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

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Alexey Makhmutov <a.makhmutov@postgrespro.ru>
Cc: Melanie Plageman <melanieplageman@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Kirill Reshke <reshkekirill@gmail.com>, Andrey Borodin <x4mmm@yandex-team.ru>, Robert Haas <robertmhaas@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>
Date: 2026-04-06T14:44:33Z
Lists: pgsql-hackers 
Hi,

On 2026-04-06 17:30:51 +0300, Alexey Makhmutov wrote:
> Sorry for the late note for the already committed patch, but I have a
> question on the last part of the 'heap_xlog_prune_freeze' function related
> to the FSM update (it was committed in add323d -'Eliminate
> XLOG_HEAP2_VISIBLE from vacuum phase III').

FWIW, I don't think it's ever too late to look at commits, and certainly not
when it's a commit from the same release.


> Currently it contains the following logic:
> 	
> ...
> Size	freespace = 0;
> ...
> if (BufferIsValid(buffer))
> {
> 	if ((xlrec.flags & (XLHP_HAS_REDIRECTIONS |
> 						XLHP_HAS_DEAD_ITEMS |
> 						XLHP_HAS_NOW_UNUSED_ITEMS)) ||
> 		(vmflags & VISIBILITYMAP_VALID_BITS))
> 		freespace = PageGetHeapFreeSpace(BufferGetPage(buffer));
> ...
> 	UnlockReleaseBuffer(buffer);
> }
> ...
> if (freespace > 0)
> 	XLogRecordPageWithFreeSpace(rlocator, blkno, freespace);
> ...
> 		
> My question is about the last check ('freespace > 0') - do we really want to
> call 'XLogRecordPageWithFreeSpace' only if 'freespace' is greater than 0? As
> I understand, the zero value is a perfectly valid output of the
> 'PageGetHeapFreeSpace' call (i.e. page has no space or no free line items
> while we mark all rows as frozen/visible), but with the current
> implementation we will skip FSM update in such case.

I don't have a strong opinion on this, but I think it's pretty defensible to
record only when there's free space. The whole goal of updating the FSM during
recovery is to make sure that free space can be found fairly quickly after
promotion (it's also beneficial in some crash recovery cases, but not that
much).

If the page filled up during an insert / update / delete, we will have updated
the FSM with that information at that point:

	/*
	 * If the page is running low on free space, update the FSM as well.
	 * Arbitrarily, our definition of "low" is less than 20%. We can't do much
	 * better than that without knowing the fill-factor for the table.
	 *
	 * XXX: Don't do this if the page was restored from full page image. We
	 * don't bother to update the FSM in that case, it doesn't need to be
	 * totally accurate anyway.
	 */
	if (action == BLK_NEEDS_REDO && freespace < BLCKSZ / 5)
		XLogRecordPageWithFreeSpace(target_locator, blkno, freespace);


The reason to update the FSM after something pruning / vacuuming related is
that there now might be *more* space available than before, it shouldn't
shrink.

Obviously the FSM is not crashsafe, so updating it with 0 during replay could
avoid some unnecessary page reads after a promotion. But I'm not sure that
that's particularly worth optimizing for.


With all that said, I'm somewhat doubtful that freespace > 0 filters out a
meaningful amount of freespace updates, it's rare for pages to be that full.

Greetings,

Andres Freund

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