fix(postgres): size the buffers from the Postgres container, in its own role

shared_buffers, work_mem and maintenance_work_mem were a quarter, half
and a twentieth of the host's RAM, while the Postgres container is
limited to 6g. On a 64 GB host that asked for 15997 MB of shared
buffers inside it. They are now the same shares of the container's
mem_limit through lookup('service_memory_mb'): 1536 MB, 3072 MB divided
by max_connections, and 307 MB.

The three variables move from group_vars/all/17_resource.yml into the
role's vars. They no longer describe the host, and only the role's
compose template reads them. Their *_MB helpers and POSTGRES_TOTAL_RAM_MB
had no other reader and are gone. An inventory can no longer override
the three values; the knob is services.postgres.mem_limit.

Discourse's db_shared_buffers was a literal 4096MB, all of the 4g the
role declares. It becomes a quarter of that limit, 1024MB. It only
applies when Discourse runs its bundled database.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

group_vars/all/17_resource.yml

@@ -43,16 +43,9 @@ RESOURCE_MEM_LIMIT: "{{ RESOURCE_MEM_LIMIT_NUM }}g"
 RESOURCE_PIDS_LIMIT: 512
 
 POSTGRES_VCPUS: "{{ ansible_facts['processor_vcpus'] | int }}"
-POSTGRES_TOTAL_RAM_MB: "{{ ansible_facts['memtotal_mb'] | int }}"
 POSTGRES_MAX_CONNECTIONS: "{{ [ ((POSTGRES_VCPUS | int) * 30 + 50), 400 ] | min }}"
 POSTGRES_ALLOWED_AVG_CONNECTIONS: "{{ (POSTGRES_MAX_CONNECTIONS | split_postgres_connections(lookup('path_absolute', 'roles'))) | int }}"
 POSTGRES_SUPERUSER_RESERVED_CONNECTIONS: 3
-POSTGRES_SHARED_BUFFERS_MB: "{{ ((POSTGRES_TOTAL_RAM_MB | int) * 25) // 100 }}"
-POSTGRES_SHARED_BUFFERS: "{{ POSTGRES_SHARED_BUFFERS_MB ~ 'MB' }}"
-POSTGRES_WORK_MEM_MB: "{{ [ ( (POSTGRES_TOTAL_RAM_MB | int) // ( [ (POSTGRES_MAX_CONNECTIONS | int), 1 ] | max ) // 2 ), 1 ] | max }}"
-POSTGRES_WORK_MEM: "{{ POSTGRES_WORK_MEM_MB ~ 'MB' }}"
-POSTGRES_MAINTENANCE_WORK_MEM_MB: "{{ [ (((POSTGRES_TOTAL_RAM_MB | int) * 5) // 100), 64 ] | max }}"
-POSTGRES_MAINTENANCE_WORK_MEM: "{{ POSTGRES_MAINTENANCE_WORK_MEM_MB ~ 'MB' }}"
 POSTGRES_MAX_LOCKS_PER_TRANSACTION: 256
 POSTGRES_DELAY: 2
 

roles/svc-db-postgres/vars/main.yml

@@ -15,3 +15,6 @@ POSTGRES_EXPOSE_LOCAL: true
 POSTGRES_LOCAL_HOST: "127.0.0.1"
 POSTGRES_LIBRARIES: "{{ lookup('postgres_libraries', lookup('service_extensions', 'postgres').values() | sum(start=[]) | unique | list) }}"
 POSTGRES_RETRIES: 5
+POSTGRES_SHARED_BUFFERS: "{{ lookup('service_memory_mb', application_id, entity_name, 0.25) }}MB"
+POSTGRES_WORK_MEM: "{{ [ (lookup('service_memory_mb', application_id, entity_name, 0.5) | int) // ([ POSTGRES_MAX_CONNECTIONS | int, 1 ] | max), 1 ] | max }}MB"
+POSTGRES_MAINTENANCE_WORK_MEM: "{{ [ lookup('service_memory_mb', application_id, entity_name, 0.05) | int, 64 ] | max }}MB"

roles/web-app-discourse/templates/config.yml.j2

@@ -22,7 +22,7 @@ params:
 
   ## Set db_shared_buffers to a max of 25% of the total memory.
   ## will be set automatically by bootstrap based on detected RAM, or you can override
-  db_shared_buffers: "4096MB"
+  db_shared_buffers: "{{ lookup('service_memory_mb', application_id, 'discourse', 0.25) }}MB"
 
   ## can improve sorting performance, but adds memory usage per-connection
   #db_work_mem: "40MB"