12. SLRU cache statistics

Monitor Simple LRU (SLRU) cache performance for PostgreSQL internal structures.

Screenshot note

SLRU statistics require PostgreSQL 13+ and specific workload patterns to generate meaningful data. In low-activity environments, these panels may show minimal data.

Purpose

SLRU caches store critical PostgreSQL metadata:

• Transaction commit status (CLOG/pg_xact)
• Subtransaction data (pg_subtrans)
• Multixact data
• Serial/notify data

Poor SLRU performance can cause system-wide slowdowns.

When to use

• Investigating unexplained latency
• High wait events on SLRU-related locks
• Transaction ID wraparound preparation
• Performance tuning for high-concurrency workloads

Key panels

The dashboard's SLRU stats row has one panel per pg_stat_slru counter, each broken down by cache type (name).

SLRU blocks hit

What it shows:

• Blocks served from each SLRU cache (cache hits)
• Higher relative to reads is better

Healthy range:

• Hits dominate reads for most caches

SLRU blocks exist

What it shows:

• blks_exists — checks for whether a block already exists in the cache

SLRU blocks read

What it shows:

• Blocks read into each SLRU cache from disk
• Breakdown by cache type

SLRU blocks written

What it shows:

• Write activity to SLRU caches
• High writes may indicate configuration issues

SLRU blocks zeroed

What it shows:

• blks_zeroed — newly initialized (zeroed) SLRU pages

SLRU truncates / SLRU flushes

What it shows:

• truncates — SLRU segment truncations (e.g. as old transaction data is removed)
• flushes — SLRU buffer flushes to disk

SLRU cache types

Cache	Purpose	Tuning parameter
CommitTs	Commit timestamps	track_commit_timestamp
Xact	Transaction status (CLOG)	N/A (auto-managed)
Subtrans	Subtransaction status	N/A
MultiXact	Multixact mappings	N/A
Notify	NOTIFY/LISTEN	max_notify_queue_pages
Serial	Serializable isolation	N/A

Variables

Variable	Purpose
cluster_name	Cluster filter
node_name	Node filter
db_name	Database filter

SLRU statistics query

select
  name,
  blks_zeroed,
  blks_hit,
  blks_read,
  blks_written,
  blks_exists,
  flushes,
  truncates,
  round(100.0 * blks_hit / nullif(blks_hit + blks_read, 0), 2) as hit_ratio
from pg_stat_slru
order by blks_read desc;

Common issues

Low Xact cache hit ratio

Symptoms:

• Slow transaction commits
• High wait events on XactBuffer

Causes:

• Long-running transactions
• Aggressive transaction ID consumption
• Transaction ID wraparound approaching

Solutions:

• Identify and terminate long transactions
• Schedule more frequent vacuums
• Consider connection pooling to reduce transaction overhead

High MultiXact activity

Symptoms:

• Slow select for update or for share
• High multiXact SLRU reads

Causes:

• Heavy use of row-level locking
• Many concurrent for update queries

Solutions:

• Review application locking patterns
• Consider advisory locks for some use cases

Notify queue issues

Symptoms:

• NOTIFY/LISTEN delays
• High notify SLRU activity

Solutions:

• Increase max_notify_queue_pages if needed
• Review NOTIFY usage patterns

Related dashboards

• Wait events — 04. Wait Events
• Node overview — 01. Node Overview

Troubleshooting

No SLRU data

SLRU statistics require PostgreSQL 13+. Check version:

select version();

Understanding SLRU impact

SLRU issues often manifest as:

• LWLock wait events (in Dashboard 04)
• Slow commit times
• Latency spikes during high concurrency

Correlate SLRU metrics with wait event data for diagnosis.