From e7cfa261c8fcb0ada57fbbdf66c8d8012d30b1ae Mon Sep 17 00:00:00 2001
From: iwanhae <iwanhae@gmail.com>
Date: Mon, 31 Aug 2026 22:21:53 +0900
Subject: [PATCH] 010: memory & stability tuning for 1GB e2-micro

max_connections 100->30, effective_cache_size 768MB, statement_timeout
30s, idle_in_transaction_session_timeout 60s, temp_file_limit 2GB via
conf.d drop-in; restart only when max_connections changes.
---
 migrations/010-tune-postgres.sh | 100 ++++++++++++++++++++++++++++++++
 1 file changed, 100 insertions(+)
 create mode 100644 migrations/010-tune-postgres.sh

diff --git a/migrations/010-tune-postgres.sh b/migrations/010-tune-postgres.sh
new file mode 100644
index 0000000..dca711e
--- /dev/null
+++ b/migrations/010-tune-postgres.sh
@@ -0,0 +1,100 @@
+#!/usr/bin/env bash
+# 010-tune-postgres - Memory & stability tuning for the 1GB e2-micro host
+# (B-tier of the 2026-08 tuning review). Settings land as a conf.d drop-in;
+# postgresql.conf itself stays pristine (include_dir = 'conf.d' is stock).
+#
+#   max_connections        100 -> 30     each backend costs MBs; real
+#                                        concurrency comes from PgBouncer
+#                                        (013). 30 = pool 20 + admin headroom
+#   effective_cache_size    4GB -> 768MB  planner hint; box has 1GB, the 4GB
+#                                        default overrates cache -> bad plans
+#   work_mem                4MB (explicit, equals default; kept for determinism)
+#   statement_timeout       0 -> 30s      runaway-query guard; long migrations:
+#                                        SET statement_timeout = 0; in-session
+#   idle_in_transaction_session_timeout 0 -> 60s   leaked transactions block
+#                                        vacuum and hold locks; kill them
+#   temp_file_limit         -1 -> 2GB     per-backend sort-spill cap so one bad
+#                                        query cannot fill the 20G data disk
+# up:    write drop-in; restart only if max_connections changes, else reload
+# down:  remove drop-in; restart (restoring max_connections needs it)
+set -euo pipefail
+
+PGBIN=/etc/postgresql/18/main
+CONF=$PGBIN/conf.d/010-tune-postgres.conf
+CLUSTER=$(pg_lsclusters --no-header | awk 'NR==1{print $1"/"$2}')
+
+# desired state as "guc value" pairs; SHOW renders exactly these strings
+WANT=(
+  'max_connections 30'
+  'effective_cache_size 768MB'
+  'work_mem 4MB'
+  'statement_timeout 30s'
+  'idle_in_transaction_session_timeout 60s'
+  'temp_file_limit 2GB'
+)
+
+pg() { runuser -u postgres -- psql -d postgres -v ON_ERROR_STOP=1 -tAc "$1"; }
+
+write_conf() {
+  install -d -m 755 "$PGBIN/conf.d"
+  cat > "$CONF" <<'EOF'
+# managed by sysmig 010-tune-postgres - memory & stability for 1GB e2-micro
+max_connections = 30
+effective_cache_size = 768MB
+work_mem = 4MB
+statement_timeout = 30s
+idle_in_transaction_session_timeout = 60s
+temp_file_limit = 2GB
+EOF
+  chown root:postgres "$CONF"
+  chmod 644 "$CONF"
+}
+
+case "${1:-}" in
+  up)
+    write_conf
+    if [[ $(pg 'SHOW max_connections') != 30 ]]; then
+      pg_ctlcluster "$CLUSTER" restart
+      pg_isready -q
+      echo "    cluster restarted (max_connections requires it)"
+    else
+      pg_ctlcluster "$CLUSTER" reload
+      echo "    cluster reloaded"
+    fi
+    for kv in "${WANT[@]}"; do
+      read -r guc want <<<"$kv"
+      got=$(pg "SHOW $guc")
+      [[ $got == "$want" ]] || { echo "FATAL: $guc is '$got', expected '$want'" >&2; exit 1; }
+    done
+    echo "    ${#WANT[@]} memory/stability settings active"
+    ;;
+  status)
+    if [[ $EUID -eq 0 ]]; then
+      q() { runuser -u postgres -- psql -d postgres -v ON_ERROR_STOP=1 -tAc "$1"; }
+    elif psql -d postgres -tAc 'SELECT 1' >/dev/null 2>&1; then
+      q() { psql -d postgres -v ON_ERROR_STOP=1 -tAc "$1"; }
+    else
+      echo "? needs root or peer DB access to verify"
+      exit 2
+    fi
+    [[ -f $CONF ]] || { echo "DRIFT drop-in $CONF missing"; exit 1; }
+    bad=()
+    for kv in "${WANT[@]}"; do
+      read -r guc want <<<"$kv"
+      got=$(q "SHOW $guc" 2>/dev/null)
+      [[ $got == "$want" ]] || bad+=("$guc=$got(want $want)")
+    done
+    if ((${#bad[@]})); then echo "DRIFT ${bad[*]}"; exit 1; fi
+    echo "OK ${#WANT[@]} settings active (max_connections=30, timeouts, temp_file_limit)"
+    ;;
+  down)
+    rm -f "$CONF"
+    if [[ $(pg 'SHOW max_connections') == 30 ]]; then
+      pg_ctlcluster "$CLUSTER" restart
+      echo "    drop-in removed, cluster restarted (defaults restored)"
+    else
+      pg_ctlcluster "$CLUSTER" reload
+      echo "    drop-in removed, cluster reloaded"
+    fi
+    ;;
+esac