  \_ postgres: background writer 
    649     641  \_ postgres: walwriter 
    650     641  \_ postgres: autovacuum launcher 
    651     641  \_ postgres: archiver 
    652     641  \_ postgres: logical replication launcher 

postgres=# SELECT pid, backend_type FROM pg_stat_activity ORDER BY pid;
 pid |         backend_type         
-----+------------------------------
 643 | io worker
 644 | io worker
 645 | io worker
 646 | checkpointer
 647 | background writer
 649 | walwriter
 650 | autovacuum launcher
 651 | archiver
 652 | logical replication launcher
 663 | client backend
(10 rows)

postgres=# SHOW io_method;
 io_method 
-----------
 worker
(1 row)

postgres=# SHOW io_workers;
 io_workers 
------------
 3
(1 row)

postgres=# ALTER SYSTEM SET io_workers = 5;
postgres=# SELECT pg_reload_conf();
 pg_reload_conf 
----------------
 t
(1 row)

$ sleep 1
$ ps -u postgres -o pid,ppid,cmd | grep 'io worker' | grep -v grep
     34      33 postgres: io worker 0
     35      33 postgres: io worker 1
     36      33 postgres: io worker 2
    141      33 postgres: io worker 3
    142      33 postgres: io worker 4

postgres=# ALTER SYSTEM RESET io_workers;
postgres=# SELECT pg_reload_conf();
 pg_reload_conf 
----------------
 t
(1 row)

$ sleep 1
$ ps -u postgres -o pid,ppid,cmd | grep 'io worker' | grep -v grep
     34      33 postgres: io worker 0
     35      33 postgres: io worker 1
     36      33 postgres: io worker 2

postgres=# SHOW shared_buffers;
 shared_buffers 
----------------
 128MB
(1 row)

postgres=# SHOW shared_memory_size;
 shared_memory_size 
--------------------
 150MB
(1 row)

postgres=# SHOW shared_memory_type;
 shared_memory_type 
--------------------
 mmap
(1 row)

postgres=# CREATE TABLE big AS SELECT g AS id, repeat('x', 100) AS pad FROM generate_series(1, 150000) g;

postgres=# SELECT pg_size_pretty(pg_relation_size('big'));
 pg_size_pretty 
----------------
 21 MB
(1 row)

$ PM=$(head -1 $PGDATA/postmaster.pid)
$ for p in $PM $(pgrep -f 'postgres: postgres postgres \[local\] idle'); do
>   echo "== PID $p  $(ps -o cmd= -p $p)"
>   grep ' rw-s ' /proc/$p/maps | awk '{print "   ", $1, $6, $7}'
> done
== PID 412  /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
    ffffaf5e3000-ffffaf5eb000 /dev/shm/PostgreSQL.1630556078 
    ffffaf5eb000-ffffaf5ec000 /SYSV00137e8a (deleted)
    ffffaf5ec000-ffffb8b9e000 /dev/zero (deleted)
== PID 583  postgres: postgres postgres [local] idle
    ffffaf450000-ffffaf481000 /dev/shm/PostgreSQL.3330517350 
    ffffaf4c2000-ffffaf5c2000 /dev/shm/PostgreSQL.3445492072 
    ffffaf5e3000-ffffaf5eb000 /dev/shm/PostgreSQL.1630556078 
    ffffaf5eb000-ffffaf5ec000 /SYSV00137e8a (deleted)
    ffffaf5ec000-ffffb8b9e000 /dev/zero (deleted)
== PID 593  postgres: postgres postgres [local] idle
    ffffaf4c2000-ffffaf5c2000 /dev/shm/PostgreSQL.3445492072 
    ffffaf5e3000-ffffaf5eb000 /dev/shm/PostgreSQL.1630556078 
    ffffaf5eb000-ffffaf5ec000 /SYSV00137e8a (deleted)
    ffffaf5ec000-ffffb8b9e000 /dev/zero (deleted)

$ PM=$(head -1 $PGDATA/postmaster.pid)
$ for p in $PM $(pgrep -f 'postgres: postgres postgres \[local\] idle'); do
>   echo "== PID $p  $(ps -o cmd= -p $p)"
>   grep -E '^(VmRSS|RssAnon|RssShmem)' /proc/$p/status
> done
== PID 412  /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
VmRSS:	   22336 kB
RssAnon:	     796 kB
RssShmem:	   13372 kB
== PID 583  postgres: postgres postgres [local] idle
VmRSS:	   34524 kB
RssAnon:	    1848 kB
RssShmem:	   24520 kB
== PID 593  postgres: postgres postgres [local] idle
VmRSS:	   11388 kB
RssAnon:	    1616 kB
RssShmem:	    3280 kB

postgres=# ALTER SYSTEM SET autovacuum_naptime = '1s';
postgres=# ALTER SYSTEM SET log_autovacuum_min_duration = 0;
postgres=# SELECT pg_reload_conf();
 pg_reload_conf 
----------------
 t
(1 row)


postgres=# CREATE TABLE av_test AS SELECT g AS id, 0 AS v FROM generate_series(1, 200000) g;
postgres=# UPDATE av_test SET v = 1;

$ for i in $(seq 1 1000); do
>   ps -u postgres -o pid,ppid,cmd | grep 'autovacuum worker' | grep -v grep && break
>   sleep 0.01
> done
    294      33 postgres: autovacuum worker postgres
$ sleep 3
$ grep -E 'autovacuum worker LOG:  automatic (vacuum|analyze) of table "postgres.public.av_test"' /home/postgres/server.log | cut -c1-120
2026-09-26 10:24:09.034 UTC [294] autovacuum worker LOG:  automatic vacuum of table "postgres.public.av_test": index sca
2026-09-26 10:24:09.079 UTC [294] autovacuum worker LOG:  automatic analyze of table "postgres.public.av_test"
2026-09-26 10:24:10.032 UTC [297] autovacuum worker LOG:  automatic vacuum of table "postgres.public.av_test": index sca
2026-09-26 10:24:11.028 UTC [299] autovacuum worker LOG:  automatic vacuum of table "postgres.public.av_test": index sca

$ kill -9 641
$ sleep 2
$ ps -u postgres -o pid,ppid,stat,cmd | grep -v -e 'ps -u' -e 'sleep infinity'
    PID    PPID STAT CMD
    664       0 Ss   bash -s

$ psql -X -c "SELECT 1"
psql: error: connection to server on socket "/tmp/.s.PGSQL.5432" failed: Connection refused
	Is the server running locally and accepting connections on that socket?
$ pg_ctl -D $PGDATA -l /home/postgres/server.log start
pg_ctl: another server might be running; trying to start server anyway
waiting for server to start.... done
server started
$ sleep 1
$ tail -n 8 $PGDATA/log/$(ls -t $PGDATA/log | head -1)
2026-09-26 10:24:29.194 UTC [689] postmaster LOG:  listening on Unix socket "/tmp/.s.PGSQL.5432"
2026-09-26 10:24:29.196 UTC [696] startup LOG:  database system was interrupted; last known up at 2026-09-26 10:24:25 UTC
2026-09-26 10:24:29.222 UTC [696] startup LOG:  database system was not properly shut down; automatic recovery in progress
2026-09-26 10:24:29.223 UTC [696] startup LOG:  unexpected pageaddr 0/26EA000 in WAL segment 000000010000000000000005, LSN 0/56EA000, offset 7249920
2026-09-26 10:24:29.223 UTC [696] startup LOG:  redo is not required
2026-09-26 10:24:29.225 UTC [694] checkpointer LOG:  checkpoint starting: end-of-recovery immediate wait
2026-09-26 10:24:29.229 UTC [694] checkpointer LOG:  checkpoint complete: wrote 0 buffers (0.0%), wrote 3 SLRU buffers; 0 WAL file(s) added, 0 removed, 0 recycled; write=0.001 s, sync=0.001 s, total=0.005 s; sync files=2, longest=0.001 s, average=0.001 s; distance=6 kB, estimate=6 kB; lsn=0/56EA048, redo lsn=0/56EA048
2026-09-26 10:24:29.230 UTC [689] postmaster LOG:  database system is ready to accept connections

2026-09-26 10:12:12.213 UTC [56] postmaster FATAL:  lock file "postmaster.pid" already exists
2026-09-26 10:12:12.213 UTC [56] postmaster HINT:  Is another postmaster (PID 26) running in data directory "/var/lib/postgresql/data"?

    PID    PPID STAT CMD
      1       0 Ss   /usr/bin/coreutils --coreutils-prog-shebang=sleep /usr/bin/sleep infinity
     26       1 Zs   [postgres] <defunct>
     27       1 Zs   [postgres] <defunct>
     28       1 Zs   [postgres] <defunct>
     29       1 Zs   [postgres] <defunct>
     30       1 Zs   [postgres] <defunct>
     31       1 Zs   [postgres] <defunct>
     33       1 Zs   [postgres] <defunct>
     34       1 Zs   [postgres] <defunct>
     35       1 Zs   [postgres] <defunct>

$ psql -X -q &
$ psql -X -q &
$ psql -X -c "SELECT pg_sleep(3600)" &

postgres=# CREATE TABLE t AS SELECT generate_series(1, 1000) AS id;

$ PM=$(head -1 $PGDATA/postmaster.pid); ps -o pid,ppid,cmd --forest -p $PM --ppid $PM
    PID    PPID CMD
    412       1 /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
    413     412  \_ postgres: io worker 0
    414     412  \_ postgres: io worker 1
    415     412  \_ postgres: io worker 2
    416     412  \_ postgres: checkpointer 
    417     412  \_ postgres: background writer 
    419     412  \_ postgres: walwriter 
    420     412  \_ postgres: autovacuum launcher 
    421     412  \_ postgres: logical replication launcher 
    430     412  \_ postgres: postgres postgres [local] idle
    439     412  \_ postgres: postgres postgres [local] idle
    447     412  \_ postgres: postgres postgres [local] SELECT

postgres=# SELECT pg_terminate_backend(430);
 pg_terminate_backend 
----------------------
 t
(1 row)

$ sleep 1
$ PM=$(head -1 $PGDATA/postmaster.pid); ps -o pid,ppid,cmd --forest -p $PM --ppid $PM
    PID    PPID CMD
    412       1 /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
...  (413-421, 위와 같은 백그라운드 프로세스 8줄)
    439     412  \_ postgres: postgres postgres [local] idle
    447     412  \_ postgres: postgres postgres [local] SELECT
$ grep '\[430\]' /home/postgres/server.log | tail -2
2026-09-26 10:24:17.448 UTC [430] client backend FATAL:  terminating connection due to administrator command
2026-09-26 10:24:17.448 UTC [430] client backend LOG:  disconnection: session time: 0:00:02.281 user=postgres database=postgres host=[local]

$ kill -9 439
$ sleep 2
$ PM=$(head -1 $PGDATA/postmaster.pid); ps -o pid,ppid,cmd --forest -p $PM --ppid $PM
    PID    PPID CMD
    412       1 /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
    484     412  \_ postgres: io worker 0
    485     412  \_ postgres: io worker 1
    486     412  \_ postgres: io worker 2
    488     412  \_ postgres: checkpointer 
    489     412  \_ postgres: background writer 
    490     412  \_ postgres: walwriter 
    491     412  \_ postgres: autovacuum launcher 
    492     412  \_ postgres: logical replication launcher 

$ sed -n '/terminated by signal 9/,$p' /home/postgres/server.log
2026-09-26 10:24:18.553 UTC [412] postmaster LOG:  client backend (PID 439) was terminated by signal 9: Killed
2026-09-26 10:24:18.553 UTC [412] postmaster LOG:  terminating any other active server processes
2026-09-26 10:24:18.553 UTC [412] postmaster LOG:  all server processes terminated; reinitializing
2026-09-26 10:24:18.564 UTC [487] startup LOG:  database system was interrupted; last known up at 2026-09-26 10:24:14 UTC
2026-09-26 10:24:18.597 UTC [487] startup LOG:  database system was not properly shut down; automatic recovery in progress
2026-09-26 10:24:18.597 UTC [487] startup LOG:  redo starts at 0/3F9BF38
2026-09-26 10:24:18.600 UTC [487] startup LOG:  invalid record length at 0/3FC3028: expected at least 24, got 0
2026-09-26 10:24:18.600 UTC [487] startup LOG:  redo done at 0/3FC2E90 system usage: CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s
2026-09-26 10:24:18.601 UTC [488] checkpointer LOG:  checkpoint starting: end-of-recovery immediate wait
2026-09-26 10:24:18.605 UTC [488] checkpointer LOG:  checkpoint complete: wrote 22 buffers (0.1%), wrote 3 SLRU buffers; 0 WAL file(s) added, 0 removed, 0 recycled; write=0.001 s, sync=0.003 s, total=0.005 s; sync files=20, longest=0.001 s, average=0.001 s; distance=156 kB, estimate=156 kB; lsn=0/3FC3028, redo lsn=0/3FC3028
2026-09-26 10:24:18.606 UTC [412] postmaster LOG:  database system is ready to accept connections

WARNING:  terminating connection because of crash of another server process
DETAIL:  The postmaster has commanded this server process to roll back the current transaction and exit, because another server process exited abnormally and possibly corrupted shared memory.
HINT:  In a moment you should be able to reconnect to the database and repeat your command.
server closed the connection unexpectedly
	This probably means the server terminated abnormally
	before or while processing the request.
connection to server was lost

postgres=# SELECT count(*) FROM t;
 count 
-------
  1000
(1 row)

SELECT pg_cancel_backend(pid);     -- 실행 중인 쿼리만 취소 (세션은 유지)
SELECT pg_terminate_backend(pid);  -- 세션 종료 (다른 세션에는 영향 없음)