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

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Peter Geoghegan <pg@bowt.ie>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Alexander Lakhin <exclusion@gmail.com>, Chao Li <li.evan.chao@gmail.com>, Kirill Reshke <reshkekirill@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Melanie Plageman <melanieplageman@gmail.com>, Matthias van de Meent <boekewurm+postgres@gmail.com>, pgsql-hackers@postgresql.org, Thomas Munro <thomas.munro@gmail.com>, Noah Misch <noah@leadboat.com>, Robert Haas <robertmhaas@gmail.com>, Michael Paquier <michael.paquier@gmail.com>
Date: 2026-01-29T19:29:27Z
Lists: pgsql-hackers 
Hi,

On 2026-01-29 13:33:02 -0500, Peter Geoghegan wrote:
> On Thu, Jan 29, 2026 at 1:06 PM Andres Freund <andres@anarazel.de> wrote:
> > Wonder if - independent of this
> > issue - it could make sense to update the FSM during nbtree WAL recovery...
> 
> Maybe that would make sense. But I tend to think that we should have a
> fully atomic, crash-safe approach to free space management.

I agree that would be nice, but realistically (as you also say below) that
would have to be embedded into the WAL records that use the page that was
acquired from the FSM.  Maybe we could accept a dedicated WAL record for the
index case, but certainly not in the heap case.

Given that we'd need to embed the record somehow anyway, just adding, for now,
a RecordUsedIndexPage() to the redo of XLOG_BTREE_SPLIT* and
XLOG_BTREE_NEWROOT or such could make sense...

It doesn't seem like it'd be great to have a completely outdated index fsm
after a failover. If the index FSM on the newly promoted node is completely
outdated, due to having been copied at a much earlier time while there were a
lot of free pages, a _bt_allocbuf() could take quite a while...

I'm somewhat surprised it doesn't cause more performance issues to keep btree
pages exclusively locked while extending the relation... If that has to write
out pages and flush the WAL...


> Particularly in index AMs, where free space can only ever come in
> BLCKSZ units -- the data structure/concurrency rules can be a lot
> simpler if it only has to accommodate index AM requirements. Maybe the
> WAL-logging could be built into existing index AM record types.

Yea, I have my doubt that makes sense to share code between the index and heap
use cases. I doubt that having one FSM implementation support variable amount
of "space tracking granularity" really makes sense.

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. 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