Subject: WAL Replay Buffer Invalidation Conflicts During Page Truncation on Read Replicas

From: Dharin Shah <[email-masked]>
Date: Tue, 8 Jul 2025 10:12:09 +0200

Hello All,

I'm experiencing significant read query blocking on Aurora PostgreSQL read
replicas during VACUUM relation truncation, particularly with TOAST tables.
This affects a high-traffic service (~3000 req/sec) and causes application
downtime.

*Problem Summary:*
  WAL replay of relation truncation operations on read replicas triggers
buffer invalidation that requires AccessExclusive locks, blocking
concurrent read queries for extended periods.

*Environment Details:*
  - Aurora PostgreSQL (read replica setup)
  - Workload: Async writes to primary, read-only queries on replica
  - TOAST table with ~4KB average compressed column size
  - maintenance_work_mem: 2087MB

* Observed Behavior:*

*  [23541]: select gzipped_dto from <table> dl1_0 where
(dl1_0.entity_id,dl1_0.language_code) in (($1,$2))  2025-06-28 11:57:34
UTC: process 23574 still waiting for AccessShareLock on relation 20655
after 1000.035 ms*

  The blocking coincides with substantial TOAST table truncation:

*2025-06-28 11:57:39 UTC::@:[8399]:LOG:  automatic vacuum of table
"delivery.pg_toast.pg_toast_20652": index scans: 1 pages: 212964 removed,
434375 remain, 78055 scanned (12.06% of total) tuples: 198922 removed,
2015440 remain, 866 are dead but not yet removable removable cutoff:
1066590201, which was 783 XIDs old when operation ended frozen: 3 pages
from table (0.00% of total) had 19 tuples frozen index scan needed: 39600
pages from table (6.12% of total) had 199413 dead item identifiers removed
index "pg_toast_20652_index": pages: 16131 in total, 35 newly deleted, 7574
currently deleted, 7539 reusable I/O timings: read: 173469.911 ms, write:
0.000 ms avg read rate: 9.198 MB/s, avg write rate: 0.000 MB/s buffer
usage: 220870 hits, 213040 misses, 0 dirtied WAL usage: 0 records, 0 full
page images, 0 bytes system usage: CPU: user: 2.97 s, system: 1.86 s,
elapsed: 180.95 s*

*Analysis:*
  The vacuum reclaimed 212,964 pages (33% of the relation), indicating
legitimate space reclamation. With maintenance_work_mem set to 2087MB,
memory constraints aren't limiting the vacuum process. However, WAL replay
of the truncation
  operation on the read replica requires invalidating these pages from
shared_buffers, which conflicts with ongoing read queries.

*Questions for Discussion:*

  1. Batch Buffer Invalidation: Could buffer invalidation during WAL replay
be batched or deferred to reduce lock contention duration?
  2. Replica-Specific Truncation Policy: Should read replicas have
different truncation thresholds
(REL_TRUNCATE_MINIMUM/REL_TRUNCATE_FRACTION) to balance space reclamation
against query availability?
  3. Cloud-Native Considerations: In cloud environments like Aurora with
separate storage layers, is immediate buffer invalidation during truncation
replay necessary, or could this be optimized?
  4. Lock Duration Optimization: The current truncation process holds
AccessExclusive locks during the entire invalidation. Could this be
shortened through incremental processing?

*Potential Approaches:*
  - Implement configurable truncation behavior for standby servers
  - Add batching/throttling to buffer invalidation during WAL replay
  - Provide a way to defer truncation replay during high read activity
periods

This issue particularly affects TOAST tables due to their chunked storage
pattern creating more opportunities for dead space, but the core problem
applies to any significant relation truncation on read replicas.

Has anyone else encountered this issue? Are there existing configuration
options or patches that address WAL replay buffer invalidation conflicts?

Thanks for any insights.

Thanks,
Dharin

---

From: Álvaro Herrera <[email-masked]>
Date: Tue, 8 Jul 2025 16:05:18 +0200

> concurrent read queries for extended periods.

Hmm, sounds like disabling truncate of the TOAST relation by vacuum
could help.  We have configuration options for that -- one is per table
and was added in Postgres 12, changed with
  ALTER TABLE ... SET (vacuum_truncate=off);
I think you can also do
  ALTER TABLE ... SET (toast.vacuum_truncate=off);
to disable it for the TOAST table.

Postgres 18 added a global parameter of the same name which you can
change in postgresql.conf, and from the commit message it sound like it
was added to cope with scenarios precisely like yours.  But if for you
it's always the same toast table (or a small number of them) then I
would think it'd be better to change the per-table param for those.
(Also, this won't require that you upgrade to Postgres 18 just yet,
which sounds particularly helpful in case Aurora doesn't offer that version.)

Here it's the commit message for the change in 18, see the "Discussion"
link for more info:

commit 0164a0f9ee12e0eff9e4c661358a272ecd65c2d4
Author:     Nathan Bossart <[email-masked]> []
AuthorDate: Thu Mar 20 10:16:50 2025 -0500
CommitDate: Thu Mar 20 10:16:50 2025 -0500

    Add vacuum_truncate configuration parameter.
    
    This new parameter works just like the storage parameter of the
    same name: if set to true (which is the default), autovacuum and
    VACUUM attempt to truncate any empty pages at the end of the table.
    It is primarily intended to help users avoid locking issues on hot
    standbys.  The setting can be overridden with the storage parameter
    or VACUUM's TRUNCATE option.
    
    Since there's presently no way to determine whether a Boolean
    storage parameter is explicitly set or has just picked up the
    default value, this commit also introduces an isset_offset member
    to relopt_parse_elt.
    
    Suggested-by: Will Storey <[email-masked]>
    Author: Nathan Bossart <[email-masked]>
    Co-authored-by: Gurjeet Singh <[email-masked]>
    Reviewed-by: Laurenz Albe <[email-masked]>
    Reviewed-by: Fujii Masao <[email-masked]>
    Reviewed-by: Robert Treat <[email-masked]>
    Discussion: https://postgr.es/m/Z2DE4lDX4tHqNGZt%40dev.null

-- 
Álvaro Herrera         PostgreSQL Developer  —  https://www.EnterpriseDB.com/
Al principio era UNIX, y UNIX habló y dijo: "Hello world\n".
No dijo "Hello New Jersey\n", ni "Hello USA\n".

---

From: Dharin Shah <[email-masked]>
Date: Tue, 8 Jul 2025 23:15:50 +0200

Thanks Alvaro,

I read the thread and actually disabled truncate on my table with a large
toast table which mitigated the issue. Unsure what happens with the empty
pages now, I guess they would be reused for new inserts.
I would like to see if there are better opportunities to improve this
truncation process. Perhaps identify why we need this arbitrary threshold
to determine a full buffer scan?

*/**
* * This is the size (in the number of blocks) above which we scan the*
* * entire buffer pool to remove the buffers for all the pages of relation*
* * being dropped. For the relations with size below this threshold, we
find*
* * the buffers by doing lookups in BufMapping table.*
* */*
*#define BUF_DROP_FULL_SCAN_THRESHOLD (uint64) (NBuffers / 32)*

https://github.com/postgres/postgres/blob/e03c95287764158941d317972a332565729b6af2/src/backend/storage/buffer/bufmgr.c#L91

As this can cause significant issues as we scale memory for shared buffers.
(Very often the case with Aurora)

Thanks,
Dharin