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Re: Buffer locking is special (hints, checksums, AIO writes)

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Andres Freund <andres@anarazel.de>, Melanie Plageman <melanieplageman@gmail.com>, Noah Misch <noah@leadboat.com>
Cc: Kirill Reshke <reshkekirill@gmail.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Michael Paquier <michael.paquier@gmail.com>
Date: 2026-02-07T12:38:53Z
Lists: pgsql-hackers 
On 03/02/2026 00:33, Andres Freund wrote:
>    - Now that we use the normal order of WAL logging, we don't need to delay
>      checkpoint starts anymore.
> 
>      I think the explanation for why that is ok is correct [1], but it needs to
>      be looked at by somebody with experience around this. Maybe Heikki?

So that's patch 0004 "bufmgr: Switch to standard order in 
MarkBufferDirtyHint()". Yes, looks correct to me.

> 	/*
> 	 * Update RedoRecPtr so that we can make the right decision. It's possible
> 	 * that a new checkpoint will start just after GetRedoRecPtr(), but that
> 	 * is ok, as the buffer is already dirty, ensuring that any BufferSync()
> 	 * started after the buffer was marked dirty cannot complete without
> 	 * flushing this buffer.  If a checkpoint started between marking the
> 	 * buffer dirty and this check, we will emit an unnecessary WAL record (as
> 	 * the buffer will be written out as part of the checkpoint), but the
> 	 * window for that is small.
> 	 */
> 	RedoRecPtr = GetRedoRecPtr();

That "small window" is actually pretty big if you think of it a little 
more loosely. Our rule is that we write the full page image if a 
checkpoint has started since the page LSN, but that's very conservative 
already. It would be sufficient to write the full page image only if the 
checkpoint has already flushed the page. This small window is just a 
special case of that conservatism.

I've been thinking of trying track that more accurately for a long time, 
because it would smoothen the WAL spike when a checkpoint begins.

That gets off-topic, but my point is that it feels a little silly to 
mention that small window when there's the other giant panoramic window 
next to it.

- Heikki

Commits

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

  1. bufmgr: Fix ordering of checks in PinBuffer()

  2. Use UnlockReleaseBuffer() in more places

  3. bufmgr: Make UnlockReleaseBuffer() more efficient

  4. bufmgr: Don't copy pages while writing out

  5. Fix use of wrong variable in _hash_kill_items()

  6. Fix bug due to confusion about what IsMVCCSnapshot means

  7. bufmgr: Switch to standard order in MarkBufferDirtyHint()

  8. bufmgr: Remove the, now obsolete, BM_JUST_DIRTIED

  9. Require share-exclusive lock to set hint bits and to flush

  10. heapam: Don't mimic MarkBufferDirtyHint() in inplace updates

  11. bufmgr: Allow conditionally locking of already locked buffer

  12. bufmgr: Avoid spurious compiler warning after fcb9c977aa5

  13. lwlock: Remove ForEachLWLockHeldByMe

  14. lwlock: Remove support for disowned lwlwocks

  15. bufmgr: Implement buffer content locks independently of lwlocks

  16. bufmgr: Change BufferDesc.state to be a 64-bit atomic

  17. lwlock: Improve local variable name

  18. lwlock: Invert meaning of LW_FLAG_RELEASE_OK

  19. bufmgr: Make definitions related to buffer descriptor easier to modify

  20. heapam: Add batch mode mvcc check and use it in page mode

  21. freespace: Don't modify page without any lock

  22. heapam: Move logic to handle HEAP_MOVED into a helper function

  23. bufmgr: Optimize & harmonize LockBufHdr(), LWLockWaitListLock()

  24. bufmgr: Add one-entry cache for private refcount

  25. bufmgr: Separate keys for private refcount infrastructure

  26. Add pg_atomic_unlocked_write_u64

  27. Rename BUFFERPIN wait event class to BUFFER

  28. bufmgr: Turn BUFFER_LOCK_* into an enum

  29. lwlock: Fix, currently harmless, bug in LWLockWakeup()

  30. bufmgr: Use atomic sub for unpinning buffers

  31. bufmgr: Allow some buffer state modifications while holding header lock

  32. bufmgr: Fix valgrind checking for buffers pinned in StrategyGetBuffer()

  33. bufmgr: Don't lock buffer header in StrategyGetBuffer()

  34. bufmgr: fewer calls to BufferDescriptorGetContentLock

  35. bufmgr: Fix signedness of mask variable in BufferSync()

  36. bufmgr: Introduce FlushUnlockedBuffer

  37. Improve ReadRecentBuffer() scalability