# max_repack_replication_slots

> max_repack_replication_slots — Sets the maximum number of replication slots for use by REPACK. Observed in PG19 Beta 3; its last measured boot default is 5 in PG19 Beta 3, with postmaster context. This is a beta-snapshot fact and can change before PostgreSQL 19 GA.
---

> [!NOTE]
> **Fact — official short description:** “Sets the maximum number of replication slots for use by REPACK.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `integer` | Upstream pg_settings type |
| Context | `postmaster` | Requires a server restart |
| Unit | — | Raw unit |
| Range | `0` – `262143` | Raw limits in the last observed version |
| Enum values | — | — for non-enum types |
| Category | Replication / Sending Servers | Upstream classification |
| Latest boot value | `5` | 5 |
{.fields meta="-"}

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG19 Beta 3 |
| Present in | PG19 Beta 3 |
| Removed in | No |
| Introduction commit | [`e76d8c749c31`](https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=e76d8c749c3152657711ed733f0aea61c0e36a91) — Reserve replication slots specifically for REPACK |
| Commit date | 2026-04-07 |
| Discussion | [thread 1](https://postgr.es/m/202604012148.nnnmyxxrr6nh@alvherre.pgsql) |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG19 Beta 3 | `5` | — | 5 |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

PostgreSQL describes max_repack_replication_slots as follows: “Sets the maximum number of replication slots for use by REPACK.” The value is fixed when the server starts, so changing it requires a controlled restart. The atlas measures it in PG19 Beta 3; boot_val is the compiled or initialized baseline, not proof of a running cluster's effective setting.

PostgreSQL 19's REPACK command can use dedicated replication slots to reorganize a relation concurrently while tracking changes. This startup ceiling reserves how many such slots the server can support; it interacts with the overall replication-slot and WAL-sender budgets and with retained-WAL disk risk.

Read it together with max_replication_slots, max_wal_senders, wal_level, max_slot_wal_keep_size. Check SHOW and pg_settings on the target server, verify the source and pending_restart fields, and compare workload, logs, and resource metrics before and after any change.

## Tuning advice {#tuning-advice}

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

| Workload | Guidance |
| --- | --- |
| OLTP | Model normal failover and worst-case lag before setting a bound. Observe sender state, retained WAL, shutdown time, and receiver catch-up; preserve enough capacity for monitoring and planned switchovers. |
| OLAP | Account for large transactions, bulk loads, slow apply, and long-distance links. A short timeout can make shutdown faster but transfer recovery work and inconsistency risk to the next startup. |
| Small nodes | Use few slots and senders, watch pg_wal disk consumption, and keep timeout behavior explicit. Test shutdown and restart with the receiver both healthy and unavailable. |
{.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 | Unmodified | — | — |
| OLAP | Unmodified | — | — |
| CRIT | Unmodified | — | — |
| TINY | Unmodified | — | — |
{.full-width}

> [!CAUTION]
> **Advice — pending human review.** Fact from the current Pigsty template projection: OLTP: PG19 Beta 3 unmodified; OLAP: PG19 Beta 3 unmodified; CRIT: PG19 Beta 3 unmodified; TINY: PG19 Beta 3 unmodified. No Pigsty-specific rationale is inferred from an absent override.

## Common pitfalls {#common-pitfalls}

- Treating the measured boot_val for max_repack_replication_slots as proof of the effective value on an initialized or managed cluster.
- Applying a change as though it were immediate while pg_settings reports postmaster context.
- Changing this setting in isolation without checking the linked limits, observability, and rollback path.
- Depending on beta behavior in production without retesting the PostgreSQL 19 final release.

## Related parameters {#related-parameters}

[`max_replication_slots`](/parameters/max-replication-slots/) · [`max_wal_senders`](/parameters/max-wal-senders/) · [`wal_level`](/parameters/wal-level/) · [`max_slot_wal_keep_size`](/parameters/max-slot-wal-keep-size/) · [`wal_sender_shutdown_timeout`](/parameters/wal-sender-shutdown-timeout/)

## References {#references}

- [PostgreSQL 19 Beta 3 — max_repack_replication_slots](https://www.postgresql.org/docs/19/runtime-config-replication.html#GUC-MAX-REPACK-REPLICATION-SLOTS)
- [PostgreSQL 19 release notes](https://www.postgresql.org/docs/19/release-19.html)
- [Machine-readable GUC export](/data/guc.jsonl)