# max_slot_wal_keep_size

> A reloadable checkpoint-time limit on how much WAL replication slots may retain. It appears from PG13 in this catalog, defaults to -1 (unlimited), and Pigsty applies a finite disk-relative cap.
---

> [!NOTE]
> **Fact — official short description:** “Sets the maximum WAL size that can be reserved by replication slots.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `integer` | Upstream pg_settings type |
| Context | `sighup` | Takes effect after configuration reload |
| Unit | `MB` | Raw unit |
| Range | `-1` – `2147483647` | Raw limits in the last observed version |
| Enum values | — | — for non-enum types |
| Category | Replication / Sending Servers | Upstream classification |
| Latest boot value | `-1` | -1 MB |
{.fields meta="-"}

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG13 |
| Present in | PG13–19 Beta 3 |
| Removed in | No |
| Introduction commit | [`c6550776394e`](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=c6550776394e25c1620bc8258427c8f1d448080d) — Allow users to limit storage reserved by replication slots |
| Commit date | 2020-04-07 |
| Discussion | [thread 1](https://postgr.es/m/20170228.122736.123383594.horiguchi.kyotaro@lab.ntt.co.jp) |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG13–19 Beta 3 | `-1` | `MB` | -1 MB |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

At checkpoint time PostgreSQL compares a slot's restart_lsn with the current WAL position. With the default -1 there is no parameter-imposed maximum; a stalled consumer can therefore retain enough WAL to fill pg_wal.

With a finite limit, required segments can be released when a slot falls too far behind. The slot can move through unreserved to lost status, and its consumer may need a new base backup or logical resynchronization. The cap protects server storage by sacrificing the guarantee that an indefinitely lagging slot remains usable.

Enforcement is checkpoint-based, so it is neither instantaneous nor an exact byte ceiling. pg_replication_slots exposes restart_lsn, wal_status, invalidation_reason, and safe_wal_size; these are the operational signals needed before the slot reaches the limit.

## Tuning advice {#tuning-advice}

> [!TIP]
> **Advice.** These are workload-specific starting points and must be validated with measurements.

| Workload | Guidance |
| --- | --- |
| OLTP | Set a finite limit within the pg_wal emergency budget and derive it from peak WAL generation times the maximum supported repair window. Alert while safe_wal_size is still positive, and document the resynchronization path for every consumer. |
| OLAP | Allow for expected bulk-WAL bursts, but do not make the cap effectively unlimited. Schedule consumers and batch jobs together, verify archive availability, and pause or repair a lagging consumer before it consumes the disk budget. |
| Small nodes | Use a stricter finite cap and a short repair objective. On a small volume, preserving the primary is usually more important than keeping a stale replica or CDC slot resumable forever. |
{.full-width}

## Pigsty {#pigsty}

Values use the fixed 8-vCPU, 32-GiB, 100-GiB SSD fixture and render the current Pigsty templates for PG19 Beta 3; this does not assert current Pigsty support for that historical or beta release.

| Template | Effective value | Versus upstream boot | Source expression |
| --- | --- | --- | --- |
| OLTP | `30GB` | different | `{{ ([pg_size_twentieth * 6, 3000])\|min }}GB` |
| OLAP | `30GB` | different | `{{ ([pg_size_twentieth * 6, 3000])\|min }}GB` |
| CRIT | `30GB` | different | `{{ ([pg_size_twentieth * 6, 3000])\|min }}GB` |
| TINY | `30GB` | different | `{{ ([pg_size_twentieth * 6, 3000])\|min }}GB` |
{.full-width}

> [!CAUTION]
> **Advice — pending human review.** Fact from the current Pigsty template projection: OLTP: PG13–19 Beta 3 = 30GB (dcs); OLAP: PG13–19 Beta 3 = 30GB (dcs); CRIT: PG13–19 Beta 3 = 30GB (dcs); TINY: PG13–19 Beta 3 = 30GB (dcs). Advice, pending human review — confirm the operational intent against the current Pigsty templates and supported PostgreSQL versions before publication.

## Common pitfalls {#common-pitfalls}

- Leaving -1 and assuming a stalled slot cannot fill the disk.
- Treating the finite value as an instantaneous hard ceiling despite checkpoint-based enforcement.
- Setting it too low and invalidating a healthy consumer during a planned bulk load.
- Assuming a consumer can always resume after its required WAL has been removed.
- Confusing this per-slot retention cap with the wal_keep_size retention floor.

## Related parameters {#related-parameters}

[`max_replication_slots`](/parameters/max-replication-slots/) · [`wal_keep_size`](/parameters/wal-keep-size/) · [`max_wal_size`](/parameters/max-wal-size/) · [`idle_replication_slot_timeout`](/parameters/idle-replication-slot-timeout/) · [`checkpoint_timeout`](/parameters/checkpoint-timeout/) · [`archive_mode`](/parameters/archive-mode/)

## References {#references}

- [PostgreSQL 19 Beta 3 — max_slot_wal_keep_size](https://www.postgresql.org/docs/19/runtime-config-replication.html#GUC-MAX-SLOT-WAL-KEEP-SIZE)
- [PostgreSQL 18: Replication Slots](https://www.postgresql.org/docs/18/warm-standby.html#STREAMING-REPLICATION-SLOTS)
- [PostgreSQL 18: pg_replication_slots](https://www.postgresql.org/docs/18/view-pg-replication-slots.html)
- [PostgreSQL 19 release notes](https://www.postgresql.org/docs/19/release-19.html)
- [Machine-readable GUC export](/data/guc.jsonl)