# bgwriter_lru_multiplier

> bgwriter_lru_multiplier — Multiple of the average buffer usage to free per round. Observed in PG9.0–19 Beta 3; its last measured boot default is 2 in PG19 Beta 3, with sighup context. This is a beta-snapshot fact and can change before PostgreSQL 19 GA.
---

> [!NOTE]
> **Fact — official short description:** “Multiple of the average buffer usage to free per round.”

## Identity {#identity}

| Field | Value | Meaning |
| --- | --- | --- |
| Type | `real` | Upstream pg_settings type |
| Context | `sighup` | Takes effect after configuration reload |
| Unit | — | Raw unit |
| Range | `0` – `10` | Raw limits in the last observed version |
| Enum values | — | — for non-enum types |
| Category | Resource Usage / Background Writer | Upstream classification |
| Latest boot value | `2` | 2 |
{.fields meta="-"}

## Lifecycle {#lifecycle}

| Fact | Value |
| --- | --- |
| First observed | PG9.0 (research boundary) |
| Present in | PG9.0–19 Beta 3 |
| Removed in | No |
| Introduction commit | Not asserted: predates the PG9.0 research boundary |
| Commit date | — |
| Discussion | — |

## Default history {#default-history}

| Versions | Raw `boot_val` | Unit | Human value |
| --- | --- | --- | --- |
| PG9.0–19 Beta 3 | `2` | — | 2 |
{.full-width caption="Measured PG9.0–19 Beta 3 boot defaults"}

## How it works {#mechanism}

The background writer averages recent buffer allocations and multiplies that demand by bgwriter_lru_multiplier to choose a target number of clean reusable buffers. A value above 1 adds cushion for bursts.

This is a forecast multiplier, not a direct number of pages. Actual writes remain capped by bgwriter_lru_maxpages and occur once per bgwriter_delay round.

More cushion can reduce foreground backend writes and latency spikes, but pages dirtied repeatedly between checkpoints may be written more times, increasing total I/O. Its SIGHUP context allows configuration reload without a server restart.

## Tuning advice {#tuning-advice}

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

| Workload | Guidance |
| --- | --- |
| OLTP | Tune bgwriter_lru_multiplier only with bgwriter and checkpoint metrics. The goal is fewer backend writes and smoother latency without excessive repeated writes; change one dimension at a time. |
| OLAP | Bulk writes can reach bgwriter_lru_multiplier limits continuously. Measure total bytes written, checkpoints, and storage queueing, not just foreground query latency. |
| Small nodes | A small host usually needs conservative write smoothing. Aggressive bgwriter_lru_multiplier can consume I/O needed by foreground work, so retain the default unless backend writes are a measured problem. |
{.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 | `5.0` | different | `5.0` |
| OLAP | `5.0` | different | `5.0` |
| CRIT | `5.0` | different | `5.0` |
| TINY | `5.0` | different | `5.0` |
{.full-width}

> [!CAUTION]
> **Advice — pending human review.** Fact from the current Pigsty template projection: OLTP: PG9.0–19 Beta 3 = 5.0 (dcs); OLAP: PG9.0–19 Beta 3 = 5.0 (dcs); CRIT: PG9.0–19 Beta 3 = 5.0 (dcs); TINY: PG9.0–19 Beta 3 = 5.0 (dcs). Advice, pending human review — Editorial inference, pending maintainer review: The larger forecast cushion is intended to absorb bursts in buffer demand, accepting possible extra background writes.

## Common pitfalls {#common-pitfalls}

- Changing bgwriter_lru_multiplier without applying its documented unit and configuration context.
- Optimizing an isolated benchmark while ignoring concurrent aggregate resource use.
- Assuming a configured value guarantees operating-system or storage behavior.
- Failing to retest startup, failover, and workload latency after the change.

## Related parameters {#related-parameters}

[`bgwriter_lru_maxpages`](/parameters/bgwriter-lru-maxpages/) · [`bgwriter_delay`](/parameters/bgwriter-delay/) · [`bgwriter_flush_after`](/parameters/bgwriter-flush-after/) · [`shared_buffers`](/parameters/shared-buffers/) · [`checkpoint_completion_target`](/parameters/checkpoint-completion-target/)

## References {#references}

- [PostgreSQL 19 Beta 3 — bgwriter_lru_multiplier](https://www.postgresql.org/docs/19/runtime-config-resource.html#GUC-BGWRITER-LRU-MULTIPLIER)
- [PostgreSQL 19 release notes](https://www.postgresql.org/docs/19/release-19.html)
- [Machine-readable GUC export](/data/guc.jsonl)