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Re: eliminate xl_heap_visible to reduce WAL (and eventually set VM on-access)

Melanie Plageman <melanieplageman@gmail.com>

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From: Melanie Plageman <melanieplageman@gmail.com>
To: Alexander Lakhin <exclusion@gmail.com>
Cc: Andres Freund <andres@anarazel.de>, Tomas Vondra <tomas@vondra.me>, David Rowley <dgrowleyml@gmail.com>, Kirill Reshke <reshkekirill@gmail.com>, Chao Li <li.evan.chao@gmail.com>, Andrey Borodin <x4mmm@yandex-team.ru>, Xuneng Zhou <xunengzhou@gmail.com>, Robert Haas <robertmhaas@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Heikki Linnakangas <hlinnaka@iki.fi>
Date: 2026-04-05T15:42:19Z
Lists: pgsql-hackers 
On Fri, Apr 3, 2026 at 1:00 AM Alexander Lakhin <exclusion@gmail.com> wrote:
>
> I've come across an interesting failure produced starting from 378a21618:
> when using a build made with CFLAGS="-DRELCACHE_FORCE_RELEASE" and
> echo "io_method = sync" >/tmp/temp.config, the test run:
> TEMP_CONFIG=/tmp/temp.config TESTS=temp make -s check-tests
>
> fails as below:
> --- .../src/test/regress/expected/temp.out   2026-02-13 06:15:55.887368624 +0200
> +++ .../src/test/regress/results/temp.out    2026-04-03 07:51:36.735504156 +0300
> @@ -493,11 +493,7 @@
>
>   -- Check that read streams deal with lower number of pins available
>   SELECT count(*), max(a) max_a, min(a) min_a, max(cnt) max_cnt FROM test_temp;
> - count | max_a | min_a | max_cnt
> --------+-------+-------+---------
> - 10000 | 10000 |     1 |       0
> -(1 row)
> -
> +ERROR:  no empty local buffer available
>   ROLLBACK;

It has to do with the query needing an additional pin for the VM
during on-access pruning and the read stream reading ahead until there
is only one remaining buffer pin in the local pin limit (the cursor
above is already consuming much of the backend local pin limit). We
could perhaps fix this test by decreasing the pages in the relation or
increasing the backend local pin limit, but I wonder if we need to do
something more invasive to ensure that we can pin at least two
buffers.

FWIW, I don't think this can be hit with the FSM, because we release
the heap buffer pin before pinning it. Though perhaps there are other
places in the code where scanning a single buffer with the read stream
entails pinning at least one other buffer.

- Melanie

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Stabilize btree_gist test against on-access VM setting

  2. Stabilize plancache test against on-access VM setting

  3. Make local buffers pin limit more conservative

  4. Update FSM during prune/freeze replay even if freespace is zero

  5. Pass down information on table modification to scan nodes

  6. Thread flags through begin-scan APIs

  7. Remove PlannedStmt->resultRelations in favor of resultRelationRelids

  8. Make it cheap to check if a relation is modified by a query

  9. Keep newest live XID up-to-date even if page not all-visible

  10. Add pruning fast path for all-visible and all-frozen pages

  11. Detect and fix visibility map corruption in more cases

  12. Save vmbuffer in heap-specific scan descriptors for on-access pruning

  13. Avoid BufferGetPage() calls in heap_update()

  14. Initialize missing fields in CreateExecutorState()

  15. Prefix PruneState->all_{visible,frozen} with set_

  16. Add PageGetPruneXid() helper

  17. Move commonly used context into PruneState and simplify helpers

  18. Remove table_scan_analyze_next_tuple unneeded parameter OldestXmin

  19. Simplify visibility check in heap_page_would_be_all_visible()

  20. Rename LVRelState VM-related logging counters

  21. Fix flakiness in the pg_visibility VM-only vacuum test by using a temporary table.

  22. Refactor lazy_scan_prune() VM clear logic into helper

  23. Eliminate use of cached VM value in lazy_scan_prune()

  24. Combine visibilitymap_set() cases in lazy_scan_prune()

  25. Fix const qualification in prune_freeze_setup()

  26. Simplify vacuum visibility assertion

  27. Split heap_page_prune_and_freeze() into helpers

  28. Assert that cutoffs are provided if freezing will be attempted

  29. Split PruneFreezeParams initializers to one field per line

  30. Refactor heap_page_prune_and_freeze() parameters into a struct

  31. Make heap_page_is_all_visible independent of LVRelState

  32. Inline TransactionIdFollows/Precedes[OrEquals]()

  33. Add helper for freeze determination to heap_page_prune_and_freeze

  34. Bump XLOG_PAGE_MAGIC after xl_heap_prune change

  35. Correct prune WAL record opcode name in comment

  36. Add error codes when vacuum discovers VM corruption

  37. Remove unused xl_heap_prune member, reason

  38. Remove unneeded VM pin from VM replay

  39. Add assert and log message to visibilitymap_set

  40. Stamp HEAD as 19devel.

  41. Add error codes to some corruption log messages