프로세스 구조

/* 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 */
    ...
}

git rev-parse --abbrev-ref HEAD
git log -1 --format='%H %s'
git archive --format=tar.gz -o postgres-src.tar.gz HEAD

REL_18_STABLE
39a0db101105eab3f4044d11c609c58b9459ea16 Fix incorrect block accounting in TID Range Scans

docker build -t pg-internals:rel18 . > build.log 2>&1; echo "exit=$?" >> build.log

#4 [1/7] FROM docker.io/library/debian:bookworm-slim@sha256:3783cc01769c7b2b1b83a5c5ad96c815348e28ed7da68e2e3687004faa906251
#6 [2/7] RUN apt-get update && apt-get install -y --no-install-recommends       build-essential bison flex pkg-config       libreadline-dev zlib1g-dev libicu-dev       procps psmisc iproute2 less ca-certificates     && rm -rf /var/lib/apt/lists/*
#6 DONE 68.2s
#7 [3/7] ADD postgres-src.tar.gz /usr/src/postgres/
#7 DONE 1.7s
#9 [5/7] RUN ./configure --prefix=/usr/local/pgsql     && make -j"$(nproc)" world-bin     && make install-world-bin
#9 DONE 23.3s
#12 naming to docker.io/library/pg-internals:rel18 done
#12 DONE 9.2s
exit=0

docker run -d --init --name pglab --hostname pglab pg-internals:rel18 sleep infinity

c799db96799db48cf177ebe53dc240cc0320725a740350f138c7db6f4e682e71
[exit=0]

postgres --version
initdb -D $PGDATA > /home/postgres/initdb.log 2>&1 && echo "initdb ok"

postgres (PostgreSQL) 18.6
initdb ok
[exit=0]

pg_ctl -D $PGDATA -l /home/postgres/server.log start

waiting for server to start.... done
server started
[exit=0]

PM=$(head -1 $PGDATA/postmaster.pid); ps -o pid,ppid,cmd --forest -p $PM --ppid $PM

    PID    PPID CMD
     37       1 /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
     38      37  \_ postgres: io worker 0
     39      37  \_ postgres: io worker 1
     40      37  \_ postgres: io worker 2
     41      37  \_ postgres: checkpointer 
     42      37  \_ postgres: background writer 
     44      37  \_ postgres: walwriter 
     45      37  \_ postgres: autovacuum launcher 
     46      37  \_ postgres: logical replication launcher 
[exit=0]

head -1 $PGDATA/postmaster.pid
cat /home/postgres/server.log

37
2026-09-24 01:41:45.623 UTC [37] LOG:  starting PostgreSQL 18.6 on aarch64-unknown-linux-gnu, compiled by gcc (Debian 12.2.0-14+deb12u1) 12.2.0, 64-bit
2026-09-24 01:41:45.623 UTC [37] LOG:  listening on IPv6 address "::1", port 5432
2026-09-24 01:41:45.623 UTC [37] LOG:  listening on IPv4 address "127.0.0.1", port 5432
2026-09-24 01:41:45.624 UTC [37] LOG:  listening on Unix socket "/tmp/.s.PGSQL.5432"
2026-09-24 01:41:45.625 UTC [43] LOG:  database system was shut down at 2026-09-24 01:41:45 UTC
2026-09-24 01:41:45.626 UTC [37] LOG:  database system is ready to accept connections
[exit=0]

psql -X -c "SELECT pid, backend_type FROM pg_stat_activity ORDER BY pid"

 pid |         backend_type         
-----+------------------------------
  38 | io worker
  39 | io worker
  40 | io worker
  41 | checkpointer
  42 | background writer
  44 | walwriter
  45 | autovacuum launcher
  46 | logical replication launcher
  70 | client backend
(9 rows)

[exit=0]

psql -X -q <<'SQL'
ALTER SYSTEM SET log_line_prefix = '%m [%p] %b ';
ALTER SYSTEM SET log_connections = 'receipt,authentication,authorization,setup_durations';
ALTER SYSTEM SET log_disconnections = on;
SELECT pg_reload_conf();
SQL

 pg_reload_conf 
----------------
 t
(1 row)

[exit=0]

docker exec -d pglab bash -c 'sleep 3600 | psql -X -q -h 127.0.0.1'
docker exec -d pglab bash -c 'sleep 3600 | psql -X -q'
docker exec -d pglab bash -c 'psql -X -c "SELECT pg_sleep(3600)"'

[exit=0]
[exit=0]
[exit=0]

PM=$(head -1 $PGDATA/postmaster.pid); ps -o pid,ppid,cmd --forest -p $PM --ppid $PM

    PID    PPID CMD
     37       1 /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
     38      37  \_ postgres: io worker 0
     39      37  \_ postgres: io worker 1
     40      37  \_ postgres: io worker 2
     41      37  \_ postgres: checkpointer 
     42      37  \_ postgres: background writer 
     44      37  \_ postgres: walwriter 
     45      37  \_ postgres: autovacuum launcher 
     46      37  \_ postgres: logical replication launcher 
     87      37  \_ postgres: postgres postgres 127.0.0.1(53408) idle
     96      37  \_ postgres: postgres postgres [local] idle
    103      37  \_ postgres: postgres postgres [local] SELECT
[exit=0]

psql -X -c "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                                                             
-----+----------------+-------------+--------+------------------------------------------------------------------------------------------------------------------------------
  87 | client backend | 127.0.0.1   | idle   | 
  96 | client backend |             | idle   | 
 103 | client backend |             | active | SELECT pg_sleep(3600)
 119 | 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)

[exit=0]

grep -E 'connection (received|authenticated|authorized|ready)' /home/postgres/server.log | head -8

2026-09-24 01:41:46.064 UTC [87] not initialized LOG:  connection received: host=127.0.0.1 port=53408
2026-09-24 01:41:46.064 UTC [87] client backend LOG:  connection authenticated: user="postgres" method=trust (/var/lib/postgresql/data/pg_hba.conf:119)
2026-09-24 01:41:46.064 UTC [87] client backend LOG:  connection authorized: user=postgres database=postgres application_name=psql
2026-09-24 01:41:46.065 UTC [87] client backend LOG:  connection ready: setup total=1.032 ms, fork=0.146 ms, authentication=0.083 ms
2026-09-24 01:41:46.113 UTC [96] not initialized LOG:  connection received: host=[local]
2026-09-24 01:41:46.113 UTC [96] client backend LOG:  connection authenticated: user="postgres" method=trust (/var/lib/postgresql/data/pg_hba.conf:117)
2026-09-24 01:41:46.113 UTC [96] client backend LOG:  connection authorized: user=postgres database=postgres application_name=psql
2026-09-24 01:41:46.113 UTC [96] client backend LOG:  connection ready: setup total=1.047 ms, fork=0.140 ms, authentication=0.083 ms
[exit=0]

psql -X -c "SHOW io_method" -c "SHOW io_workers"

 io_method 
-----------
 worker
(1 row)

 io_workers 
------------
 3
(1 row)

[exit=0]

psql -X -q -c "ALTER SYSTEM SET io_workers = 5" -c "SELECT pg_reload_conf()"
sleep 1
ps -u postgres -o pid,ppid,cmd | grep 'io worker' | grep -v grep

 pg_reload_conf 
----------------
 t
(1 row)

     38      37 postgres: io worker 0
     39      37 postgres: io worker 1
     40      37 postgres: io worker 2
    145      37 postgres: io worker 3
    146      37 postgres: io worker 4
[exit=0]

psql -X -q -c "ALTER SYSTEM RESET io_workers" -c "SELECT pg_reload_conf()"
sleep 1
ps -u postgres -o pid,ppid,cmd | grep 'io worker' | grep -v grep

 pg_reload_conf 
----------------
 t
(1 row)

     38      37 postgres: io worker 0
     39      37 postgres: io worker 1
     40      37 postgres: io worker 2
[exit=0]

psql -X -q <<'SQL'
ALTER SYSTEM SET autovacuum_naptime = '1s';
ALTER SYSTEM SET log_autovacuum_min_duration = 0;
SELECT pg_reload_conf();
CREATE TABLE av_test AS SELECT g AS id, 0 AS v FROM generate_series(1, 200000) g;
UPDATE av_test SET v = 1;
SQL
for i in $(seq 1 40); do
  ps -u postgres -o pid,ppid,cmd | grep 'autovacuum worker' | grep -v grep && break
  sleep 0.25
done
sleep 3
grep -E 'autovacuum worker LOG:  automatic (vacuum|analyze) of table "postgres.public.av_test"' /home/postgres/server.log | cut -c1-120

 pg_reload_conf 
----------------
 t
(1 row)

    213      37 postgres: autovacuum worker postgres
2026-09-24 01:41:51.298 UTC [179] autovacuum worker LOG:  automatic vacuum of table "postgres.public.av_test": index sca
2026-09-24 01:41:51.352 UTC [179] autovacuum worker LOG:  automatic analyze of table "postgres.public.av_test"
2026-09-24 01:41:52.291 UTC [197] autovacuum worker LOG:  automatic vacuum of table "postgres.public.av_test": index sca
2026-09-24 01:41:53.289 UTC [213] autovacuum worker LOG:  automatic vacuum of table "postgres.public.av_test": index sca
2026-09-24 01:41:54.297 UTC [217] autovacuum worker LOG:  automatic vacuum of table "postgres.public.av_test": index sca
2026-09-24 01:41:55.285 UTC [219] autovacuum worker LOG:  automatic vacuum of table "postgres.public.av_test": index sca
[exit=0]

psql -X -q -c "ALTER SYSTEM SET max_connections = 5" -c "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
[exit=0]

for i in 1 2 3 4 5; do docker exec -d pglab bash -c 'sleep 3600 | 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
[exit=2]

grep -B1 'too many clients' /home/postgres/server.log

2026-09-24 01:41:58.942 UTC [309] not initialized LOG:  connection received: host=[local]
2026-09-24 01:41:58.942 UTC [309] client backend FATAL:  sorry, too many clients already
[exit=0]

docker exec -d pglab bash -c 'sleep 3600 | psql -X -q'
docker exec -d pglab bash -c 'sleep 3600 | psql -X -q'
docker exec -d pglab bash -c 'psql -X -c "SELECT pg_sleep(3600)" > /home/postgres/neighbor.out 2>&1'

psql -X -q -c "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
    332       1 /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
    333     332  \_ postgres: io worker 0
    334     332  \_ postgres: io worker 1
    335     332  \_ postgres: io worker 2
    336     332  \_ postgres: checkpointer 
    337     332  \_ postgres: background writer 
    339     332  \_ postgres: walwriter 
    340     332  \_ postgres: autovacuum launcher 
    341     332  \_ postgres: logical replication launcher 
    350     332  \_ postgres: postgres postgres [local] idle
    359     332  \_ postgres: postgres postgres [local] idle
    367     332  \_ postgres: postgres postgres [local] SELECT
[exit=0]

VICTIM=$(pgrep -f 'postgres: postgres postgres \[local\] idle' | head -1)
echo "pg_terminate_backend($VICTIM)"
psql -X -c "SELECT pg_terminate_backend($VICTIM)"
sleep 1
PM=$(head -1 $PGDATA/postmaster.pid); ps -o pid,ppid,cmd --forest -p $PM --ppid $PM
grep "\[$VICTIM\]" /home/postgres/server.log | tail -2

pg_terminate_backend(350)
 pg_terminate_backend 
----------------------
 t
(1 row)

    PID    PPID CMD
    332       1 /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
    333     332  \_ postgres: io worker 0
    334     332  \_ postgres: io worker 1
    335     332  \_ postgres: io worker 2
    336     332  \_ postgres: checkpointer 
    337     332  \_ postgres: background writer 
    339     332  \_ postgres: walwriter 
    340     332  \_ postgres: autovacuum launcher 
    341     332  \_ postgres: logical replication launcher 
    359     332  \_ postgres: postgres postgres [local] idle
    367     332  \_ postgres: postgres postgres [local] SELECT
[exit=0]
2026-09-24 01:42:01.680 UTC [350] client backend FATAL:  terminating connection due to administrator command
2026-09-24 01:42:01.680 UTC [350] client backend LOG:  disconnection: session time: 0:00:02.272 user=postgres database=postgres host=[local]
[exit=0]

VICTIM=$(pgrep -f 'postgres: postgres postgres \[local\] idle' | head -1)
echo "kill -9 $VICTIM"
kill -9 $VICTIM
sleep 2
PM=$(head -1 $PGDATA/postmaster.pid); ps -o pid,ppid,cmd --forest -p $PM --ppid $PM

kill -9 359
    PID    PPID CMD
    332       1 /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
    404     332  \_ postgres: io worker 0
    405     332  \_ postgres: io worker 1
    406     332  \_ postgres: io worker 2
    408     332  \_ postgres: checkpointer 
    409     332  \_ postgres: background writer 
    410     332  \_ postgres: walwriter 
    411     332  \_ postgres: autovacuum launcher 
    412     332  \_ postgres: logical replication launcher 
[exit=0]

sed -n '/terminated by signal 9/,$p' /home/postgres/server.log

2026-09-24 01:42:02.788 UTC [332] postmaster LOG:  client backend (PID 359) was terminated by signal 9: Killed
2026-09-24 01:42:02.788 UTC [332] postmaster LOG:  terminating any other active server processes
2026-09-24 01:42:02.789 UTC [332] postmaster LOG:  all server processes terminated; reinitializing
2026-09-24 01:42:02.798 UTC [407] startup LOG:  database system was interrupted; last known up at 2026-09-24 01:41:59 UTC
2026-09-24 01:42:02.828 UTC [407] startup LOG:  database system was not properly shut down; automatic recovery in progress
2026-09-24 01:42:02.829 UTC [407] startup LOG:  redo starts at 0/3F9F888
2026-09-24 01:42:02.832 UTC [407] startup LOG:  invalid record length at 0/3FC6978: expected at least 24, got 0
2026-09-24 01:42:02.832 UTC [407] startup LOG:  redo done at 0/3FC67E0 system usage: CPU: user: 0.00 s, system: 0.00 s, elapsed: 0.00 s
2026-09-24 01:42:02.834 UTC [408] checkpointer LOG:  checkpoint starting: end-of-recovery immediate wait
2026-09-24 01:42:02.838 UTC [408] 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=0/3FC6978, redo lsn=0/3FC6978
2026-09-24 01:42:02.838 UTC [332] postmaster LOG:  database system is ready to accept connections
[exit=0]

cat /home/postgres/neighbor.out

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
[exit=0]

psql -X -c "SELECT count(*) FROM t"

 count 
-------
  1000
(1 row)

[exit=0]

docker exec -d pglab bash -c '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
ps -o pid,ppid,cmd -C pg_receivewal

    PID    PPID CMD
    332       1 /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
    404     332  \_ postgres: io worker 0
    405     332  \_ postgres: io worker 1
    406     332  \_ postgres: io worker 2
    408     332  \_ postgres: checkpointer 
    409     332  \_ postgres: background writer 
    410     332  \_ postgres: walwriter 
    411     332  \_ postgres: autovacuum launcher 
    412     332  \_ postgres: logical replication launcher 
    450     332  \_ postgres: walsender postgres [local] streaming 0/3FD1BB8
    PID    PPID CMD
    443       0 pg_receivewal -D /home/postgres/wal
[exit=0]

WS=$(pgrep -f 'postgres: walsender')
grep "\[$WS\]" /home/postgres/server.log
psql -X -c "SELECT pid, backend_type, application_name, state FROM pg_stat_activity WHERE backend_type = 'walsender'"

2026-09-24 01:42:05.115 UTC [450] not initialized LOG:  connection received: host=[local]
2026-09-24 01:42:05.115 UTC [450] walsender LOG:  connection authenticated: user="postgres" method=trust (/var/lib/postgresql/data/pg_hba.conf:124)
2026-09-24 01:42:05.115 UTC [450] walsender LOG:  replication connection authorized: user=postgres application_name=pg_receivewal
2026-09-24 01:42:05.115 UTC [450] walsender LOG:  connection ready: setup total=0.710 ms, fork=0.107 ms, authentication=0.112 ms
 pid | backend_type | application_name | state  
-----+--------------+------------------+--------
 450 | walsender    | pg_receivewal    | active
(1 row)

[exit=0]

psql -X -c "SHOW shared_buffers" -c "SHOW shared_memory_size" -c "SHOW shared_memory_type"

 shared_buffers 
----------------
 128MB
(1 row)

 shared_memory_size 
--------------------
 150MB
(1 row)

 shared_memory_type 
--------------------
 mmap
(1 row)

[exit=0]

psql -X -q -c "CREATE TABLE big AS SELECT g AS id, repeat('x', 100) AS pad FROM generate_series(1, 150000) g"
psql -X -c "SELECT pg_size_pretty(pg_relation_size('big'))"

 pg_size_pretty 
----------------
 21 MB
(1 row)

[exit=0]

docker exec -d pglab bash -c '(echo "SELECT count(*) FROM big;"; sleep 3600) | psql -X -q > /dev/null'
docker exec -d pglab bash -c 'sleep 3600 | psql -X -q'

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 332  /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
    ffff7cb27000-ffff860d9000 /dev/zero (deleted)
    ffff88938000-ffff88940000 /dev/shm/PostgreSQL.2214466610 
    ffff889a9000-ffff889aa000 /SYSV000fb191 (deleted)
== PID 502  postgres: postgres postgres [local] idle
    ffff7c994000-ffff7c9c5000 /dev/shm/PostgreSQL.3907697562 
    ffff7ca06000-ffff7cb06000 /dev/shm/PostgreSQL.2641106026 
    ffff7cb27000-ffff860d9000 /dev/zero (deleted)
    ffff88938000-ffff88940000 /dev/shm/PostgreSQL.2214466610 
    ffff889a9000-ffff889aa000 /SYSV000fb191 (deleted)
== PID 512  postgres: postgres postgres [local] idle
    ffff7ca06000-ffff7cb06000 /dev/shm/PostgreSQL.2641106026 
    ffff7cb27000-ffff860d9000 /dev/zero (deleted)
    ffff88938000-ffff88940000 /dev/shm/PostgreSQL.2214466610 
    ffff889a9000-ffff889aa000 /SYSV000fb191 (deleted)
[exit=0]

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 332  /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
VmRSS:	   22792 kB
RssAnon:	     956 kB
RssShmem:	   13372 kB
== PID 502  postgres: postgres postgres [local] idle
VmRSS:	   34768 kB
RssAnon:	    2028 kB
RssShmem:	   24640 kB
== PID 512  postgres: postgres postgres [local] idle
VmRSS:	   11684 kB
RssAnon:	    1776 kB
RssShmem:	    3472 kB
[exit=0]

mkdir -p /home/postgres/archive
psql -X -q <<'SQL'
ALTER SYSTEM SET logging_collector = on;
ALTER SYSTEM SET archive_mode = on;
ALTER SYSTEM SET archive_command = 'cp %p /home/postgres/archive/%f';
SQL
pg_ctl -D $PGDATA -l /home/postgres/server.log restart -m fast
sleep 1
PM=$(head -1 $PGDATA/postmaster.pid); ps -o pid,ppid,cmd --forest -p $PM --ppid $PM

waiting for server to shut down.... done
server stopped
waiting for server to start.... done
server started
    PID    PPID CMD
    560       1 /usr/local/pgsql/bin/postgres -D /var/lib/postgresql/data
    561     560  \_ postgres: logger 
    562     560  \_ postgres: io worker 0
    563     560  \_ postgres: io worker 1
    564     560  \_ postgres: io worker 2
    565     560  \_ postgres: checkpointer 
    566     560  \_ postgres: background writer 
    568     560  \_ postgres: walwriter 
    569     560  \_ postgres: autovacuum launcher 
    570     560  \_ postgres: archiver 
    571     560  \_ postgres: logical replication launcher 
[exit=0]

psql -X -c "SELECT pid, backend_type FROM pg_stat_activity ORDER BY pid"

 pid |         backend_type         
-----+------------------------------
 562 | io worker
 563 | io worker
 564 | io worker
 565 | checkpointer
 566 | background writer
 568 | walwriter
 569 | autovacuum launcher
 570 | archiver
 571 | logical replication launcher
 582 | client backend
(10 rows)

[exit=0]

PM=$(head -1 $PGDATA/postmaster.pid)
echo "kill -9 $PM (postmaster)"
kill -9 $PM
sleep 2
ps -u postgres -o pid,ppid,stat,cmd | grep -v -e 'ps -u' -e 'sleep infinity'

kill -9 560 (postmaster)
    PID    PPID STAT CMD
    583       0 Ss   bash -s
[exit=0]

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?
[exit=2]

pg_ctl -D $PGDATA -l /home/postgres/server.log start
sleep 1
tail -n 8 $PGDATA/log/$(ls -t $PGDATA/log | head -1)

pg_ctl: another server might be running; trying to start server anyway
waiting for server to start.... done
server started
2026-09-24 01:42:13.447 UTC [609] postmaster LOG:  listening on Unix socket "/tmp/.s.PGSQL.5432"
2026-09-24 01:42:13.449 UTC [616] startup LOG:  database system was interrupted; last known up at 2026-09-24 01:42:10 UTC
2026-09-24 01:42:13.473 UTC [616] startup LOG:  database system was not properly shut down; automatic recovery in progress
2026-09-24 01:42:13.475 UTC [616] startup LOG:  unexpected pageaddr 0/26EC000 in WAL segment 000000010000000000000005, LSN 0/56EC000, offset 7258112
2026-09-24 01:42:13.475 UTC [616] startup LOG:  redo is not required
2026-09-24 01:42:13.476 UTC [614] checkpointer LOG:  checkpoint starting: end-of-recovery immediate wait
2026-09-24 01:42:13.478 UTC [614] 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=0.003 s; sync files=2, longest=0.001 s, average=0.001 s; distance=0 kB, estimate=0 kB; lsn=0/56EC048, redo lsn=0/56EC048
2026-09-24 01:42:13.478 UTC [609] postmaster LOG:  database system is ready to accept connections
[exit=0]

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

2026-09-24 01:42:02.788 UTC [332] postmaster LOG:  terminating any other active server processes
2026-09-24 01:42:02.789 UTC [332] postmaster LOG:  all server processes terminated; reinitializing

2026-09-24 01:39:36.364 UTC [642] postmaster FATAL:  lock file "postmaster.pid" already exists
2026-09-24 01:39:36.364 UTC [642] postmaster HINT:  Is another postmaster (PID 615) running in data directory "/var/lib/postgresql/data"?

    PID    PPID STAT CMD
      1       0 Ss   sleep infinity
    615       1 Zs   [postgres] <defunct>
    616       1 Zs   [postgres] <defunct>
    617       1 Zs   [postgres] <defunct>