Follow-up to the previous two commits: the ALTER DATABASE ... SET
autovacuum_* commands fail at boot with
ERROR: parameter "autovacuum_vacuum_scale_factor" cannot be changed now
and take matrix-synapse-db-tune.service (and nextcloud-db-init.service)
down with them.
Root cause: ALTER DATABASE/ROLE ... SET validates through
set_config_option() with an interactive context, and guc.c rejects any
PGC_SIGHUP parameter set that way. All autovacuum_* GUCs are
SIGHUP-context, so per-database scoping is impossible for them — only
USERSET-level parameters (e.g. work_mem, statement_timeout) can be set
per-database.
Nothing is lost: the same values are already set cluster-wide in
configuration.nix, which covers both nextclouddb and matrix-synapse.
Remove the ALTERs from nextcloud-db-init and delete the now-purposeless
matrix-synapse-db-tune service.
Monolith Synapse spends most of its RAM on caches to avoid Postgres
round-trips, but Sovran ships stock cache settings (global_factor 0.5,
10K event cache, no autotuning) and a default 5-connection DB pool.
- caches.global_factor 4.0 + 100K event cache + autotuning capped at
2G (target 1G), with boosts for the /sync and room-join hot paths.
- DB pool cp_min 5 / cp_max 15, txn_limit 10000 (fewer reconnects).
- gc_thresholds raised to cut GC pauses on a 32 GB box.
- cache-memory extra for cache-size statistics.
- Per-database autovacuum (ALTER DATABASE, scoped to matrix-synapse)
matching the nextclouddb tuning.
Deliberately unchanged: presence and URL previews stay enabled
(disabling them is faster but user-visible), and no workers — monolith
is the right call under ~100 users. Workers would need Redis
replication, the redis extra, and Caddy reverse-proxy rework; revisit
if federation load ever justifies it.
Nextcloud 35's Database checks flag three Performance issues out of the
box: buffer cache hit ratio ~96% (wants 99%+), 100k+ dead tuples, and
million-plus sequential scans on oc_mail_tags / oc_guests_users.
Root causes in Sovran: stock 128MB shared_buffers, stock 60s autovacuum
naptime, APCu file locking, and db:add-missing-indices running exactly
once at install time (never on upgrades or app installs).
Size Postgres for the README's Server + Desktop recommendation (32 GB
RAM, NVMe): 2GB shared_buffers, 12GB effective_cache_size, 512MB
maintenance_work_mem, 32MB work_mem, 4GB max_wal_size, 30s autovacuum
naptime with 4 workers. shared_buffers stays below the 25% rule because
Postgres shares the box with bitcoind, Electrs, LND, MariaDB and PHP-FPM.
Scope the aggressive autovacuum to nextclouddb via ALTER DATABASE so the
shared matrix-synapse DB keeps the milder cluster defaults.
Add a local Redis (127.0.0.1:6379, Nextcloud only) and move
memcache.distributed/locking to Redis; migrate existing installs with a
one-shot since nextcloud-init never re-runs.
Add a weekly nextcloud-db-maintenance timer (VACUUM ANALYZE +
db:add-missing-*) so upgrades and later app installs can't regress the
checks again.
Note: shared_buffers needs one 'systemctl restart postgresql', which
briefly takes down both Nextcloud and Matrix. Everything else is
reload-only or scoped to nextclouddb.
Sovran Contributor