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

Kirill Reshke <reshkekirill@gmail.com>

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From: Kirill Reshke <reshkekirill@gmail.com>
To: Andres Freund <andres@anarazel.de>
Cc: Heikki Linnakangas <hlinnaka@iki.fi>, Melanie Plageman <melanieplageman@gmail.com>, Noah Misch <noah@leadboat.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: 2026-02-09T19:54:27Z
Lists: pgsql-hackers 
On Sun, 8 Feb 2026 at 23:38, Andres Freund <andres@anarazel.de> wrote:
>

> Consider:
>
> 1) modify page w/ FPI
> 2) redo pointer determined at X
> 3) modify page w/o FPI, as the page hasn't yet been flushed at X+1
> 4) checkpointer flushes page
> 5) checkpoint completes, at X+2
> 6) page is dirtied, w/o FPI X+3, as X+1 > X
> 7) in the middle of writing out the page, we crash, the page is torn
>
> For recovery we will replay starting from position X. Then will replay the
> record from 3), which will be skipped due to the LSN. Then we will replay X+3,
> which either will be skipped due to the LSN condition (if the page header
> survived the torn page), leading to the changes to the "old portion" of the
> torn page not being replayed, or we will replay the WAL record, applying it to
> a torn page (or failing to read in the page due to checksum errors).
>
> If we only needed to think about buffers that stay in memory, we could "just"
> tackle this by remember that the page will need to be FPId during the next
> modification in the BufferDesc, but that doesn't help us if the page is
> evicted and reread...
>
>

Hmm, after thinking about this, I wonder if we can actually have a TAP
test for this sequence of events?
Maybe it would be desirable to execute some rare recovery code path.
But I'm unsure if there is any reliable way to have an OS to have a
buffer in page cache, but not on disk when evicted.


-- 
Best regards,
Kirill Reshke

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